Re: [PATCH 3/5] page allocator: Wait on both sync and asynccongestion after direct reclaim

[Jens Axboe]

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

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