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Date: Mon, 12 Nov 2007 17:48:20 +0100
From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
To: Benny Halevy <bhalevy@...asas.com>
Cc: "Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
LKML <linux-kernel@...r.kernel.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
aneesh.kumar@...ux.vnet.ibm.com
Subject: Re: iozone write 50% regression in kernel 2.6.24-rc1
On Mon, 2007-11-12 at 17:05 +0200, Benny Halevy wrote:
> On Nov. 12, 2007, 15:26 +0200, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> > Single socket, dual core opteron, 2GB memory
> > Single SATA disk, ext3
> >
> > x86_64 kernel and userland
> >
> > (dirty_background_ratio, dirty_ratio) tunables
> >
> > ---- (5,10) - default
> >
> > 2.6.23.1-42.fc8 #1 SMP
> >
> > 524288 4 59580 60356
> > 524288 4 59247 61101
> > 524288 4 61030 62831
> >
> > 2.6.24-rc2 #28 SMP PREEMPT
> >
> > 524288 4 49277 56582
> > 524288 4 50728 61056
> > 524288 4 52027 59758
> > 524288 4 51520 62426
> >
> >
> > ---- (20,40) - similar to your 8GB
> >
> > 2.6.23.1-42.fc8 #1 SMP
> >
> > 524288 4 225977 447461
> > 524288 4 232595 496848
> > 524288 4 220608 478076
> > 524288 4 203080 445230
> >
> > 2.6.24-rc2 #28 SMP PREEMPT
> >
> > 524288 4 54043 83585
> > 524288 4 69949 516253
> > 524288 4 72343 491416
> > 524288 4 71775 492653
> >
> > ---- (60,80) - overkill
> >
> > 2.6.23.1-42.fc8 #1 SMP
> >
> > 524288 4 208450 491892
> > 524288 4 216262 481135
> > 524288 4 221892 543608
> > 524288 4 202209 574725
> > 524288 4 231730 452482
> >
> > 2.6.24-rc2 #28 SMP PREEMPT
> >
> > 524288 4 49091 86471
> > 524288 4 65071 217566
> > 524288 4 72238 492172
> > 524288 4 71818 492433
> > 524288 4 71327 493954
> >
> >
> > While I see that the write speed as reported under .24 ~70MB/s is much
> > lower than the one reported under .23 ~200MB/s, I find it very hard to
> > believe my poor single SATA disk could actually do the 200MB/s for
> > longer than its cache 8/16 MB (not sure).
> >
> > vmstat shows that actual IO is done, even though the whole 512MB could
> > fit in cache, hence my suspicion that the ~70MB/s is the most realistic
> > of the two.
>
> Even 70 MB/s seems too high. What throughput do you see for the
> raw disk partition/
>
> Also, are the numbers above for successive runs?
> It seems like you're seeing some caching effects so
> I'd recommend using a file larger than your cache size and
> the -e and -c options (to include fsync and close in timings)
> to try to eliminate them.
------ iozone -i 0 -r 4k -s 512m -e -c
.23 (20,40)
524288 4 31750 33560
524288 4 29786 32114
524288 4 29115 31476
.24 (20,40)
524288 4 25022 32411
524288 4 25375 31662
524288 4 26407 33871
------ iozone -i 0 -r 4k -s 4g -e -c
.23 (20,40)
4194304 4 39699 35550
4194304 4 40225 36099
.24 (20,40)
4194304 4 39961 41656
4194304 4 39244 39673
Yanmin, for that benchmark you ran, what was it meant to measure?
From what I can make of it its just write cache benching.
One thing I don't understand is how the write numbers are so much lower
than the rewrite numbers. The iozone code (which gives me headaches,
damn what a mess) seems to suggest that the only thing that is different
is the lack of block allocation.
Linus posted a patch yesterday fixing up a regression in the ext3 bitmap
block allocator, /me goes apply that patch and rerun the tests.
> > ---- (20,40) - similar to your 8GB
> >
> > 2.6.23.1-42.fc8 #1 SMP
> >
> > 524288 4 225977 447461
> > 524288 4 232595 496848
> > 524288 4 220608 478076
> > 524288 4 203080 445230
> >
> > 2.6.24-rc2 #28 SMP PREEMPT
> >
> > 524288 4 54043 83585
> > 524288 4 69949 516253
> > 524288 4 72343 491416
> > 524288 4 71775 492653
2.6.24-rc2 +
patches/wu-reiser.patch
patches/writeback-early.patch
patches/bdi-task-dirty.patch
patches/bdi-sysfs.patch
patches/sched-hrtick.patch
patches/sched-rt-entity.patch
patches/sched-watchdog.patch
patches/linus-ext3-blockalloc.patch
524288 4 179657 487676
524288 4 173989 465682
524288 4 175842 489800
Linus' patch is the one that makes the difference here. So I'm unsure
how you bisected it down to:
04fbfdc14e5f48463820d6b9807daa5e9c92c51f
These results seem to point to
7c9e69faa28027913ee059c285a5ea8382e24b5d
as being the offending patch.
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