Re: Do not overload dispatch queue (Was: Re: IO scheduler based IO controller V10)

[Mike Galbraith <efault@gmx.de>]

On Sat, 2009-10-03 at 08:40 -0400, Vivek Goyal wrote:
> On Sat, Oct 03, 2009 at 07:29:15AM -0400, Vivek Goyal wrote:
> > On Sat, Oct 03, 2009 at 07:56:18AM +0200, Mike Galbraith wrote:
> > > On Sat, 2009-10-03 at 07:49 +0200, Mike Galbraith wrote:
> > > > On Fri, 2009-10-02 at 20:19 +0200, Jens Axboe wrote:
> > > > 
> > > > > If you could do a cleaned up version of your overload patch based on
> > > > > this:
> > > > > 
> > > > > http://git.kernel.dk/?p=linux-2.6-block.git;a=commit;h=1d2235152dc745c6d94bedb550fea84cffdbf768
> > > > > 
> > > > > then lets take it from there.
> > > 
> > 
> > > Note to self: build the darn thing after last minute changes.
> > > 
> > > Block:  Delay overloading of CFQ queues to improve read latency.
> > > 
> > > Introduce a delay maximum dispatch timestamp, and stamp it when:
> > >         1. we encounter a known seeky or possibly new sync IO queue.
> > >         2. the current queue may go idle and we're draining async IO.
> > >         3. we have sync IO in flight and are servicing an async queue.
> > >         4  we are not the sole user of disk.
> > > Disallow exceeding quantum if any of these events have occurred recently.
> > > 
> > 
> > So it looks like primarily the issue seems to be that we done lot of
> > dispatch from async queue and if some sync queue comes in now, it will
> > experience latencies.
> > 
> > For a ongoing seeky sync queue issue will be solved up to some extent
> > because previously we did not choose to idle for that queue now we will
> > idle, hence async queue will not get a chance to overload the dispatch
> > queue.
> > 
> > For the sync queues where we choose not to enable idle, we still will see
> > the latencies. Instead of time stamping on all the above events, can we 
> > just keep track of last sync request completed in the system and don't
> > allow async queue to flood/overload the dispatch queue with-in certain 
> > time limit of that last sync request completion. This just gives a buffer
> > period to that sync queue to come back and submit more requests and
> > still not suffer large latencies?
> > 
> > Thanks
> > Vivek
> > 
> 
> Hi Mike,
> 
> Following is a quick hack patch for the above idea. It is just compile and
> boot tested. Can you please see if it helps in your scenario.

Box sends hugs and kisses.  s/desktop/latency and ship 'em :)

perf stat      1.70     1.94     1.32     1.89     1.87    1.7     fairness=1 overload_delay=1
               1.55     1.79     1.38     1.53     1.57    1.5     desktop=1 +last_end_sync

perf stat testo.sh                               Avg
108.12   106.33    106.34    97.00    106.52   104.8  1.000 fairness=0 overload_delay=0
 93.98   102.44     94.47    97.70     98.90    97.4   .929 fairness=0 overload_delay=1
 90.87    95.40     95.79    93.09     94.25    93.8   .895 fairness=1 overload_delay=0
 89.93    90.57     89.13    93.43     93.72    91.3   .871 fairness=1 overload_delay=1
 89.81    88.82     91.56    96.57     89.38    91.2   .870 desktop=1 +last_end_sync

	-Mike