multi-queue scheduler update

multi-queue scheduler update

[Mike Kravetz]

I just posted an updated version of the multi-queue scheduler
for the 2.4.0 kernel.  This version also contains support for
realtime tasks.  The patch can be found at:

http://lse.sourceforge.net/scheduling/

Here are some very preliminary numbers from sched_test_yield
(which was previously posted to this (lse-tech) list by Bill
Hartner).  Tests were run on a system with 8 700 MHz Pentium
III processors.

                           microseconds/yield
# threads      2.2.16-22           2.4        2.4-multi-queue
------------   ---------         --------     ---------------
16               18.740            4.603         1.455
32               17.702            5.134         1.456
64               23.300            5.586         1.466
128              47.273           18.812         1.480
256             105.701           71.147         1.517
512               FRC            143.500         1.661
1024              FRC            196.425         6.166
2048              FRC              FRC          23.291
4096              FRC              FRC          47.117

*FRC = failed to reach confidence level

-- 
Mike Kravetz                                 mkravetz@sequent.com
IBM Linux Technology Center
15450 SW Koll Parkway
Beaverton, OR 97006-6063                     (503)578-3494
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Re: multi-queue scheduler update

[Andrea Arcangeli]

On Thu, Jan 18, 2001 at 03:53:11PM -0800, Mike Kravetz wrote:
> Here are some very preliminary numbers from sched_test_yield
> (which was previously posted to this (lse-tech) list by Bill
> Hartner).  Tests were run on a system with 8 700 MHz Pentium
> III processors.
> 
>                            microseconds/yield
> # threads      2.2.16-22           2.4        2.4-multi-queue
> ------------   ---------         --------     ---------------
> 16               18.740            4.603         1.455

I remeber the O(1) scheduler from Davide Libenzi was beating the mainline O(N)
scheduler with over 7 tasks in the runqueue (actually I'm not sure if the
number was 7 but certainly it was under 10). So if you also use a O(1)
scheduler too as I guess (since you have a chance to run fast on the lots of
tasks running case) the most interesting thing is how you score with 2/4/8
tasks in the runqueue (I think the tests on the O(1) scheduler patch was done
at max on a 2-way SMP btw). (the argument for which Davide's patch wasn't
included is that most machines have less than 4/5 tasks in the runqueue at the
same time)

Andrea
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Re: multi-queue scheduler update

[Gerhard Mack]

What affect does this scheduler have on 1 - 5 tasks??

	Gerhard


On Thu, 18 Jan 2001, Mike Kravetz wrote:

> I just posted an updated version of the multi-queue scheduler
> for the 2.4.0 kernel.  This version also contains support for
> realtime tasks.  The patch can be found at:
> 
> http://lse.sourceforge.net/scheduling/
> 
> Here are some very preliminary numbers from sched_test_yield
> (which was previously posted to this (lse-tech) list by Bill
> Hartner).  Tests were run on a system with 8 700 MHz Pentium
> III processors.
> 
>                            microseconds/yield
> # threads      2.2.16-22           2.4        2.4-multi-queue
> ------------   ---------         --------     ---------------
> 16               18.740            4.603         1.455
> 32               17.702            5.134         1.456
> 64               23.300            5.586         1.466
> 128              47.273           18.812         1.480
> 256             105.701           71.147         1.517
> 512               FRC            143.500         1.661
> 1024              FRC            196.425         6.166
> 2048              FRC              FRC          23.291
> 4096              FRC              FRC          47.117
> 
> *FRC = failed to reach confidence level
> 
> -- 
> Mike Kravetz                                 mkravetz@sequent.com
> IBM Linux Technology Center
> 15450 SW Koll Parkway
> Beaverton, OR 97006-6063                     (503)578-3494
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> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> Please read the FAQ at http://www.tux.org/lkml/
> 

--
Gerhard Mack

gmack@innerfire.net

<>< As a computer I find your faith in technology amusing.

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Re: [Lse-tech] Re: multi-queue scheduler update

[Andi Kleen]

On Fri, Jan 19, 2001 at 01:26:16AM +0100, Andrea Arcangeli wrote:
> I remeber the O(1) scheduler from Davide Libenzi was beating the mainline O(N)
> scheduler with over 7 tasks in the runqueue (actually I'm not sure if the
> number was 7 but certainly it was under 10). So if you also use a O(1)
> scheduler too as I guess (since you have a chance to run fast on the lots of
> tasks running case) the most interesting thing is how you score with 2/4/8
> tasks in the runqueue (I think the tests on the O(1) scheduler patch was done
> at max on a 2-way SMP btw). (the argument for which Davide's patch wasn't
> included is that most machines have less than 4/5 tasks in the runqueue at the
> same time)

They seem to have tried that in a separate patch:
http://lse.sourceforge.net/scheduling/PrioScheduler.html

Very nice literate programming style btw @-)


-Andi 
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Re: [Lse-tech] Re: multi-queue scheduler update

[Mike Kravetz]

On Fri, Jan 19, 2001 at 01:26:16AM +0100, Andrea Arcangeli wrote:
> On Thu, Jan 18, 2001 at 03:53:11PM -0800, Mike Kravetz wrote:
> > Here are some very preliminary numbers from sched_test_yield
> > (which was previously posted to this (lse-tech) list by Bill
> > Hartner).  Tests were run on a system with 8 700 MHz Pentium
> > III processors.
> > 
> >                            microseconds/yield
> > # threads      2.2.16-22           2.4        2.4-multi-queue
> > ------------   ---------         --------     ---------------
> > 16               18.740            4.603         1.455
> 
> I remeber the O(1) scheduler from Davide Libenzi was beating the mainline O(N)
> scheduler with over 7 tasks in the runqueue (actually I'm not sure if the
> number was 7 but certainly it was under 10). So if you also use a O(1)
> scheduler too as I guess (since you have a chance to run fast on the lots of
> tasks running case) the most interesting thing is how you score with 2/4/8
> tasks in the runqueue (I think the tests on the O(1) scheduler patch was done
> at max on a 2-way SMP btw). (the argument for which Davide's patch wasn't
> included is that most machines have less than 4/5 tasks in the runqueue at the
> same time)
> 
> Andrea

Thanks for the suggestion.  The only reason I hesitated to test with
a small number of threads is because I was under the assumption that
this particular benchmark may have problems if the number of threads
was less than the number of processors.  I'll give the tests a try
with a smaller number of threads.  I'm also open to suggestions for
what benchmarks/test methods I could use for scheduler testing.  If
you remember what people have used in the past, please let me know.

