Re: Jiffy is not able to measure the fraction of time a process runs a processor

["Mauricio Lin" <mauriciolin@gmail.com>]

Hi,

On 3/15/06, linux-os (Dick Johnson) <linux-os@analogic.com> wrote:
>
> On Wed, 15 Mar 2006, Mauricio Lin wrote:
>
> > Hi all,
> >
> > I have managed to measure the cpu time in nanoseconds. On i386 I have used
> > the monotonic_clock() to measure the cpu time accurately.
> >
> > The cpu time measurements were based on t->sched_info.cpu_time, but
> > instead of accumulate
> > the all cpu time, I needed just the diff=jiffies
> > -t->sched_info.last_arrival in the sched_info_depart().
> >
> > The problem was most of time the diff was zero. So to solve this
> > problem I used the monotonic_clock() function that provides more accurate
> > way to measure cpu time.
> >
> > Any comments?
> >
> > On 14 Mar 2006 15:54:17 -0500, Frank Ch. Eigler <fche@redhat.com> wrote:
> >> "Mauricio Lin" <mauriciolin@gmail.com> writes:
> >>
> >>>  I am trying to measure the fraction of time a process runs on a
> >>> processor, but the jiffies is not able to provide an accurate value.
> >>
> >> See sched_clock().
> >
> > I have checked it. It helped me to reach the monotonic_clock()
> > function after hacking the code.
> >
> >>
> >>>  The example below [...]
> >>> PID  : NAME : LAST ARRIVAL : CPU TIME : CALLER
> >>> 4544 : kmix : 6170433 : 0 : work_resched+0x6c
> >>> 4078 : lpd : 6170433 : 0 : __down_interruptible+0x5
> >>> 4544 : kmix : 6170433 : 0 : schedule_timeout+0xb8
> >>
> >> What tool/patchset are you using to generate this trace?
> >
> > I am using the relayfs to report the information I need among the
> > processors. I just put some klog in some key points in the code.
> >
> > BR,
> >
> > Mauricio Lin.
>
> Jiffies changes only 1000 times per second if it's set to 1000.
> On a fast CPU this kind of coarse resolution will not give you
> any useful information when trying to determine execution time.
>
> You need to use the number of CPU clocks that it has taken.
> This is obtained using rdtsc on ix86 machines.

Yes. The monotonic_clock() can use it, if the timer selected is TSC. I
have checked how monotonic_clock() is implemented and it uses
monotonic_clock_tsc() if the timer used is TSC. monotonic_clock_tsc()
calls rdtscll() that reads the time stamp counter as rdstc() also
does. So probably monotonic_clock() can help me.

Any comments?

BR,

Mauricio Lin.

> Cheers,
> Dick Johnson
> Penguin : Linux version 2.6.15.4 on an i686 machine (5589.54 BogoMips).
> Warning : 98.36% of all statistics are fiction, book release in April.
> _
> \x1a\x04
>
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