Re: [PATCH 12/14] Add a tunable that decides when memory should be compacted and when it should be reclaimed

[Mel Gorman <mel@csn.ul.ie>]

On Tue, Apr 06, 2010 at 05:06:13PM -0700, Andrew Morton wrote:
> On Fri,  2 Apr 2010 17:02:46 +0100
> Mel Gorman <mel@csn.ul.ie> wrote:
> 
> > The kernel applies some heuristics when deciding if memory should be
> > compacted or reclaimed to satisfy a high-order allocation. One of these
> > is based on the fragmentation. If the index is below 500, memory will
> > not be compacted. This choice is arbitrary and not based on data. To
> > help optimise the system and set a sensible default for this value, this
> > patch adds a sysctl extfrag_threshold. The kernel will only compact
> > memory if the fragmentation index is above the extfrag_threshold.
> 
> Was this the most robust, reliable, no-2am-phone-calls thing we could
> have done?
> 
> What about, say, just doing a bit of both until something worked? 

I guess you could but that is not a million miles away from what
currently happens.

This heuristic is basically "based on free memory layout, how likely is
compaction to succeed?". It makes a decision based on that. A later
patch then checks if the guess was right. If not, just try direct
reclaim for a bit before trying compaction again.

> For
> extra smarts we could remember what worked best last time, and make
> ourselves more likely to try that next time.
> 

With the later patch, this is essentially what we do. Granted we
remember the opposite "If the kernel guesses wrong, then don't compact
for a short while before trying again".

> Or whatever, but extfrag_threshold must die!  And replacing it with a
> hardwired constant doesn't count ;)
> 

I think what you have in mind is "just try compaction every time" but my
concern about that is we'll hit a corner case where a lot of CPU time is
taken scanning zones uselessly. That is what this heuristic and the
back-off logic in a later patch was meant to avoid. I haven't thought of
a better alternative :/

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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