-- 
Mike Kravetz                                 mkravetz@sequent.com
IBM Linux Technology Center
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Re: multi-queue scheduler update

[Mark Hahn]

> >                            microseconds/yield
> > # threads      2.2.16-22           2.4        2.4-multi-queue
> > ------------   ---------         --------     ---------------
> > 16               18.740            4.603         1.455
> 
> I remeber the O(1) scheduler from Davide Libenzi was beating the mainline O(N)

isn't the normal case (as in "The Right Case to optimize") 
where there are close to zero runnable tasks?  what realistic/sane
scenarios have very large numbers of spinning threads?  all server
situations I can think of do not.  not volanomark -loopback, surely!

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Re: multi-queue scheduler update

[Andi Kleen]

On Thu, Jan 18, 2001 at 08:00:16PM -0500, Mark Hahn wrote:
> > >                            microseconds/yield
> > > # threads      2.2.16-22           2.4        2.4-multi-queue
> > > ------------   ---------         --------     ---------------
> > > 16               18.740            4.603         1.455
> > 
> > I remeber the O(1) scheduler from Davide Libenzi was beating the mainline O(N)
> 
> isn't the normal case (as in "The Right Case to optimize") 
> where there are close to zero runnable tasks?  what realistic/sane
> scenarios have very large numbers of spinning threads?  all server
> situations I can think of do not.  not volanomark -loopback, surely!

I think the main point of Mike's patch is decreasing locking and cache line
bouncing overhead of multi cpu scheduling, not optimizing lots of runnable tasks.


-Andi
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Re: multi-queue scheduler update

[John Clemens]

While I agree that this is probably only a win for very specialized tasks,
I'd be interested in seeing this patch implemented on a NUMA machine, with
one runqueue per node... anybody willing to try it? I don't have access to
one. How about from the Linux Scalability project at SGI? any comments?

john.c

-- 
John Clemens          http://www.deater.net/john
john@deater.net     ICQ: 7175925, IM: PianoManO8
      "I Hate Quotes" -- Samuel L. Clemens


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Re: multi-queue scheduler update

[Mike Kravetz]

On Fri, Jan 19, 2001 at 02:08:52AM +0100, Andi Kleen wrote:
> On Thu, Jan 18, 2001 at 08:00:16PM -0500, Mark Hahn wrote:
> > > >                            microseconds/yield
> > > > # threads      2.2.16-22           2.4        2.4-multi-queue
> > > > ------------   ---------         --------     ---------------
> > > > 16               18.740            4.603         1.455
> > > 
> > > I remeber the O(1) scheduler from Davide Libenzi was beating the mainline O(N)
> > 
> > isn't the normal case (as in "The Right Case to optimize") 
> > where there are close to zero runnable tasks?  what realistic/sane
> > scenarios have very large numbers of spinning threads?  all server
> > situations I can think of do not.  not volanomark -loopback, surely!
> 
> I think the main point of Mike's patch is decreasing locking and cache line
> bouncing overhead of multi cpu scheduling, not optimizing lots of runnable tasks.
> 
> 
> -Andi

Andi is correct.  Although the results I posted may seem to indicate
we are concentrating on high thread counts, this is really secondary
to reducing lock contention within the scheduler.  A co-worker down
the hall just ran pgbench (a postgresql db) benchmark and saw
contention on the runqueue lock at 57%.  Now, I know nothing about this
benchmark, but it will be interesting to see what happens after
applying my patch.

-- 
Mike Kravetz                                 mkravetz@sequent.com
IBM Linux Technology Center
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Re: [Lse-tech] Re: multi-queue scheduler update

[Andrea Arcangeli]

On Thu, Jan 18, 2001 at 04:52:25PM -0800, Mike Kravetz wrote:
> was less than the number of processors.  I'll give the tests a try
> with a smaller number of threads.  I'm also open to suggestions for

OK!

> what benchmarks/test methods I could use for scheduler testing.  If
> you remember what people have used in the past, please let me know.

It was this one IIRC (it spawns threads calling sched_yield() in loop).

/*
  Tester for the kernel's speed in scheduling.
  (C) 1999 / Willy Tarreau <willy@meta-x.org>

  Modified by Davide Libenzi <davidel@maticad.it>


  You can do whatever you want with this program, but I'm not
  responsible for any misuse. Be aware that it can heavily load
  a host. As it is multithreaded, it might take advantages of SMP.

  It basically creates a growing amount of threads and measures
  their cumulative work (i.e. loop iterations/second). The output
  is easily useable by gnuplot.

