From: malc <av1474@comtv.ru>
To: Con Kolivas <kernel@kolivas.org>
Cc: linux-kernel@vger.kernel.org
Subject: Re: CPU load
Date: Mon, 12 Feb 2007 21:05:38 +0300 (MSK) [thread overview]
Message-ID: <Pine.LNX.4.64.0702122101001.2345@home.oyster.ru> (raw)
In-Reply-To: <8cd998d50702112144y38958d27saec4196f6f5d5236@mail.gmail.com>
On Mon, 12 Feb 2007, Con Kolivas wrote:
> On 12/02/07, Vassili Karpov <av1474@comtv.ru> wrote:
>> Hello,
[..snip..]
>
> The kernel looks at what is using cpu _only_ during the timer
> interrupt. Which means if your HZ is 1000 it looks at what is running
> at precisely the moment those 1000 timer ticks occur. It is
> theoretically possible using this measurement system to use >99% cpu
> and record 0 usage if you time your cpu usage properly. It gets even
> more inaccurate at lower HZ values for the same reason.
And indeed it appears to be possible to do just that. Example:
/* gcc -o hog smallhog.c */
#include <time.h>
#include <limits.h>
#include <signal.h>
#include <sys/time.h>
#define HIST 10
static 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);
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;
}
/* end smallhog.c */
Might need some adjustment for a particular system but ran just fine here
on:
2.4.30 + Athlon tbird (1Ghz)
2.6.19.2 + Athlon X2 3800+ (2Ghz)
Showing next to zero load in top(1) and a whole lot more in APC.
http://www.boblycat.org/~malc/apc/load-tbird-hog.png
http://www.boblycat.org/~malc/apc/load-x2-hog.png
Not quite 99% but nevertheless scary.
--
vale
next prev parent reply other threads:[~2007-02-12 18:05 UTC|newest]
Thread overview: 25+ 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
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 [this message]
-- strict thread matches above, loose matches on Subject: below --
2007-02-12 16:57 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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