Re: [PATCH 1/2] mm: vmscan: Remove lumpy reclaim

[Mel Gorman <mgorman@suse.de>]

On Tue, Apr 10, 2012 at 09:24:54AM +0100, Mel Gorman wrote:
> On Fri, Apr 06, 2012 at 04:52:09PM -0700, Ying Han wrote:
> > On Wed, Mar 28, 2012 at 9:06 AM, Mel Gorman <mgorman@suse.de> wrote:
> > > Lumpy reclaim had a purpose but in the mind of some, it was to kick
> > > the system so hard it trashed. For others the purpose was to complicate
> > > vmscan.c. Over time it was giving softer shoes and a nicer attitude but
> > > memory compaction needs to step up and replace it so this patch sends
> > > lumpy reclaim to the farm.
> > >
> > > Here are the important notes related to the patch.
> > >
> > > 1. The tracepoint format changes for isolating LRU pages.
> > >
> > > 2. This patch stops reclaim/compaction entering sync reclaim as this
> > >   was only intended for lumpy reclaim and an oversight. Page migration
> > >   has its own logic for stalling on writeback pages if necessary and
> > >   memory compaction is already using it. This is a behaviour change.
> > >
> > > 3. RECLAIM_MODE_SYNC no longer exists. pageout() does not stall
> > >   on PageWriteback with CONFIG_COMPACTION has been this way for a while.
> > >   I am calling it out in case this is a surpise to people.
> > 
> > Mel,
> > 
> > Can you point me the commit making that change? I am looking at
> > v3.4-rc1 where set_reclaim_mode() still set RECLAIM_MODE_SYNC for
> > COMPACTION_BUILD.
> > 
> 
> You're right.
> 
> There is only one call site that passes sync==true for set_reclaim_mode() in
> vmscan.c and that is only if should_reclaim_stall() returns true. It had the
> comment "Only stall on lumpy reclaim" but the comment is not accurate
> and that mislead me.
> 
> Thanks, I'll revisit the patch.
> 

Just to be clear, I think the patch is right in that stalling on page
writeback was intended just for lumpy reclaim. I've split out the patch
that stops reclaim/compaction entering sync reclaim but the end result
of the series is the same. Unfortunately we do not have tracing to record
how often reclaim waited on writeback during compaction so my historical
data does not indicate how often it happened. However, it may partially
explain occasionaly complaints about interactivity during heavy writeback
when THP is enabled (the bulk of the stalls were due to something else but
on rare occasions disabling THP was reported to make a small unquantifable
difference). I'll enable ftrace to record how often mm_vmscan_writepage()
used RECLAIM_MODE_SYNC during tests for this series and include that
information in the changelog.

-- 
Mel Gorman
SUSE Labs

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