  To compile, you need libpthread :

     gcc -O2 -fomit-frame-pointer -o threads threads.c -lpthread

  Output on stdout is :
     <nb_threads> <average_work> <zero_work_threads> <std_deviation>

*/

#include <stdio.h>
#include <pthread.h>
#include <signal.h>
#include <unistd.h>
#include <time.h>



#define MAXTHREADS	450
#define MEASURE_TIME	60



pthread_t       thr[MAXTHREADS];
int             nbthreads = MAXTHREADS;
int             measure_time = MEASURE_TIME;
volatile        actthreads = 0;

long long int   totalwork[MAXTHREADS];
volatile int    stop = 0,
                start = 0,
                count = 0;

void            oneatwork(int thr)
{
    int             i;
    while (!start)              /* don't disturb pthread_create() */
        usleep(10000);

    actthreads++;
    while (!stop)
    {
        if (count)
            totalwork[thr]++;

        syscall(158); /* sys_sched_yield() */
    }
    actthreads--;
    pthread_exit(0);
}

main(int argc, char **argv)
{

    int             i,
                    err,
                    avgwork,
                    thrzero;
    long long int   value,
                    avgvalue;
    double          sqrdev;
    time_t          ts,
                    te;

    if (argc < 3)
    {
        printf("usage: %s  threads  time\n", argv[0]);
        exit(1);
    }    

    nbthreads = atoi(argv[1]);
    measure_time = atoi(argv[2]);
    
    
    start = 0;
    count = 0;
    stop = 0;
    actthreads = 0;
    thrzero = 0;
    value = 0;
    sqrdev = 0.0;

    fprintf(stderr, "\nCreating %d threads ...", nbthreads);
    for (i = 0; i < nbthreads; i++)
    {
        if ((err = pthread_create(&thr[i], NULL, (void *) &oneatwork, (void *) i)) != 0)
        {
            fprintf(stderr, "thread %d pthread_create=%d -> ", i, err);
            perror("");
            exit(1);
        }
        pthread_detach(thr[i]);
    }

    for (i = 0; i < nbthreads; i++)
        totalwork[i] = 0;

    fprintf(stderr, " OK !\nWaiting for all threads to start ...");

    start = 1;
    while (actthreads != nbthreads)
        usleep(10000);         /* waiting for a bit of stability */

    fprintf(stderr, "Go !\n");

    count = 1;
    time(&ts);

    sleep(measure_time);

    count = 0;
    stop = 1;
    time(&te);


    for (i = 0; i < nbthreads; i++)
    {
        value += totalwork[i];
        if (totalwork[i] == 0)
            ++thrzero;
    }
    avgvalue = value / nbthreads;
    value /= (int) difftime(te, ts);
    avgwork = (int) (value / nbthreads);

    for (i = 0; i < nbthreads; i++)
    {
        double          difvv = (double) (totalwork[i] - avgvalue);

        sqrdev += difvv * difvv;
    }

    while (actthreads > 0)
        usleep(10000);

    printf("%d\t\t%lld\t\t%d\t\t%d\t\t%f\n", nbthreads, value, avgwork, thrzero,
            sqrdev / ((double) nbthreads * avgvalue * avgvalue));

    exit(0);

}

Andrea
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Re: [Lse-tech] Re: multi-queue scheduler update

[Mike Kravetz]

On Fri, Jan 19, 2001 at 02:30:41AM +0100, Andrea Arcangeli wrote:
> On Thu, Jan 18, 2001 at 04:52:25PM -0800, Mike Kravetz wrote:
> > was less than the number of processors.  I'll give the tests a try
> > with a smaller number of threads.  I'm also open to suggestions for
> 
> OK!
> 
> > what benchmarks/test methods I could use for scheduler testing.  If
> > you remember what people have used in the past, please let me know.
> 
> It was this one IIRC (it spawns threads calling sched_yield() in loop).

Thanks!

At first glance this looks to be the same type of test/benchmark
I have been using.

-
Mike
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Re: multi-queue scheduler update

[Andrea Arcangeli]

On Thu, Jan 18, 2001 at 08:00:16PM -0500, Mark Hahn wrote:
> > >                            microseconds/yield
> > > # threads      2.2.16-22           2.4        2.4-multi-queue
> > > ------------   ---------         --------     ---------------
> > > 16               18.740            4.603         1.455
> > 
> > I remeber the O(1) scheduler from Davide Libenzi was beating the mainline O(N)
> 
> isn't the normal case (as in "The Right Case to optimize") 
> where there are close to zero runnable tasks?  what realistic/sane
> scenarios have very large numbers of spinning threads?  all server
> situations I can think of do not.  not volanomark -loopback, surely!

This is why the numbers with 2/4/8 threads in the runqueue are the most
interesting ones 8)

Andrea
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Re: multi-queue scheduler update

[Davide Libenzi]

On Thursday 18 January 2001 17:33, Mike Kravetz wrote:
> On Fri, Jan 19, 2001 at 02:08:52AM +0100, Andi Kleen wrote:
> > On Thu, Jan 18, 2001 at 08:00:16PM -0500, Mark Hahn wrote:
> > > > >                            microseconds/yield
> > > > > # threads      2.2.16-22           2.4        2.4-multi-queue
> > > > > ------------   ---------         --------     ---------------
> > > > > 16               18.740            4.603         1.455
> > > >
> > > > I remeber the O(1) scheduler from Davide Libenzi was beating the
> > > > mainline O(N)
> > >
> > > isn't the normal case (as in "The Right Case to optimize")
> > > where there are close to zero runnable tasks?  what realistic/sane
> > > scenarios have very large numbers of spinning threads?  all server
> > > situations I can think of do not.  not volanomark -loopback, surely!
> >
> > I think the main point of Mike's patch is decreasing locking and cache
> > line bouncing overhead of multi cpu scheduling, not optimizing lots of
> > runnable tasks.
> >
> >
> > -Andi
>
> Andi is correct.  Although the results I posted may seem to indicate
> we are concentrating on high thread counts, this is really secondary
> to reducing lock contention within the scheduler.  A co-worker down
> the hall just ran pgbench (a postgresql db) benchmark and saw
> contention on the runqueue lock at 57%.  Now, I know nothing about this
> benchmark, but it will be interesting to see what happens after
> applying my patch.

Yep, the patch work in a different way and if these are the numbers it seems 
to be interesting.
Could You post results for a fewer number of tasks ?
I mean what is the performance loss for 1,2,..,5 tasks ?

To test You can use lmbench ( I don't remember the link ) and I should have 
the program I've used to test my patch somewhere.


