Bug: 2.6.15 Process accounting fails to account for small time slice loads (still)

Bug: 2.6.15 Process accounting fails to account for small time slice loads (still)

[Tom Burns]

The bug described back in 2004 in the following message still seems to exist:

http://linux.derkeiler.com/Mailing-Lists/Kernel/2004-05/4313.html

Essentially, it appears that if a process sleeps before it uses up a
complete jiffy then no charge is made to its process accounting table.

This problem still exists in the same manner in kernel:

 2.6.15-1.2054_FC5smp #1 SMP Tue Mar 14 16:05:46 EST 2006 i686 i686
i386 GNU/Linux

The linked message above contains the necessary code to easily test
for the problem.  It compiles cleanly, just be sure to include -lrt on
the command line.

Cheers,
Tom Burns

Re: Bug: 2.6.15 Process accounting fails to account for small time slice loads (still)

[Tom Burns]

Problem also verified to exist on kernel:
2.6.17-5mdv #1 SMP Wed Sep 13 14:28:02 EDT 2006 x86_64 Intel(R)
Core(TM)2 CPU          6400  @ 2.13GHz GNU/Linux

Cheers,
Tom Burns

On 1/30/07, Tom Burns <tom.i.burns@gmail.com> wrote:
> The bug described back in 2004 in the following message still seems to exist:
>
> http://linux.derkeiler.com/Mailing-Lists/Kernel/2004-05/4313.html
>
> .. snipped..

Re: Bug: 2.6.15 Process accounting fails to account for small time slice loads (still)

[Tim Schmielau]

On Tue, 30 Jan 2007, Tom Burns wrote:

> The bug described back in 2004 in the following message still seems to exist:
> 
> http://linux.derkeiler.com/Mailing-Lists/Kernel/2004-05/4313.html
> 
> Essentially, it appears that if a process sleeps before it uses up a
> complete jiffy then no charge is made to its process accounting table.

That's not a bug, it's a feature. ;-)

More seriously, it's a design decision. The benefit of exactly correct 
micro bookkeeping does not seem to warrant the extra cost in terms of cpu 
cycles spent in the accounting code.

IIRC there was a patch for that some years ago, if you are interested I 
will try to find it. No idea how much work it would be to port it to 
current kernels, though.

Tim

Re: Bug: 2.6.15 Process accounting fails to account for small time slice loads (still)

[Tom Burns]

Hi Tim.

That would be great if you could find that patch - I can worry about
porting it to the current kernel.  We're having an issue where our
device, while routing large amounts of network traffic, reports 0% CPU
utilisation when it cannot be the case, and I think this is the cause.

Cheers,
Tom Burns

On 1/30/07, Tim Schmielau <tim@physik3.uni-rostock.de> wrote:
> On Tue, 30 Jan 2007, Tom Burns wrote:
>
> > The bug described back in 2004 in the following message still seems to exist:
> >
> > http://linux.derkeiler.com/Mailing-Lists/Kernel/2004-05/4313.html
> >
> > Essentially, it appears that if a process sleeps before it uses up a
> > complete jiffy then no charge is made to its process accounting table.
>
> That's not a bug, it's a feature. ;-)
>
> More seriously, it's a design decision. The benefit of exactly correct
> micro bookkeeping does not seem to warrant the extra cost in terms of cpu
> cycles spent in the accounting code.
>
> IIRC there was a patch for that some years ago, if you are interested I
> will try to find it. No idea how much work it would be to port it to
> current kernels, though.
>
> Tim
>

Re: Bug: 2.6.15 Process accounting fails to account for small time slice loads (still)

[Christian Borntraeger]

On Tuesday 30 January 2007 17:24, you wrote:
> The bug described back in 2004 in the following message still seems to 
exist:
> 
> http://linux.derkeiler.com/Mailing-Lists/Kernel/2004-05/4313.html
> 
> Essentially, it appears that if a process sleeps before it uses up a
> complete jiffy then no charge is made to its process accounting table.
> 
> This problem still exists in the same manner in kernel:

This is a known limitation of tick based accounting used on most 
architectures. 
It was fixed on s390 and using a cpu timer on every context switch. (IIRC ppc 
is now using this infrastructure as well)

see for a http://www.kernel-traffic.org/kernel-traffic/kt20041117_284.html#8
for the first patch in the series.

I dont know if there are plans of using the infrastructure on x86 compatible 
platforms. I also dont know it that is actually easy to implement with x86 
hardware. 




-- 
Mit freundlichen Grüßen / Best Regards

Christian Borntraeger
Linux Software Engineer zSeries Linux & Virtualization

Re: Bug: 2.6.15 Process accounting fails to account for small time slice loads (still)

[Tim Schmielau]

On Tue, 30 Jan 2007, Tom Burns wrote:

> That would be great if you could find that patch - I can worry about
> porting it to the current kernel.  We're having an issue where our
> device, while routing large amounts of network traffic, reports 0% CPU
> utilisation when it cannot be the case, and I think this is the cause.

Here is one patch for CPU cycle accounting: (although I'm not sure that's 
really the one I had in mind)

  http://lkml.org/lkml/2004/6/23/78

It might also be worth having a look at oprofile:

  http://oprofile.sourceforge.net/

If you are on IA64 or PPC rather than i386, ppacc might help you:

  http://lkml.org/lkml/2006/1/13/308
  http://ppacc.sourceforge.net/about.html

Although I'd guess you are not on PPC or S390 as these already to correct 
CPU cycle accounting (IIRC).

Hope that helps,
Tim