linux-kernel - Re: [PATCH 1/3] mm/slab: use percpu allocator for cpu cache

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Date:	Tue, 26 Aug 2014 11:19:04 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
	Tejun Heo <htejun@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] mm/slab: use percpu allocator for cpu cache

On Mon, Aug 25, 2014 at 08:13:58AM -0500, Christoph Lameter wrote:
> On Mon, 25 Aug 2014, Joonsoo Kim wrote:
> 
> > On Thu, Aug 21, 2014 at 09:21:30AM -0500, Christoph Lameter wrote:
> > > On Thu, 21 Aug 2014, Joonsoo Kim wrote:
> > >
> > > > So, this patch try to use percpu allocator in SLAB. This simplify
> > > > initialization step in SLAB so that we could maintain SLAB code more
> > > > easily.
> > >
> > > I thought about this a couple of times but the amount of memory used for
> > > the per cpu arrays can be huge. In contrast to slub which needs just a
> > > few pointers, slab requires one pointer per object that can be in the
> > > local cache. CC Tj.
> > >
> > > Lets say we have 300 caches and we allow 1000 objects to be cached per
> > > cpu. That is 300k pointers per cpu. 1.2M on 32 bit. 2.4M per cpu on
> > > 64bit.
> >
> > Amount of memory we need to keep pointers for object is same in any case.
> 
> What case? SLUB uses a linked list and therefore does not have these
> storage requirements.

I misunderstand that you mentioned just memory usage. My *any case*
means memory usage of previous SLAB and SLAB with this percpu alloc
change. Sorry for confusion.

> 
> > I know that percpu allocator occupy vmalloc space, so maybe we could
> > exhaust vmalloc space on 32 bit. 64 bit has no problem on it.
> > How many cores does largest 32 bit system have? Is it possible
> > to exhaust vmalloc space if we use percpu allocator?
> 
> There were NUMA systems on x86 a while back (not sure if they still
> exists) with 128 or so processors.
> 
> Some people boot 32 bit kernels on contemporary servers. The Intel ones
> max out at 18 cores (36 hyperthreaded). I think they support up to 8
> scokets. So 8 * 36?
> 
> 
> Its different on other platforms with much higher numbers. Power can
> easily go up to hundreds of hardware threads and SGI Altixes 7 yearsago
> where at 8000 or so.

Okay... These large systems with 32 bit kernel could be break with this
change. I will do more investigation. Possibly, I will drop this patch. :)

Thanks.
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