- Davide
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Re: [Lse-tech] Re: multi-queue scheduler update

[Davide Libenzi]

On Thursday 18 January 2001 17:39, Andrea Arcangeli wrote:
> On Thu, Jan 18, 2001 at 04:52:25PM -0800, Mike Kravetz wrote:
> > was less than the number of processors.  I'll give the tests a try
> > with a smaller number of threads.  I'm also open to suggestions for
>
> OK!
>
> > what benchmarks/test methods I could use for scheduler testing.  If
> > you remember what people have used in the past, please let me know.
>
> It was this one IIRC (it spawns threads calling sched_yield() in loop).
>
> /*
>   Tester for the kernel's speed in scheduling.
>   (C) 1999 / Willy Tarreau <willy@meta-x.org>
>
>   Modified by Davide Libenzi <davidel@maticad.it>
>
>
>   You can do whatever you want with this program, but I'm not
>   responsible for any misuse. Be aware that it can heavily load
>   a host. As it is multithreaded, it might take advantages of SMP.
>
>   It basically creates a growing amount of threads and measures
>   their cumulative work (i.e. loop iterations/second). The output
>   is easily useable by gnuplot.
>
>   To compile, you need libpthread :
>
>      gcc -O2 -fomit-frame-pointer -o threads threads.c -lpthread
>
>   Output on stdout is :
>      <nb_threads> <average_work> <zero_work_threads> <std_deviation>
>
> */
>
> #include <stdio.h>
> #include <pthread.h>
> #include <signal.h>
> #include <unistd.h>
> #include <time.h>
>
>
>
> #define MAXTHREADS	450
> #define MEASURE_TIME	60
>
>
>
> pthread_t       thr[MAXTHREADS];
> int             nbthreads = MAXTHREADS;
> int             measure_time = MEASURE_TIME;
> volatile        actthreads = 0;
>
> long long int   totalwork[MAXTHREADS];
> volatile int    stop = 0,
>                 start = 0,
>                 count = 0;
>
> void            oneatwork(int thr)
> {
>     int             i;
>     while (!start)              /* don't disturb pthread_create() */
>         usleep(10000);
>
>     actthreads++;
>     while (!stop)
>     {
>         if (count)
>             totalwork[thr]++;
>
>         syscall(158); /* sys_sched_yield() */
>     }
>     actthreads--;
>     pthread_exit(0);
> }
>
> main(int argc, char **argv)
> {
>
>     int             i,
>                     err,
>                     avgwork,
>                     thrzero;
>     long long int   value,
>                     avgvalue;
>     double          sqrdev;
>     time_t          ts,
>                     te;
>
>     if (argc < 3)
>     {
>         printf("usage: %s  threads  time\n", argv[0]);
>         exit(1);
>     }
>
>     nbthreads = atoi(argv[1]);
>     measure_time = atoi(argv[2]);
>
>
>     start = 0;
>     count = 0;
>     stop = 0;
>     actthreads = 0;
>     thrzero = 0;
>     value = 0;
>     sqrdev = 0.0;
>
>     fprintf(stderr, "\nCreating %d threads ...", nbthreads);
>     for (i = 0; i < nbthreads; i++)
>     {
>         if ((err = pthread_create(&thr[i], NULL, (void *) &oneatwork, (void
> *) i)) != 0) {
>             fprintf(stderr, "thread %d pthread_create=%d -> ", i, err);
>             perror("");
>             exit(1);
>         }
>         pthread_detach(thr[i]);
>     }
>
>     for (i = 0; i < nbthreads; i++)
>         totalwork[i] = 0;
>
>     fprintf(stderr, " OK !\nWaiting for all threads to start ...");
>
>     start = 1;
>     while (actthreads != nbthreads)
>         usleep(10000);         /* waiting for a bit of stability */
>
>     fprintf(stderr, "Go !\n");
>
>     count = 1;
>     time(&ts);
>
>     sleep(measure_time);
>
>     count = 0;
>     stop = 1;
>     time(&te);
>
>
>     for (i = 0; i < nbthreads; i++)
>     {
>         value += totalwork[i];
>         if (totalwork[i] == 0)
>             ++thrzero;
>     }
>     avgvalue = value / nbthreads;
>     value /= (int) difftime(te, ts);
>     avgwork = (int) (value / nbthreads);
>
>     for (i = 0; i < nbthreads; i++)
>     {
>         double          difvv = (double) (totalwork[i] - avgvalue);
>
>         sqrdev += difvv * difvv;
>     }
>
>     while (actthreads > 0)
>         usleep(10000);
>
>     printf("%d\t\t%lld\t\t%d\t\t%d\t\t%f\n", nbthreads, value, avgwork,
> thrzero, sqrdev / ((double) nbthreads * avgvalue * avgvalue));
>
>     exit(0);
>
> }
>

Andrea found it before me :)


- Davide
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Re: multi-queue scheduler update

[Andi Kleen]

On Fri, Jan 19, 2001 at 02:35:02AM +0100, Andrea Arcangeli wrote:
> On Thu, Jan 18, 2001 at 08:00:16PM -0500, Mark Hahn wrote:
> > > >                            microseconds/yield
> > > > # threads      2.2.16-22           2.4        2.4-multi-queue
> > > > ------------   ---------         --------     ---------------
> > > > 16               18.740            4.603         1.455
> > > 
> > > I remeber the O(1) scheduler from Davide Libenzi was beating the mainline O(N)
> > 
> > isn't the normal case (as in "The Right Case to optimize") 
> > where there are close to zero runnable tasks?  what realistic/sane
> > scenarios have very large numbers of spinning threads?  all server
> > situations I can think of do not.  not volanomark -loopback, surely!
> 
> This is why the numbers with 2/4/8 threads in the runqueue are the most
> interesting ones 8)

