Re: [PATCH 0/4] Reclaim page capture v3

[Andy Whitcroft <apw@shadowen.org>]

On Mon, Sep 08, 2008 at 04:08:03PM +1000, Nick Piggin wrote:
> On Friday 05 September 2008 20:19, Andy Whitcroft wrote:
> > For sometime we have been looking at mechanisms for improving the
> > availability of larger allocations under load.  One of the options we have
> > explored is the capturing of pages freed under direct reclaim in order to
> > increase the chances of free pages coelescing before they are subject to
> > reallocation by racing allocators.
> >
> > Following this email is a patch stack implementing page capture during
> > direct reclaim.  It consits of four patches.  The first two simply pull
> > out existing code into helpers for reuse.  The third makes buddy's use
> > of struct page explicit.  The fourth contains the meat of the changes,
> > and its leader contains a much fuller description of the feature.
> >
> > This update represents a rebase to -mm and incorporates feedback from
> > KOSAKI Motohiro.  It also incorporates an accounting fix which was
> > preventing some captures.
> >
> > I have done a lot of comparitive testing with and without this patch
> > set and in broad brush I am seeing improvements in hugepage allocations
> > (worst case size) success on all of my test systems.  These tests consist
> > of placing a constant stream of high order allocations on the system,
> > at varying rates.  The results for these various runs are then averaged
> > to give an overall improvement.
> >
> > 		Absolute	Effective
> > x86-64		2.48%		 4.58%
> > powerpc		5.55%		25.22%
> 
> These are the numbers for the improvement of hugepage allocation success?
> Then what do you mean by absolute and effective?

The absolute improvement is the literal change in success percentage,
the literal percentage of all memory which may be allocated as huge
pages.  The effective improvement is percentage of the baseline success
rates that this change represents; for the powerpc results we get some
20% of memory allocatable without the patches, and 25% with, 25% more
pages are allocatable with the patches.

> What sort of constant stream of high order allocations are you talking
> about? In what "real" situations are you seeing higher order page allocation
> failures, and in those cases, how much do these patches help?

The test case simulates a constant demand for huge pages, at various
rates.  This is intended to replicate the scenario where a system is
used with mixed small page and huge page applications, with a dynamic
huge page pool.  Particularly we are examining the effect of starting a
very large huge page job on a busy system.  Obviously starting hugepage
applications depends on hugepage availability as they are not swappable.
This test was chosen because it both tests the initial large page demand
and then pushes the system to the limit.

> I must say I don't really like this approach. IMO it might be better to put
> some sort of queue in the page allocator, so if memory becomes low, then
> processes will start queueing up and not be allowed to jump the queue and
> steal memory that has been freed by hard work of a direct reclaimer. That
> would improve a lot of fairness problems as well as improve coalescing for
> higher order allocations without introducing this capture stuff.

The problem with queuing all allocators is two fold.  Firstly allocations
are obviously required for reclaim and those would have to be exempt
from the queue, and these are as likely to prevent coelesce of pages
as any other.  Secondly where a very large allocation is requested
all allocators would be held while reclaim at that size is performed,
majorly increasing latency for those allocations.  Reclaim for an order
0 page may target of the order of 32 pages, whereas reclaim for x86_64
hugepages is 1024 pages minimum.  It would be grosly unfair for a single
large allocation to hold up normal allocations.

> > x86-64 has a relatively small huge page size and so is always much more
> > effective at allocating huge pages.  Even there we get a measurable
> > improvement.  On powerpc the huge pages are much larger and much harder
> > to recover.  Here we see a full 25% increase in page recovery.
> >
> > It should be noted that these are worst case testing, and very agressive
> > taking every possible page in the system.  It would be helpful to get
> > wider testing in -mm.
> >
> > Against: 2.6.27-rc1-mm1
> >
> > Andrew, please consider for -mm.
> >
> > -apw
> >
> > Changes since V2:
> >  - Incorporates review feedback from Christoph Lameter,
> >  - Incorporates review feedback from Peter Zijlstra, and
> >  - Checkpatch fixes.
> >
> > Changes since V1:
> >  - Incorporates review feedback from KOSAKI Motohiro,
> >  - fixes up accounting when checking watermarks for captured pages,
> >  - rebase 2.6.27-rc1-mm1,
> >  - Incorporates review feedback from Mel.
> >
> >
> > Andy Whitcroft (4):
> >   pull out the page pre-release and sanity check logic for reuse
> >   pull out zone cpuset and watermark checks for reuse
> >   buddy: explicitly identify buddy field use in struct page
> >   capture pages freed during direct reclaim for allocation by the
> >     reclaimer
> >
> >  include/linux/mm_types.h   |    4 +
> >  include/linux/page-flags.h |    4 +
> >  mm/internal.h              |    8 +-
> >  mm/page_alloc.c            |  263
> > ++++++++++++++++++++++++++++++++++++++------ mm/vmscan.c                | 
> > 115 ++++++++++++++++----
> >  5 files changed, 338 insertions(+), 56 deletions(-)
> >
> > --
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-apw