linux-kernel - Re: [PATCH] sched: Provide iowait counters

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Message-Id: <20090724214006.7380c3b4.akpm@linux-foundation.org>
Date:	Fri, 24 Jul 2009 21:40:06 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	"Kok, Auke-jan H" <auke-jan.h.kok@...el.com>
Subject: Re: [PATCH] sched: Provide iowait counters

On Fri, 24 Jul 2009 21:33:02 -0700 Arjan van de Ven <arjan@...ux.intel.com> wrote:

> Andrew Morton wrote:
> > On Mon, 20 Jul 2009 11:31:47 -0700 Arjan van de Ven <arjan@...ux.intel.com> wrote:
> > 
> >> For counting how long an application has been waiting for (disk) IO,
> >> there currently is only the HZ sample driven information available, while
> >> for all other counters in this class, a high resolution version is
> >> available via CONFIG_SCHEDSTATS.
> >>
> >> In order to make an improved bootchart tool possible, we also need
> >> a higher resolution version of the iowait time.
> >>
> >> This patch below adds this scheduler statistic to the kernel.
> > 
> > Doesn't this duplicate the delay accounting already available via the
> > taskstats interface?
> 
> we have how long we wait. we do not have how long we iowait afaik...
> at least not in nanosecond granularity. (We do have the sampled data, but that
> is milisecond sampled data, not very useful for making charts based on time
> to show the sequence of events)

See include/linux/sched.h's definition of task_delay_info - u64
blkio_delay is in nanoseconds.  It uses
do_posix_clock_monotonic_gettime() internally.



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