With Arjan's patch to use prefetching for the runqueue scan the numbers
will be likely different [at least on cpus that can benefit from prefetching
like p2+ or athlon] 

-Andi
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Re: [Lse-tech] Re: multi-queue scheduler update

[David Lang]

another thing that would be interesting is what is the overhead on UP or
small (2-4 way) SMP machines

David Lang

On Thu, 18 Jan 2001, Mike Kravetz wrote:

> Date: Thu, 18 Jan 2001 16:52:25 -0800
> From: Mike Kravetz <mkravetz@sequent.com>
> To: Andrea Arcangeli <andrea@suse.de>
> Cc: lse-tech@lists.sourceforge.net, linux-kernel@vger.kernel.org
> Subject: Re: [Lse-tech] Re: multi-queue scheduler update
>
> On Fri, Jan 19, 2001 at 01:26:16AM +0100, Andrea Arcangeli wrote:
> > On Thu, Jan 18, 2001 at 03:53:11PM -0800, Mike Kravetz wrote:
> > > Here are some very preliminary numbers from sched_test_yield
> > > (which was previously posted to this (lse-tech) list by Bill
> > > Hartner).  Tests were run on a system with 8 700 MHz Pentium
> > > III processors.
> > >
> > >                            microseconds/yield
> > > # threads      2.2.16-22           2.4        2.4-multi-queue
> > > ------------   ---------         --------     ---------------
> > > 16               18.740            4.603         1.455
> >
> > I remeber the O(1) scheduler from Davide Libenzi was beating the mainline O(N)
> > scheduler with over 7 tasks in the runqueue (actually I'm not sure if the
> > number was 7 but certainly it was under 10). So if you also use a O(1)
> > scheduler too as I guess (since you have a chance to run fast on the lots of
> > tasks running case) the most interesting thing is how you score with 2/4/8
> > tasks in the runqueue (I think the tests on the O(1) scheduler patch was done
> > at max on a 2-way SMP btw). (the argument for which Davide's patch wasn't
> > included is that most machines have less than 4/5 tasks in the runqueue at the
> > same time)
> >
> > Andrea
>
> Thanks for the suggestion.  The only reason I hesitated to test with
> a small number of threads is because I was under the assumption that
> this particular benchmark may have problems if the number of threads
> was less than the number of processors.  I'll give the tests a try
> with a smaller number of threads.  I'm also open to suggestions for
> what benchmarks/test methods I could use for scheduler testing.  If
> you remember what people have used in the past, please let me know.
>
> --
> Mike Kravetz                                 mkravetz@sequent.com
> IBM Linux Technology Center
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Re: [Lse-tech] Re: multi-queue scheduler update

[Mike Kravetz]

On Thu, Jan 18, 2001 at 05:34:35PM -0800, Mike Kravetz wrote:
> On Fri, Jan 19, 2001 at 02:30:41AM +0100, Andrea Arcangeli wrote:
> > On Thu, Jan 18, 2001 at 04:52:25PM -0800, Mike Kravetz wrote:
> > > was less than the number of processors.  I'll give the tests a try
> > > with a smaller number of threads.  I'm also open to suggestions for
> > 
> > OK!
> > 
> > > what benchmarks/test methods I could use for scheduler testing.  If
> > > you remember what people have used in the past, please let me know.
> > 
> > It was this one IIRC (it spawns threads calling sched_yield() in loop).
> 
> Thanks!

It was my intention to post IIRC numbers for small thread counts today.
However, the benchmark (not the system) seems to hang on occasion.  This
occurs on both the unmodified 2.4.0 kernel and the one which contains
my multi-queue patch.  Therefore, I'm pretty sure it is not something
I did. :)

Anyone else see anything like this before?  I'll look into the reason
for the hang, but it will delay my posting of these numbers.

-- 
Mike Kravetz                                 mkravetz@sequent.com
IBM Linux Technology Center
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Re: [Lse-tech] Re: multi-queue scheduler update

[Mike Kravetz]

On Fri, Jan 19, 2001 at 12:49:21PM -0800, Mike Kravetz showed his lack
of internet slang understanding and wrote:
> 
> It was my intention to post IIRC numbers for small thread counts today.
> However, the benchmark (not the system) seems to hang on occasion.  This
> occurs on both the unmodified 2.4.0 kernel and the one which contains
> my multi-queue patch.  Therefore, I'm pretty sure it is not something
> I did. :)
> 
> Anyone else see anything like this before?  I'll look into the reason
> for the hang, but it will delay my posting of these numbers.

I think I have found the problem.  Here is a code snippet from the
benchmark Andrea posted.

void            oneatwork(int thr)
{
    int             i;
    while (!start)              /* don't disturb pthread_create() */
        usleep(10000);                                                          

    actthreads++;
    while (!stop)
    {
        if (count)
            totalwork[thr]++;

        syscall(158); /* sys_sched_yield() */                                   
    }                                                                           
    actthreads--;                                                               
    pthread_exit(0);
}                                                                               

Note that actthreads is a global variable which is being updated
by multiple threads without any form of synchronization.  Because
of this actthreads sometimes never goes to zero after all worker
threads have finished.  I changed actthreads to be an atomic and
used atomic operations to manipulate it.  With this change, I was
able to complete one round of testing which I had not been able to
do in the past.

Does anyone maintain this benchmark code?  The changes I indicate
above should be made.  If you need more specifics I can provide
them.

