linux-kernel - Re: [patch] SLQB slab allocator

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Date:	Thu, 22 Jan 2009 11:33:12 +0200
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Cc:	Christoph Lameter <cl@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>, Lin Ming <ming.m.lin@...el.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch] SLQB slab allocator

On Thu, 2009-01-22 at 17:30 +0800, Zhang, Yanmin wrote:
> On Thu, 2009-01-22 at 11:27 +0200, Pekka Enberg wrote:
> > Hi Christoph,
> > 
> > On Mon, 19 Jan 2009, Nick Piggin wrote:
> > > > > > You only go to the allocator when the percpu queue goes empty though, so
> > > > > > if memory policy changes (eg context switch or something), then subsequent
> > > > > > allocations will be of the wrong policy.
> > > > >
> > > > > The per cpu queue size in SLUB is limited by the queues only containing
> > > > > objects from the same page. If you have large queues like SLAB/SLQB(?)
> > > > > then this could be an issue.
> > > >
> > > > And it could be a problem in SLUB too. Chances are that several allocations
> > > > will be wrong after every policy switch. I could describe situations in which
> > > > SLUB will allocate with the _wrong_ policy literally 100% of the time.
> > 
> > On Wed, 2009-01-21 at 19:13 -0500, Christoph Lameter wrote:
> > > No it cannot because in SLUB objects must come from the same page.
> > > Multiple objects in a queue will only ever require a single page and not
> > > multiple like in SLAB.
> > 
> > There's one potential problem with "per-page queues", though. The bigger
> > the object, the smaller the "queue" (i.e. less objects per page). Also,
> > partial lists are less likely to help for big objects because they get
> > emptied so quickly and returned to the page allocator. Perhaps we should
> > do a small "full list" for caches with large objects?
> That helps definitely. We could use a batch to control the list size.

s/full list/empty list/g

That is, a list of pages that could be returned to the page allocator
but are pooled in SLUB to avoid the page allocator overhead. Note that
this will not help allocators that trigger page allocator pass-through.

		Pekka

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