linux-kernel - Re: [PATCH 1/3] Slab infrastructure for array operations

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Date:	Fri, 13 Feb 2015 13:20:10 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Christoph Lameter <cl@...ux.com>
cc:	Joonsoo Kim <iamjoonsoo.kim@....com>, akpm@...uxfoundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	penberg@...nel.org, iamjoonsoo@....com,
	Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: [PATCH 1/3] Slab infrastructure for array operations

On Fri, 13 Feb 2015, Christoph Lameter wrote:

> > I also think that this implementation is slub-specific. For example,
> > in slab case, it is always better to access local cpu cache first than
> > page allocator since slab doesn't use list to manage free objects and
> > there is no cache line overhead like as slub. I think that,
> > in kmem_cache_alloc_array(), just call to allocator-defined
> > __kmem_cache_alloc_array() is better approach.
> 
> What do you mean by "better"? Please be specific as to where you would see
> a difference. And slab definititely manages free objects although
> differently than slub. SLAB manages per cpu (local) objects, per node
> partial lists etc. Same as SLUB. The cache line overhead is there but no
> that big a difference in terms of choosing objects to get first.
> 

I think because we currently lack a non-fallback implementation for slab 
that it may be premature to discuss what would be unified if such an 
implementation were to exist.  That unification can always happen later 
if/when the slab implementation is proposed, but I don't think we should 
be unifying an implementation that doesn't exist.  

In other words, I think it would be much cleaner to do just define the 
generic array alloc and array free functions in mm/slab_common.c along 
with their EXPORT_SYMBOL()'s as simple callbacks to per-allocator 
__kmem_cache_{alloc,free}_array() implementations.  I think it's also 
better from a source code perspective to avoid reading two different 
functions and then realizing that nothing is actually unified between them 
(and the absence of an unnecessary #ifdef is currently helpful).
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