-- 
Mike Kravetz                                 mkravetz@sequent.com
IBM Linux Technology Center
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Re: [Lse-tech] Re: multi-queue scheduler update

[Davide Libenzi]

On Friday 19 January 2001 13:59, Mike Kravetz wrote:
> On Fri, Jan 19, 2001 at 12:49:21PM -0800, Mike Kravetz showed his lack
>
> of internet slang understanding and wrote:
> > It was my intention to post IIRC numbers for small thread counts today.
> > However, the benchmark (not the system) seems to hang on occasion.  This
> > occurs on both the unmodified 2.4.0 kernel and the one which contains
> > my multi-queue patch.  Therefore, I'm pretty sure it is not something
> > I did. :)
> >
> > Anyone else see anything like this before?  I'll look into the reason
> > for the hang, but it will delay my posting of these numbers.
>
> I think I have found the problem.  Here is a code snippet from the
> benchmark Andrea posted.
>
> void            oneatwork(int thr)
> {
>     int             i;
>     while (!start)              /* don't disturb pthread_create() */
>         usleep(10000);
>
>     actthreads++;
>     while (!stop)
>     {
>         if (count)
>             totalwork[thr]++;
>
>         syscall(158); /* sys_sched_yield() */
>     }
>     actthreads--;
>     pthread_exit(0);
> }
>
> Note that actthreads is a global variable which is being updated
> by multiple threads without any form of synchronization.  Because
> of this actthreads sometimes never goes to zero after all worker
> threads have finished. 

If all threads complete successfully actthreads has to be zero.
If some thread dies, this won't be true.



- Davide
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Re: [Lse-tech] Re: multi-queue scheduler update

[Mike Kravetz]

On Fri, Jan 19, 2001 at 02:03:06PM -0800, Davide Libenzi wrote:
<stuff deleted>
> > void            oneatwork(int thr)
> > {
> >     int             i;
> >     while (!start)              /* don't disturb pthread_create() */
> >         usleep(10000);
> >
> >     actthreads++;
> >     while (!stop)
> >     {
> >         if (count)
> >             totalwork[thr]++;
> >
> >         syscall(158); /* sys_sched_yield() */
> >     }
> >     actthreads--;
> >     pthread_exit(0);
> > }
> >
> > Note that actthreads is a global variable which is being updated
> > by multiple threads without any form of synchronization.  Because
> > of this actthreads sometimes never goes to zero after all worker
> > threads have finished. 
> 
> If all threads complete successfully actthreads has to be zero.

Not as currently coded.  If two threads try to decrement actthreads
at the same time, there is no guarantee that it will be decremented
twice.  That is why you need to put some type of synchronization in
place.

-- 
Mike Kravetz                                 mkravetz@sequent.com
IBM Linux Technology Center
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Re: [Lse-tech] Re: multi-queue scheduler update

[Davide Libenzi]

On Friday 19 January 2001 15:23, Mike Kravetz wrote:
> On Fri, Jan 19, 2001 at 02:03:06PM -0800, Davide Libenzi wrote:
> <stuff deleted>
>
> > > void            oneatwork(int thr)
> > > {
> > >     int             i;
> > >     while (!start)              /* don't disturb pthread_create() */
> > >         usleep(10000);
> > >
> > >     actthreads++;
> > >     while (!stop)
> > >     {
> > >         if (count)
> > >             totalwork[thr]++;
> > >
> > >         syscall(158); /* sys_sched_yield() */
> > >     }
> > >     actthreads--;
> > >     pthread_exit(0);
> > > }
> > >
> > > Note that actthreads is a global variable which is being updated
> > > by multiple threads without any form of synchronization.  Because
> > > of this actthreads sometimes never goes to zero after all worker
> > > threads have finished.
> >
> > If all threads complete successfully actthreads has to be zero.
>
> Not as currently coded.  If two threads try to decrement actthreads
> at the same time, there is no guarantee that it will be decremented
> twice.  That is why you need to put some type of synchronization in
> place.

Right, inc & dec are not atomic w/o #LOCK.


- Davide
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Re: multi-queue scheduler update

[Mike Kravetz]

As promised, here are some numbers for low thread counts from the
benchmark Andrew and Davide provided.  I ran the benchmark for
1,2,4 and 8 threads.  I ran the test 5 times for each thread count
and used 60 seconds as the measure time in each case.

2.4.0
-----
1               1785408         1785408         0               0.000000
1               1786130         1786130         0               0.000000
1               1786156         1786156         0               0.000000
1               1781575         1781575         0               0.000000
1               1780079         1780079         0               0.000000
2               1873405         936702          0               0.000000
2               2006473         1003236         0               0.000001
2               1953842         976921          0               0.000004
2               1951338         975669          0               0.000000
2               1887887         943943          0               0.000004
4               1936350         484087          0               0.000055
4               1814430         453607          0               0.000087
4               1972681         493170          0               0.000055
4               1951748         487937          0               0.000206
4               1862182         465545          0               0.000283
8               2917216         364652          0               0.000008
8               2655834         331979          0               0.000018
8               3026734         378341          0               0.000005
8               3010204         376275          0               0.000004
8               2569647         321205          0               0.000014

2.4.0-multi-queue
-----------------
1               1295498         1295498         0               0.000000
1               1295011         1295011         0               0.000000
1               1296768         1296768         0               0.000000
1               1296053         1296053         0               0.000000
1               1296472         1296472         0               0.000000
2               1999043         999521          0               0.000000
2               1410636         705318          0               0.000000
2               1414476         707238          0               0.000000
2               2014664         1007332         0               0.000001
2               1414509         707254          0               0.000000
4               2046182         511545          0               0.000232
4               2101535         525383          0               0.000115
4               2094828         523707          0               0.000155
4               2097406         524351          0               0.000144
4               2057331         514332          0               0.000132
8               3795829         474478          0               0.000185
8               4058329         507291          0               0.001871
8               3845934         480741          0               0.000248
8               3715243         464405          0               0.000084
8               3777303         472162          0               0.000194

As expected the single thread numbers for the multi-queue scheduler
are not as good as those of the existing scheduler.  However, at 2
threads it is getting pretty close and from 4 threads up, the
multi-queue scheduler does better.

