On Mon, 25 Aug 2014, Joonsoo Kim wrote:

On Thu, Aug 21, 2014 at 09:21:30AM -0500, Christoph Lameter wrote:

quoted

On Thu, 21 Aug 2014, Joonsoo Kim wrote:

quoted

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 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.

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