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SubjectRe: [RFC PATCH 0/3] Avoid the use of congestion_wait under zone pressure
Mel Gorman wrote:
> On Mon, Mar 15, 2010 at 01:09:35PM -0700, Andrew Morton wrote:
>> On Mon, 15 Mar 2010 13:34:50 +0100
>> Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com> wrote:
>>
[...]
>
> 120+ kernels and a lot of hurt later;

Thanks for all your effort in searching the real cause behind
congestion_wait becoming such a time sink for some benchmarks.

>
> 2.6.32 0 0 0 3 2 44437 221753 2760857 132517 0
> 2.6.32-revertevict 0 0 0 3 2 35 14 1570 460 0
> 2.6.32-ttyfix 0 0 0 2 0 60770 303206 3659254 166293 0
> 2.6.32-ttyfix-revertevict 0 0 0 3 0 55 62 2496 494 0
> 2.6.32.10 0 0 0 2 1 90769 447702 4251448 234868 0
> 2.6.32.10-revertevict 0 0 0 3 2 148 597 8642 478 0
> 2.6.32.10-ttyfix 0 0 0 3 0 91729 453337 4374070 238593 0
> 2.6.32.10-ttyfix-revertevict 0 0 0 3 1 65 146 3408 347 0
>
> Again, fixing tty and reverting evict-once helps bring figures more in line
> with 2.6.29.
>
> 2.6.33 0 0 0 3 0 152248 754226 4940952 267214 0
> 2.6.33-revertevict 0 0 0 3 0 883 4306 28918 507 0
> 2.6.33-ttyfix 0 0 0 3 0 157831 782473 5129011 237116 0
> 2.6.33-ttyfix-revertevict 0 0 0 2 0 1056 5235 34796 519 0
> 2.6.33.1 0 0 0 3 1 156422 776724 5078145 234938 0
> 2.6.33.1-revertevict 0 0 0 2 0 1095 5405 36058 477 0
> 2.6.33.1-ttyfix 0 0 0 3 1 136324 673148 4434461 236597 0
> 2.6.33.1-ttyfix-revertevict 0 0 0 1 1 1339 6624 43583 466 0
>

[...]

>
> Christian, can you test the following amalgamated patch on 2.6.32.10 and
> 2.6.33 please? Note it's 2.6.32.10 because the patches below will not apply
> cleanly to 2.6.32 but it will against 2.6.33. It's a combination of ttyfix
> and revertevict. If your problem goes away, it implies that the stalls I
> can measure are roughly correlated to the more significant problem you have.

While your tty&evict patch might fix something as seen by your numbers,
it unfortunately doesn't affect my big throughput loss.

Again the scenario was 4,8 and 16 threads iozone sequential read with
2Gb files and one disk per process, running on a s390x machine with 4
cpus and 256m.
My table shows the throughput deviation to plain 2.6.32 git in percent.

percentage 4thr 8thr 16thr
2.6.32 0.00% 0.00% 0.00%
2.6.32.10 (stable) 4.44% 7.97% 4.11%
2.6.32.10-ttyfix-revertevict 3.33% 6.64% 5.07%
2.6.33 5.33% -2.82% -10.87%
2.6.33-ttyfix-revertevict 3.33% -3.32% -10.51%
2.6.32-watermarkwait 40.00% 58.47% 42.03%

In terms of throughput for my load your patch doesn't change anything
significantly above the noise level of the test case (which is around
~1%). The fix probably even has a slight performance decrease in low
thread cases.

For better comparison I added a 2.6.32 run with your watermark wait
patch which is still the only one fixing the issue.

That said I'd still love to see watermark wait getting accepted :-)

--

Grüsse / regards, Christian Ehrhardt
IBM Linux Technology Center, System z Linux Performance
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