In this multi-queue implementation, the amount of overhead is
related to the number of processors in the system.  Therefore,
I would expect the numbers to 'be better' for low thread counts
on systems with lower (<8) processor counts.  It would be
interesting to see if the point at which the multi-queue does
better stays at aprox CPUs/2 as we change system configurations.
Hopefully we will have some more extensive benchmark results in
the not too distant future.  Until then, we'll be looking into
optimizations to help out the multi-queue scheduler at low
thread counts.

-- 
Mike Kravetz                                 mkravetz@sequent.com
IBM Linux Technology Center
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Re: [Lse-tech] multi-queue scheduler update

[Jun Nakajima]

I tried to run SDET (Software Development Environment Throughput), which
basically is a system level, throughput oriented benchmark, on the 2.4.0
kernel and 2.4.0 kernel with this patch. 

I guess many (old?) Unix guys are familiar with it, and it is (was?)
sometimes used to check some aspects of scalability of the system. The
details of this bechmark are not so important in this mail (available
upon request).

The following are very preliminary numbers from the benchmark. Tests
were run on a system with 8 550 MHz Pentium III processors. I think
those results are encouraging.

# of Scripts	Throughput 	Throughput
                2.4		2.4-multi-queue
---------	----------	--------
1       	2057.1		1978.0
2       	4114.3		4067.8
4       	7700.5		7700.5
6       	10746.3		10746.3
8       	12973.0		12576.4
10      	13186.8		13235.3
15      	13138.7		13235.3
20      	12996.4		13043.5
25      	13005.8		13005.8
30      	12811.4		13059.3
40      	12676.1		12732.1
50      	12121.2		12676.1
60      	12314.7		12442.4
70      	12051.6		11954.5
80      	11871.4		11985.0
90      	11608.7		11777.5
100     	10849.9		11523.7
125     	10678.7		10940.9
150     	10416.7		10503.8
175     	10187.6		10314.3
200     	9749.5		10106.7
250     	8343.4		8787.3

I also checked hot-spots with the 2.4.0 kernel (not with multi-queue)
with lockmeter (http://oss.sgi.com/projects/lockmeter/). The data were
sampled when the number of scripte is 175.

SPINLOCKS       HOLD          WAIT
   UTIL   CON   MEAN (MAX)    MEAN (MAX)        TOTAL   NAME
...
10.56%  26.89%  7.4us(175us)  3.4us(692us)  1569304   runqueue_lock
 2.23%  29.75%  4.5us(20us)   4.4us(646us)   550505    __wake_up+0x7c
 0.01%  11.62%  6.6us(15us)   1.0us(65us)      2056    __wake_up+0x128
 0.00%  14.29%  0.4us(2.6us)  3.0us(332us)     1393   
deliver_signal+0x58
 0.00%   9.94%  7.2us(16us)   1.2us(56us)       332   
process_timeout+0x14
 0.01%  26.70%  4.7us(16us)   5.0us(296us)     1457   
schedule_tail+0x58
 7.53%  23.28%   11us(175us)  3.0us(692us)   781676    schedule+0xd0
 0.66%  35.42%  3.5us(23us)   2.8us(486us)   206008    schedule+0x458
 0.00%  11.79%  4.2us(78us)   1.1us(56us)       560    schedule+0x504
 0.11%   9.42%  5.0us(21us)   2.3us(420us)    25317   
wake_up_process+0x14

The above result basically tells utilization of runqueue_lock is about
10% of all spinlocks held during the benchmarck and nealy 27% of the
requests for this lock need to spin and wait for the lock (The NAMEs
below the lock are the locations where that lock is used). This might
explain the throughput improvements gained by the multi-queue scheduler.

Now who has the largest utilization? Of course it's kernel_flag.
SPINLOCKS       HOLD          WAIT
   UTIL   CON   MEAN (MAX)    MEAN (MAX)      TOTAL  NAME
...
43.15%  33.08%  13us(95971us) 12us(95997us) 3558789  kernel_flag
 0.02%  38.26%  0.7us(29us)   34us(94975us)   23788   acct_process+0x1c
 0.02%  44.63%  8.3us(43us)    23us(675us)     2012   chrdev_open+0x4c
 0.00%  22.26%  0.9us(2.5us)   16us(525us)      283   de_put+0x28
 5.26%  38.34%  244us(1184us) 21us(53127us)   23788   do_exit+0xf8
 0.99%  36.22%   11us(840us)  12us(53195us)   96205   
ext2_delete_inode+0x20
 0.46%  29.64%  1.2us(159us)  9.1us(53249us) 430421  
ext2_discard_prealloc+0x20
 1.28%  40.60%  9.7us(152us)   22us(43404us) 146014  
ext2_get_block+0x54
 0.00%  40.00%  0.4us(0.7us)   8.6us(34us)        5  
locks_remove_flock+0x34
 0.00%  40.00%  0.6us(1.2us)   4.5us(14us)        5  
locks_remove_posix+0x38
 0.92%  40.80%  12us(572us)    16us(47804us)  84618   lookup_hash+0x84
 0.16%  37.35%  1.0us(178us)   13us(53173us) 175002   notify_change+0x68
 7.78%  15.00%  46us(2523us)   3.1us(27213us)188485   permission+0x38
20.34%  32.99%  12us(1981us)   12us(95997us)1927065   real_lookup+0x64
 0.05%  47.31%  595us(51910us) 22us(270us)       93   schedule+0x490
 0.56%  42.11%  32861us(95971us)41us(405us)      19  
sync_old_buffers+0x20
 0.83%  40.22%  19us(1473us)   19us(41614us)  48081   sys_fcntl64+0x44
 0.01%  38.05%  1.3us(37us)    22us(49506us)  12422   sys_ioctl+0x4c
 0.06%  33.12%  0.5us(62us)    15us(49778us) 132230   sys_llseek+0x88
 0.00%  39.64%  0.9us(4.9us)   19us(849us)     5401   sys_lseek+0x6c
 0.00%  37.50%  28us(48us)     12us(222us)      200   sys_rename+0x1a0
 0.02%  42.29%  6.2us(22us)    81us(93181us)   3802   sys_sysctl+0x4c
 0.00%  52.27%   6.4us(29us)   13us(156us)      132   tty_read+0xbc
 0.01%  41.36%  13us(37us)     16us(434us)      810   tty_release+0x1c
 0.00%  48.12%  17us(143us)    22us(497us)      133   tty_write+0x1bc
 2.08%  41.32%  25us(309us)    18us(29470us)  92009   vfs_create+0x98
 0.52%  38.57%  85us(227us)    12us(698us)     6800   vfs_mkdir+0x90
 1.10%  38.40%  20us(317us)    14us(1100us)   60359   vfs_readdir+0x68
 0.07%  41.66%  12us(78us)    18us(1120us)     6800   vfs_rmdir+0x188
 0.00% 100.00%  24us(24us)    21us(27us)          2   vfs_statfs+0x4c
 0.60%  36.52%  7.2us(104us)  9.4us(904us)    91805   vfs_unlink+0x110

