Hello, Glauber.
On Thu, Sep 27, 2012 at 10:30:36PM +0400, Glauber Costa wrote:
> > But that happens only when pages enter and leave slab and if it still
> > is significant, we can try to further optimize charging. Given that
> > this is only for cases where memcg is already in use and we provide a
> > switch to disable it globally, I really don't think this warrants
> > implementing fully hierarchy configuration.
>
> Not totally true. We still have to match every allocation to the right
> cache, and that is actually our heaviest hit, responsible for the 2, 3 %
> we're seeing when this is enabled. It is the kind of path so hot that
> people frown upon branches being added, so I don't think we'll ever get
> this close to being free.
Sure, depening on workload, any addition to alloc/free could be
noticeable. I don't know. I'll write more about it when replying to
Michal's message. BTW, __memcg_kmem_get_cache() does seem a bit
heavy. I wonder whether indexing from cache side would make it
cheaper? e.g. something like the following.
kmem_cache *__memcg_kmem_get_cache(cachep, gfp)
{
struct kmem_cache *c;
c = cachep->memcg_params->caches[percpu_read(kmemcg_slab_idx)];
if (likely(c))
return c;
/* try to create and then fall back to cachep */
}
where kmemcg_slab_idx is updated from sched notifier (or maybe add and
use current->kmemcg_slab_idx?). You would still need __GFP_* and
in_interrupt() tests but current->mm and PF_KTHREAD tests can be
rolled into index selection.
Thanks.
--
tejun
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