On Thu, Sep 29, 2011 at 04:49:57PM +0800, Peter Zijlstra wrote:
> On Thu, 2011-09-29 at 11:32 +0800, Wu Fengguang wrote:
> > > Now I guess the only problem is when nr_bdi * MIN_WRITEBACK_PAGES ~
> > > limit, at which point things go pear shaped.
> >
> > Yes. In that case the global @dirty will always be drove up to @limit.
> > Once @dirty dropped reasonably below, whichever bdi task wakeup first
> > will take the chance to fill the gap, which is not fair for bdi's of
> > different speed.
> >
> > Let me retry the thresh=1M,10M test cases without MIN_WRITEBACK_PAGES.
> > Hopefully the removal of it won't impact performance a lot.
>
>
> Right, so alternatively we could try an argument that this is
> sufficiently rare and shouldn't happen. People with lots of disks tend
> to also have lots of memory, etc.
Right.
> If we do find it happens we can always look at it again.
Sure. Now I got the results for single disk thresh=1M,8M,100M cases
and find no big differences if removing MIN_WRITEBACK_PAGES:
3.1.0-rc4-bgthresh3+ 3.1.0-rc4-bgthresh4+
------------------------ ------------------------
3988742 +1.9% 4063217 thresh=100M/ext4-10dd-4k-8p-4096M-100M:10-X
4758884 +1.5% 4829320 thresh=100M/ext4-1dd-4k-8p-4096M-100M:10-X
4621240 +1.6% 4693525 thresh=100M/ext4-2dd-4k-8p-4096M-100M:10-X
3420717 +0.1% 3423712 thresh=100M/xfs-10dd-4k-8p-4096M-100M:10-X
4361830 +1.4% 4423554 thresh=100M/xfs-1dd-4k-8p-4096M-100M:10-X
3964043 +0.2% 3972057 thresh=100M/xfs-2dd-4k-8p-4096M-100M:10-X
2937926 +0.6% 2956870 thresh=1M/ext4-10dd-4k-8p-4096M-1M:10-X
4472552 -1.9% 4387457 thresh=1M/ext4-1dd-4k-8p-4096M-1M:10-X
4085707 -3.0% 3961155 thresh=1M/ext4-2dd-4k-8p-4096M-1M:10-X
2206897 +2.1% 2253839 thresh=1M/xfs-10dd-4k-8p-4096M-1M:10-X
4207336 -2.1% 4119821 thresh=1M/xfs-1dd-4k-8p-4096M-1M:10-X
3739888 -3.6% 3604315 thresh=1M/xfs-2dd-4k-8p-4096M-1M:10-X
3279302 -0.2% 3273310 thresh=8M/ext4-10dd-4k-8p-4096M-8M:10-X
4834878 +1.6% 4912372 thresh=8M/ext4-1dd-4k-8p-4096M-8M:10-X
4511120 -1.7% 4435193 thresh=8M/ext4-2dd-4k-8p-4096M-8M:10-X
2443874 -0.5% 2432188 thresh=8M/xfs-10dd-4k-8p-4096M-8M:10-X
4308416 -0.6% 4283110 thresh=8M/xfs-1dd-4k-8p-4096M-8M:10-X
3739810 +0.6% 3763320 thresh=8M/xfs-2dd-4k-8p-4096M-8M:10-X
Or lowering the largest promotion ratio from 128 to 8:
3.1.0-rc4-bgthresh4+ 3.1.0-rc4-bgthresh5+
------------------------ ------------------------
4063217 -0.0% 4062022 thresh=100M/ext4-10dd-4k-8p-4096M-100M:10-X
4829320 +1.1% 4882829 thresh=100M/ext4-1dd-4k-8p-4096M-100M:10-X
4693525 +0.1% 4700537 thresh=100M/ext4-2dd-4k-8p-4096M-100M:10-X
3423712 +0.2% 3431603 thresh=100M/xfs-10dd-4k-8p-4096M-100M:10-X
4423554 -0.3% 4408912 thresh=100M/xfs-1dd-4k-8p-4096M-100M:10-X
3972057 -0.1% 3968535 thresh=100M/xfs-2dd-4k-8p-4096M-100M:10-X
2956870 -0.9% 2929605 thresh=1M/ext4-10dd-4k-8p-4096M-1M:10-X
4387457 -0.2% 4378233 thresh=1M/ext4-1dd-4k-8p-4096M-1M:10-X
3961155 -0.5% 3940075 thresh=1M/ext4-2dd-4k-8p-4096M-1M:10-X
2253839 -0.9% 2232976 thresh=1M/xfs-10dd-4k-8p-4096M-1M:10-X
4119821 -2.1% 4031983 thresh=1M/xfs-1dd-4k-8p-4096M-1M:10-X
3604315 -3.1% 3493042 thresh=1M/xfs-2dd-4k-8p-4096M-1M:10-X
3273310 -1.1% 3237060 thresh=8M/ext4-10dd-4k-8p-4096M-8M:10-X
4912372 -0.0% 4911287 thresh=8M/ext4-1dd-4k-8p-4096M-8M:10-X
4435193 +0.1% 4441581 thresh=8M/ext4-2dd-4k-8p-4096M-8M:10-X
2432188 +1.1% 2459249 thresh=8M/xfs-10dd-4k-8p-4096M-8M:10-X
4283110 +0.1% 4289456 thresh=8M/xfs-1dd-4k-8p-4096M-8M:10-X
3763320 -0.1% 3758938 thresh=8M/xfs-2dd-4k-8p-4096M-8M:10-X
As for the thresh=100M JBOD cases, I don't see much occurrences of promotion
ratio > 2. So the simplification should make no difference, too.
Thus the finalized code will be:
+ x_intercept = bdi_thresh / 2;
+ if (bdi_dirty < x_intercept) {
+ if (bdi_dirty > x_intercept / 8) {
+ pos_ratio *= x_intercept;
+ do_div(pos_ratio, bdi_dirty);
+ } else
+ pos_ratio *= 8;
+ }
Thanks,
Fengguang
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