This tells many things, but 
- utilization of kernel_flag is about 43% and more than half of that 
  utilization is done by real_lookup.
- its average hold-time is not relatively significant, but max wait-time 
  is.
- The location sync_old_buffers+0x20 looks responsible for the longest 
  wait-time (95997us).
- sync_old_buffers is responsible only for 0.83% of lock utilization,
but
  it has the largest average (32861us) and max (95971us) hold-time.

So if we replace the big kernel lock with a fine-grained lock in the
real_lookup function, we would see more throughput improvements at
leaset for this benchmarck. 

But I guess the reason for holding the big kernel in real_lookup() is
that not all filesystems don't implement an MP-safe lookup routine. Is
that correct assumption?

For sync_old_buffers, we could hold the big kernel lock per filesystem,
for example. 

static struct dentry * real_lookup(struct dentry * parent, struct qstr *
name, int flags)
{
 ...

        result = d_lookup(parent, name);
        if (!result) {
                struct dentry * dentry = d_alloc(parent, name);
                result = ERR_PTR(-ENOMEM);
                if (dentry) {
                        lock_kernel();
                        result = dir->i_op->lookup(dir, dentry);
                        unlock_kernel();
                        if (result)
                                dput(dentry);
                        else
                                result = dentry;
                }
                up(&dir->i_sem);
                return result;
        }

...
}

static int sync_old_buffers(void)
{
        lock_kernel();
        sync_supers(0);
        sync_inodes(0);
        unlock_kernel();

        flush_dirty_buffers(1);
        /* must really sync all the active I/O request to disk here */
        run_task_queue(&tq_disk);
        return 0;
}



Mike Kravetz wrote:
> 
> I just posted an updated version of the multi-queue scheduler
> for the 2.4.0 kernel.  This version also contains support for
> realtime tasks.  The patch can be found at:
> 
> http://lse.sourceforge.net/scheduling/
> 
> Here are some very preliminary numbers from sched_test_yield
> (which was previously posted to this (lse-tech) list by Bill
> Hartner).  Tests were run on a system with 8 700 MHz Pentium
> III processors.
> 
>                            microseconds/yield
> # threads      2.2.16-22           2.4        2.4-multi-queue
> ------------   ---------         --------     ---------------
> 16               18.740            4.603         1.455
> 32               17.702            5.134         1.456
> 64               23.300            5.586         1.466
> 128              47.273           18.812         1.480
> 256             105.701           71.147         1.517
> 512               FRC            143.500         1.661
> 1024              FRC            196.425         6.166
> 2048              FRC              FRC          23.291
> 4096              FRC              FRC          47.117
> 
> *FRC = failed to reach confidence level
> 
> --
> Mike Kravetz                                 mkravetz@sequent.com
> IBM Linux Technology Center
> 15450 SW Koll Parkway
> Beaverton, OR 97006-6063                     (503)578-3494
> 
> _______________________________________________
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-- 
Jun U Nakajima
Core OS Development
SCO/Murray Hill, NJ
Email: jun@sco.com, Phone: 908-790-2352 Fax: 908-790-2426
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Re: [Lse-tech] multi-queue scheduler update

[Mike Kravetz]

On Tue, Jan 23, 2001 at 11:49:27AM -0500, Jun Nakajima wrote:
> I tried to run SDET (Software Development Environment Throughput), which
> basically is a system level, throughput oriented benchmark, on the 2.4.0
> kernel and 2.4.0 kernel with this patch. 

Thanks for running this.  I too remember SDET, but I won't claim
to be old. :)

We were doing some more analysis on the multi-queue scheduler and
noticed that performance has regressed since posting preliminary
numbers with the 2.4.0-test10 kernel.  After comparing the code,
it looks like I have over-engineered for the worst case of lock
contention.  This was done at the expense of the normal case.
I'm currently working on this situation and expect to have a new
patch out in the not too distant future.

I expect the numbers will get better.
-- 
Mike Kravetz                                 mkravetz@sequent.com
IBM Linux Technology Center
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Re: multi-queue scheduler update

[Jesse Pollard]

Mark Hahn <hahn@coffee.psychology.mcmaster.ca>:
> 
> > >                            microseconds/yield
> > > # threads      2.2.16-22           2.4        2.4-multi-queue
> > > ------------   ---------         --------     ---------------
> > > 16               18.740            4.603         1.455
> > 
> > I remeber the O(1) scheduler from Davide Libenzi was beating the mainline O(N)
> 
> isn't the normal case (as in "The Right Case to optimize") 
> where there are close to zero runnable tasks?  what realistic/sane
> scenarios have very large numbers of spinning threads?  all server
> situations I can think of do not.  not volanomark -loopback, surely!

How about massively parallel compute jobs when synchronizing.

-------------------------------------------------------------------------
Jesse I Pollard, II
Email: pollard@navo.hpc.mil

Any opinions expressed are solely my own.
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