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Message-ID: <0100016679e3c96f-c78df4e2-9ab8-48db-8796-271c4b439f16-000000@email.amazonses.com>
Date: Mon, 15 Oct 2018 22:41:03 +0000
From: Christopher Lameter <cl@...ux.com>
To: Andrew Morton <akpm@...ux-foundation.org>
cc: David Rientjes <rientjes@...gle.com>,
Pekka Enberg <penberg@...nel.org>,
Joonsoo Kim <iamjoonsoo.kim@....com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [patch] mm, slab: avoid high-order slab pages when it does not
reduce waste
On Fri, 12 Oct 2018, Andrew Morton wrote:
> > If the amount of waste is the same at higher cachep->gfporder values,
> > there is no significant benefit to allocating higher order memory. There
> > will be fewer calls to the page allocator, but each call will require
> > zone->lock and finding the page of best fit from the per-zone free areas.
There is a benefit because the management overhead is halved.
> > Instead, it is better to allocate order-0 memory if possible so that pages
> > can be returned from the per-cpu pagesets (pcp).
Have a benchmark that shows this?
>
> > There are two reasons to prefer this over allocating high order memory:
> >
> > - allocating from the pcp lists does not require a per-zone lock, and
> >
> > - this reduces stranding of MIGRATE_UNMOVABLE pageblocks on pcp lists
> > that increases slab fragmentation across a zone.
The slab allocators generally buffer pages from the page allocator to
avoid this effect given the slowness of page allocator operations anyways.
> Confused. Higher-order slab pages never go through the pcp lists, do
> they? I'd have thought that by tending to increase the amount of
> order-0 pages which are used by slab, such stranding would be
> *increased*?
Potentially.
> > We are particularly interested in the second point to eliminate cases
> > where all other pages on a pageblock are movable (or free) and fallback to
> > pageblocks of other migratetypes from the per-zone free areas causes
> > high-order slab memory to be allocated from them rather than from free
> > MIGRATE_UNMOVABLE pages on the pcp.
Well does this actually do some good?
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