Re: [RFC 1/3] Slab infrastructure for array operations

From: Jesper Dangaard Brouer <brouer@redhat.com>
To: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Christoph Lameter <cl@linux.com>,
	akpm@linuxfoundation.org, LKML <linux-kernel@vger.kernel.org>,
	Linux Memory Management List <linux-mm@kvack.org>,
	Pekka Enberg <penberg@kernel.org>,
	brouer@redhat.com
Subject: Re: [RFC 1/3] Slab infrastructure for array operations
Date: Tue, 3 Feb 2015 23:55:12 +0100	[thread overview]
Message-ID: <20150203235512.62738c3c@redhat.com> (raw)
In-Reply-To: <20150129074443.GA19607@js1304-P5Q-DELUXE>

On Thu, 29 Jan 2015 16:44:43 +0900
Joonsoo Kim <iamjoonsoo.kim@lge.com> wrote:

> On Wed, Jan 28, 2015 at 09:30:56AM -0600, Christoph Lameter wrote:
> > On Wed, 28 Jan 2015, Joonsoo Kim wrote:
> > 
[...]
> > 
> > The default when no options are specified is to first exhaust the node
> > partial objects, then allocate new slabs as long as we have more than
> > objects per page left and only then satisfy from cpu local object. I think
> > that is satisfactory for the majority of the cases.
> > 
> > The detailed control options were requested at the meeting in Auckland at
> > the LCA. I am fine with dropping those if they do not make sense. Makes
> > the API and implementation simpler. Jesper, are you ok with this?

Yes, I'm okay with dropping the allocation flags. 

We might want to keep the flag "GFP_SLAB_ARRAY_FULL_COUNT" for allowing
allocator to return less-than the requested elements (but I'm not 100%
sure).  The idea behind this is, if the allocator can "see" that it
needs to perform a (relativly) expensive operation, then I would rather
want it to return current elements (even if it's less than requested).
As this is likely very performance sensitive code using this API.

> IMHO, it'd be better to choose a proper way of allocation by slab
> itself and not to expose options to API user. We could decide the
> best option according to current status of kmem_cache and requested
> object number and internal implementation.
> 
> Is there any obvious example these option are needed for user?

The use-cases were, if the subsystem/user know about their use-case e.g.
1) needing a large allocation which does not need to be cache hot,
2) needing a smaller (e.g 8-16 elems) allocation that should be cache hot.

But, as you argue, I guess it is best to leave this up to the slab
implementation as the status of the kmem_cache is only known to the
allocator itself.

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