| Re: [PATCH] Show slab memory usage on OOM and SysRq-M [message #12165 is a reply to message #12123] |
Wed, 18 April 2007 12:07   |
Eric Dumazet
Messages: 36
Registered: July 2006
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On Wed, 18 Apr 2007 09:17:19 +0300 (EEST)
Pekka J Enberg <penberg@cs.helsinki.fi> wrote:
> On Tue, 17 Apr 2007, Eric Dumazet wrote:
> > This nr_pages should be in struct kmem_list3, not in struct kmem_cache,
> > or else you defeat NUMA optimizations if touching a field in kmem_cache
> > at kmem_getpages()/kmem_freepages() time.
>
> We already touch ->flags, ->gfpflags, and ->gfporder in kmem_getpages().
> Sorry for my ignorance, but how is this different?
>
Those fields are read. Thats OK, because several CPUS might share all those without problem.
But modifying one field in kmem_cache would invalidate one cache line for all cpus that would have to reload it later.
This is what we call "false sharing" or cache line ping pongs
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