Re: [RFC PATCH 2/2] slab: implement bulk free in SLAB allocator

[Jesper Dangaard Brouer <brouer@redhat.com>]

On Tue, 8 Dec 2015 17:56:35 +0300
Vladimir Davydov <vdavydov@virtuozzo.com> wrote:

> On Tue, Dec 08, 2015 at 08:15:21AM -0600, Christoph Lameter wrote:
> > On Tue, 8 Dec 2015, Vladimir Davydov wrote:
> > 
> > > If producers are represented by different processes, they can belong to
> > > different memory cgroups, so that objects passed to the consumer will
> > > come from different kmem caches (per memcg caches), although they are
> > > all of the same kind. This means, we must call cache_from_obj() on each
> > > object passed to kmem_cache_free_bulk() in order to free each object to
> > > the cache it was allocated from.
> > 
> > The we should change the API so that we do not specify kmem_cache on bulk
> > free. Do it like kfree without any cache spec.
> > 
> 
> Don't think so, because AFAIU the whole kmem_cache_free_bulk
> optimization comes from the assumption that objects passed to it are
> likely to share the same slab page. So they must be of the same kind,
> otherwise no optimization would be possible and the function wouldn't
> perform any better than calling kfree directly in a for-loop. By
> requiring the caller to specify the cache we emphasize this.

I agree with Vladimir here.  The performance gain for SLUB is
especially depended on that objects are likely to share the same slab
page.  It might not hurt SLAB as much.

> Enabling kmemcg might break the assumption and neglect the benefit of
> using kmem_cache_free_bulk, but it is to be expected, because kmem
> accounting does not come for free. Callers who do care about the
> performance and count every cpu cycle will surely disable it, in which
> case cache_from_obj() will be a no-op and kmem_cache_free_bulk will bear
> fruit.

True, compiler does realize, when CONFIG_MEMCG_KMEM is disabled, that it
can optimize the call to cache_from_obj() away.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer

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