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Date: Tue, 26 Aug 2014 16:22:06 -0500 (CDT)
From: Christoph Lameter <cl@linux.com>
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To: Joonsoo Kim <iamjoonsoo.kim@lge.com>
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Subject: Re: [PATCH 1/3] mm/slab: use percpu allocator for cpu cache
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On Tue, 26 Aug 2014, Joonsoo Kim wrote:

> > 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.

Ok. True the total amount of memory used does not increase.

> > > 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. :)

Wait the last system mentioned are 64 bit. SGI definitely. Power probably
too.