Re: iozone regression with 2.6.29-rc6

Lin Ming
2009-03-02T02:23:47Z

On Fri, 2009-02-27 at 17:49 +0800, Peter Zijlstra wrote:
> On Fri, 2009-02-27 at 17:13 +0800, Lin Ming wrote:
> > bisect locates below commits,
> >
> > commit 1cf6e7d83bf334cc5916137862c920a97aabc018
> > Author: Nick Piggin <npiggin@suse.de>
> > Date: Wed Feb 18 14:48:18 2009 -0800
> >
> > mm: task dirty accounting fix
> >
> > YAMAMOTO-san noticed that task_dirty_inc doesn't seem to be called properly for
> > cases where set_page_dirty is not used to dirty a page (eg. mark_buffer_dirty).
> >
> > Additionally, there is some inconsistency about when task_dirty_inc is
> > called. It is used for dirty balancing, however it even gets called for
> > __set_page_dirty_no_writeback.
> >
> > So rather than increment it in a set_page_dirty wrapper, move it down to
> > exactly where the dirty page accounting stats are incremented.
> >
> > Cc: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
> > Signed-off-by: Nick Piggin <npiggin@suse.de>
> > Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> > Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
> >
> >
> > below data in parenthesis is the result after above commit reverted, for example,
> > -10% (+2%) means,
> > iozone has ~10% regression with 2.6.29-rc6 compared with 2.6.29-rc5.
> > and
> > iozone has ~2% improvement with 2.6.29-rc6-revert-1cf6e7d compared with 2.6.29-rc5.
> >
> >
> > 4P dual-core HT 2P qual-core 2P qual-core HT
> > tulsa stockley Nehalem
> > --------------------------------------------------------
> > iozone-rewrite -10% (+2%) -8% (0%) -10% (-7%)
> > iozone-rand-write -50% (0%) -20% (+10%)
> > iozone-read -13% (0%)
> > iozone-write -28% (-1%)
> > iozone-reread -5% (-1%)
> > iozone-mmap-read -7% (+2%)
> > iozone-mmap-reread -7% (+2%)
> > iozone-mmap-rand-read -7% (+3%)
> > iozone-mmap-rand-write -5% (0%)
>
> Ugh, that's unexpected..
>
> So 'better' accounting leads to worse performance, which would indicate
> we throttle more.
>
> I take it you machine has gobs of memory.
>
> Does something like the below help any?

It helps some as below test result,
The data in second parenthesis means 2.6.29-rc6-with-peter's-patch
compared with 2.6.29-rc5.

4P dual-core HT 2P qual-core 2P qual-core HT
tulsa stockley Nehalem
--------------------------------------------------------
iozone-rewrite -10% (+2%)(-3%) -8% (0%)(0%) -10% (-7%)(-2%)
iozone-rand-write -50% (0%)(-10%) -20% (+10%)(+3%)
iozone-read -13% (0%)(-8%)
iozone-write -28% (-1%)(+35%)
iozone-reread -5% (-1%)(-1%)
iozone-mmap-read -7% (+2%)(-7%)
iozone-mmap-reread -7% (+2%)(-7%)
iozone-mmap-rand-read -7% (+3%)(-7%)
iozone-mmap-rand-write -5% (0%)(+27%)

Lin Ming

>
> ---
> Subject: mm: bdi: tweak task dirty penalty
> From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Date: Fri Feb 27 10:41:22 CET 2009
>
> Penalizing heavy dirtiers with 1/8-th the total dirty limit might be rather
> excessive on large memory machines. Use sqrt to scale it sub-linearly.
>
> Update the comment while we're there.
>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
> mm/page-writeback.c | 12 ++++++++----
> 1 file changed, 8 insertions(+), 4 deletions(-)
>
> Index: linux-2.6/mm/page-writeback.c
> ===================================================================
> --- linux-2.6.orig/mm/page-writeback.c
> +++ linux-2.6/mm/page-writeback.c
> @@ -293,17 +293,21 @@ static inline void task_dirties_fraction
> }
>
> /*
> - * scale the dirty limit
> + * Task specific dirty limit:
> *
> - * task specific dirty limit:
> + * dirty -= 8 * sqrt(dirty) * p_{t}
> *
> - * dirty -= (dirty/8) * p_{t}
> + * Penalize tasks that dirty a lot of pages by lowering their dirty limit. This
> + * avoids infrequent dirtiers from getting stuck in this other guys dirty
> + * pages.
> + *
> + * Use a sub-linear function to scale the penalty, we only need a little room.
> */
> static void task_dirty_limit(struct task_struct *tsk, long *pdirty)
> {
> long numerator, denominator;
> long dirty = *pdirty;
> - u64 inv = dirty >> 3;
> + u64 inv = 8*int_sqrt(dirty);
>
> task_dirties_fraction(tsk, &numerator, &denominator);
> inv *= numerator;
>
>