Re: [PATCH] mm: Do not stall in synchronous compaction for THP allocations

[Mel Gorman <mgorman@suse.de>]

On Tue, Nov 15, 2011 at 03:00:09AM +0100, Andrea Arcangeli wrote:
> On Mon, Nov 14, 2011 at 04:03:45PM -0800, Andrew Morton wrote:
> > On Fri, 11 Nov 2011 10:01:56 +0000
> > Mel Gorman <mgorman@suse.de> wrote:
> > > A 1000-hour compute job will have its pages collapsed into hugepages by
> > > khugepaged so they might not have the huge pages at the very beginning
> > > but they get them. With khugepaged in place, there should be no need for
> > > an additional tuneable.
> > 
> > OK...
> 
> It's good idea to keep it monitored. But I guess the reduced rate will
> only materialize at temporary VM stress times.
> 
> > Fair enough.  One slight problem though:
> > 
> > akpm:/usr/src/25> grep -r thp_collapse_alloc_failed Documentation 
> > akpm:/usr/src/25> 
> 
> I didn't fill that gap but I was reading the code again and I don't
> see why we keep retrying for -EAGAIN in the !sync case. Maybe the
> below is good (untested). I doubt it's good to spend cpu to retry the
> trylock or to retry the migrate on a pinned page by O_DIRECT.

The retry happens on the expectation that the page is not locked
for very long. Pinning for O_DIRECT would be an exception. Readahead
pages would be another. I don't have data on how often we encounter
a page that is locked for a very short period of time versus being
locked for IO and even if I did, it would be depend on the workload
and the amount of RAM.

> In fact
> as far as THP success rate is concerned maybe we should "goto out"
> instead of "goto fail" but I didn't change to that as compaction even
> if it fails a subpage may still be successful at creating order
> 1/2/3/4...8 pages. I only avoid 9 loops to retry a trylock or a page
> under O_DIRECT. Maybe that will save a bit of CPU, I doubt it can
> decrease the success rate in any significant way. I'll test it at the
> next build...
> 
> ====
> From: Andrea Arcangeli <aarcange@redhat.com>
> Subject: [PATCH] migrate: !sync don't retry
> 
> For !sync it's not worth retrying because we won't lock_page even
> after the second pass.

Except in the cases where the page was locked for a very short
period of time. I think this will have an impact on success rates for
minimal latency savings but I cannot predict how much of an impact.

-- 
Mel Gorman
SUSE Labs