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:

quoted

quoted

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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