From: malc <av1474@comtv.ru>
To: Pavel Machek <pavel@ucw.cz>
Cc: Con Kolivas <kernel@kolivas.org>, linux-kernel@vger.kernel.org
Subject: Re: CPU load
Date: Sun, 25 Feb 2007 13:35:47 +0300 (MSK) [thread overview]
Message-ID: <Pine.LNX.4.64.0702251334200.17241@linmac.oyster.ru> (raw)
In-Reply-To: <20070214204515.GA26153@elf.ucw.cz>
On Wed, 14 Feb 2007, Pavel Machek wrote:
> Hi!
[..snip..]
>> The current situation ought to be documented. Better yet some flag
>> can
>
> It probably _is_ documented, somewhere :-). If you find nice place
> where to document it (top manpage?) go ahead with the patch.
How about this:
<Documentation/load.txt>
CPU load
--------
Linux exports various bits of information via `/proc/stat' and
`/proc/uptime' that userland tools, such as top(1), use to calculate
the average time system spent in a particular state, for example:
<transcript>
$ iostat
Linux 2.6.18.3-exp (linmac) 02/20/2007
avg-cpu: %user %nice %system %iowait %steal %idle
10.01 0.00 2.92 5.44 0.00 81.63
...
</transcript>
Here the system thinks that over the default sampling period the
system spent 10.01% of the time doing work in user space, 2.92% in the
kernel, and was overall 81.63% of the time idle.
In most cases the `/proc/stat' information reflects the reality quite
closely, however due to the nature of how/when the kernel collects
this data sometimes it can not be trusted at all.
So how is this information collected? Whenever timer interrupt is
signalled the kernel looks what kind of task was running at this
moment and increments the counter that corresponds to this tasks
kind/state. The problem with this is that the system could have
switched between various states multiple times between two timer
interrupts yet the counter is incremented only for the last state.
Example
-------
If we imagine the system with one task that periodically burns cycles
in the following manner:
time line between two timer interrupts
|--------------------------------------|
^ ^
|_ something begins working |
|_ something goes to sleep
(only to be awaken quite soon)
In the above situation the system will be 0% loaded according to the
`/proc/stat' (since the timer interrupt will always happen when the
system is executing the idle handler), but in reality the load is
closer to 99%.
One can imagine many more situations where this behavior of the kernel
will lead to quite erratic information inside `/proc/stat'.
/* gcc -o hog smallhog.c */
#include <time.h>
#include <limits.h>
#include <signal.h>
#include <sys/time.h>
#define HIST 10
static volatile sig_atomic_t stop;
static void sighandler (int signr)
{
(void) signr;
stop = 1;
}
static unsigned long hog (unsigned long niters)
{
stop = 0;
while (!stop && --niters);
return niters;
}
int main (void)
{
int i;
struct itimerval it = { .it_interval = { .tv_sec = 0, .tv_usec = 1 },
.it_value = { .tv_sec = 0, .tv_usec = 1 } };
sigset_t set;
unsigned long v[HIST];
double tmp = 0.0;
unsigned long n;
signal (SIGALRM, &sighandler);
setitimer (ITIMER_REAL, &it, NULL);
hog (ULONG_MAX);
for (i = 0; i < HIST; ++i) v[i] = ULONG_MAX - hog (ULONG_MAX);
for (i = 0; i < HIST; ++i) tmp += v[i];
tmp /= HIST;
n = tmp - (tmp / 3.0);
sigemptyset (&set);
sigaddset (&set, SIGALRM);
for (;;) {
hog (n);
sigwait (&set, &i);
}
return 0;
}
References
----------
http://lkml.org/lkml/2007/2/12/6
Documentation/filesystems/proc.txt (1.8)
</Documentation/load.txt>
--
vale
next prev parent reply other threads:[~2007-02-25 10:46 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-02-12 5:33 CPU load Vassili Karpov
2007-02-12 5:44 ` Con Kolivas
2007-02-12 5:54 ` malc
2007-02-12 6:12 ` Con Kolivas
2007-02-12 7:10 ` malc
2007-02-12 7:29 ` Con Kolivas
2007-02-12 5:55 ` Stephen Rothwell
2007-02-12 6:08 ` Con Kolivas
2007-02-12 14:32 ` Pavel Machek
2007-02-13 22:01 ` malc
2007-02-13 22:08 ` Con Kolivas
2007-02-14 7:28 ` malc
2007-02-14 8:09 ` Con Kolivas
2007-02-14 20:45 ` Pavel Machek
2007-02-25 10:35 ` malc [this message]
2007-02-26 9:28 ` Pavel Machek
2007-02-26 10:42 ` malc
2007-02-26 16:38 ` Randy Dunlap
2007-02-12 18:05 ` malc
-- strict thread matches above, loose matches on Subject: below --
2018-03-27 11:44 CPU Load Ryan Meulenkamp
2018-03-29 1:24 ` Andre McCurdy
2018-03-29 6:42 ` Jussi Laako
2007-02-12 16:57 CPU load Andrew Burgess
2007-02-12 18:15 ` malc
2002-07-10 14:50 David Chow
2002-07-10 16:54 ` William Lee Irwin III
2002-07-10 17:49 ` Robert Love
2002-07-26 17:38 ` David Chow
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