On Fri, Nov 13, 2009 at 02:32:12PM +0100, Jens Axboe wrote: > On Fri, Nov 13 2009, Mel Gorman wrote: > > On Fri, Nov 13, 2009 at 12:55:58PM +0100, Jens Axboe wrote: > > > On Fri, Nov 13 2009, KOSAKI Motohiro wrote: > > > > (cc to Jens) > > > > > > > > > Testing by Frans Pop indicated that in the 2.6.30..2.6.31 window at least > > > > > that the commits 373c0a7e 8aa7e847 dramatically increased the number of > > > > > GFP_ATOMIC failures that were occuring within a wireless driver. Reverting > > > > > this patch seemed to help a lot even though it was pointed out that the > > > > > congestion changes were very far away from high-order atomic allocations. > > > > > > > > > > The key to why the revert makes such a big difference is down to timing and > > > > > how long direct reclaimers wait versus kswapd. With the patch reverted, > > > > > the congestion_wait() is on the SYNC queue instead of the ASYNC. As a > > > > > significant part of the workload involved reads, it makes sense that the > > > > > SYNC list is what was truely congested and with the revert processes were > > > > > waiting on congestion as expected. Hence, direct reclaimers stalled > > > > > properly and kswapd was able to do its job with fewer stalls. > > > > > > > > > > This patch aims to fix the congestion_wait() behaviour for SYNC and ASYNC > > > > > for direct reclaimers. Instead of making the congestion_wait() on the SYNC > > > > > queue which would only fix a particular type of workload, this patch adds a > > > > > third type of congestion_wait - BLK_RW_BOTH which first waits on the ASYNC > > > > > and then the SYNC queue if the timeout has not been reached. In tests, this > > > > > counter-intuitively results in kswapd stalling less and freeing up pages > > > > > resulting in fewer allocation failures and fewer direct-reclaim-orientated > > > > > stalls. > > > > > > > > Honestly, I don't like this patch. page allocator is not related to > > > > sync block queue. vmscan doesn't make read operation. > > > > This patch makes nearly same effect of s/congestion_wait/io_schedule_timeout/. > > > > > > > > Please don't make mysterious heuristic code. > > > > > > > > > > > > Sidenode: I doubt this regression was caused from page allocator. > > > > Probably not. As noted, the major change is really in how long callers > > are waiting on congestion_wait. The tarball includes graphs from an > > instrumented kernel that shows how long callers are waiting due to > > congestion_wait(). This has changed significantly. > > > > I'll queue up tests over the weekend that test without dm-crypt being involved. > > > > > > Probably we need to confirm caller change.... > > > > > > See the email from Chris from yesterday, he nicely explains why this > > > change made a difference with dm-crypt. > > > > Indeed. > > > > But bear in mind that it also possible that direct reclaimers are also > > congesting the queue due to swap-in. > > Are you speculating, or has this been observed? While I don't contest > that that could happen, it's also not a new thing. And it should be an > unlikely event. > > > > dm-crypt needs fixing, not a hack like this added. > > > > > > > As noted by Chris in the same mail, dm-crypt has not changed. What has > > changed is how long callers wait in congestion_wait. > > Right dm-crypt didn't change, it WAS ALREADY BUGGY. > On a different note, I tried without dm-crypt and found by far the greatest different to page allocator success or failure was the value of the low_latency tunable. low_latency == 0 2.6.32-rc6-0000000-force-highorder Elapsed:17:50.935(stddev:002.535) Failures:0 2.6.32-rc6-0000006-dm-crypt-unplug Elapsed:18:44.610(stddev:002.236) Failures:1 2.6.32-rc6-0000012-pgalloc-2.6.30 Elapsed:17:18.330(stddev:002.258) Failures:2 2.6.32-rc6-0000123-congestion-both Elapsed:16:17.370(stddev:002.167) Failures:1 2.6.32-rc6-0001234-kswapd-quick-recheck Elapsed:15:54.880(stddev:002.234) Failures:0 2.6.32-rc6-0012345-kswapd-stay-awake-when-min Elapsed:19:26.417(stddev:002.237) Failures:1 2.6.32-rc6-0123456-dm-crypt-unplug Elapsed:20:19.135(stddev:001.516) Failures:0 low_latency == 1 2.6.32-rc6-0000000-force-highorder Elapsed:20:04.755(stddev:078.551) Failures:22 2.6.32-rc6-0000006-dm-crypt-unplug Elapsed:25:05.608(stddev:053.224) Failures:12 2.6.32-rc6-0000012-pgalloc-2.6.30 Elapsed:19:01.530(stddev:002.146) Failures:14 2.6.32-rc6-0000123-congestion-both Elapsed:18:01.938(stddev:002.171) Failures:2 2.6.32-rc6-0001234-kswapd-quick-recheck Elapsed:19:52.833(stddev:064.168) Failures:7 2.6.32-rc6-0012345-kswapd-stay-awake-when-min Elapsed:26:25.767(stddev:050.827) Failures:1 2.6.32-rc6-0123456-dm-crypt-unplug Elapsed:22:56.850(stddev:053.914) Failures:1 Setting low_latency both regresses performance of the test and causes a boatload of allocation failures. Note also the deviations. With low_latency == 0, gitk performs predictably +/- around 2 seconds. With low_latency == 1, there are huge varianes +/- about a minute. Sampling writeback, I found that the number of pages in writeback with low_latency == 1 was higher for longer. Any theories as to why enabling low_latency has such a negative impact? Should it be disabled by default? -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- To unsubscribe from this list: send the line "unsubscribe kernel-testers" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
