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SubjectRe: [RFC][PATCH] iowait statistics
Rik van Riel wrote:
>
> Hi,
>
> the following patch implements iowait statistics in a simple way:
>
> 1) if we go to sleep while waiting on a page or buffer, we
> increment nr_iowait_tasks, note that this is done only in
> the slow path so overhead shouldn't even be measurable
>
> 2) if no process is running, the timer interrupt adds a jiffy
> to the iowait time
>
> 3) iowait time is counted separately from user/system/idle and
> can overlap with either system or idle (when no process is
> running the system can still be busy processing interrupts)
>
> 4) on SMP systems the iowait time can be overestimated, no big
> deal IMHO but cheap suggestions for improvement are welcome

I suspect that a number of these statistical accounting mechanisms
are going to break. The new irq-affinity code works awfully well.

The kernel profiler in 2.5 doesn't work very well at present.
When investigating this, I ran a busy-wait process. It attached
itself to CPU #3 and that CPU received precisely zero interrupts
across a five minute period. So the profiler cunningly avoids profiling
busy CPUs, which is rather counter-productive. Fortunate that oprofile
uses NMI.

> ...
> ===== fs/buffer.c 1.64 vs edited =====
> --- 1.64/fs/buffer.c Mon May 13 19:04:59 2002
> +++ edited/fs/buffer.c Mon May 13 19:16:57 2002
> @@ -156,8 +156,10 @@
> get_bh(bh);
> add_wait_queue(&bh->b_wait, &wait);
> do {
> + atomic_inc(&nr_iowait_tasks);
> run_task_queue(&tq_disk);
> set_task_state(tsk, TASK_UNINTERRUPTIBLE);
> + atomic_dec(&nr_iowait_tasks);
> if (!buffer_locked(bh))
> break;
> schedule();

Shouldn't the atomic_inc cover the schedule()?


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