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 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. dm-crypt needs fixing, not a hack like this added. The vm needs to drop congestion hints and usage, not increase it. The above changelog is mostly hand-wavy nonsense, imho. -- Jens Axboe -- 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
