linux-kernel - Re: [patch] mm, slab: avoid high-order slab pages when it does not reduce waste

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Date:   Fri, 12 Oct 2018 15:13:41 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Christoph Lameter <cl@...ux.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 14:24:57 -0700 (PDT) David Rientjes <rientjes@...gle.com> wrote:

> The slab allocator has a heuristic that checks whether the internal
> fragmentation is satisfactory and, if not, increases cachep->gfporder to
> try to improve this.
> 
> 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.
> 
> Instead, it is better to allocate order-0 memory if possible so that pages
> can be returned from the per-cpu pagesets (pcp).
> 
> 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.

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*?

> 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.
> 
>  mm/slab.c | 15 +++++++++++++++

Do slub and slob also suffer from this effect?