linux-kernel - Re: SLUB defrag pull request?

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Message-ID: <4900A7C8.9020707@cosmosbay.com>
Date:	Thu, 23 Oct 2008 18:35:20 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Miklos Szeredi <miklos@...redi.hu>, nickpiggin@...oo.com.au,
	hugh@...itas.com, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org
Subject: Re: SLUB defrag pull request?

Christoph Lameter a écrit :
> On Thu, 23 Oct 2008, Eric Dumazet wrote:
> 
>> At alloc time, I remember I added a prefetchw() call in SLAB in 
>> __cache_alloc(),
>> this could explain some differences between SLUB and SLAB too, since SLAB
>> gives a hint to processor to warm its cache.
> 
> SLUB touches objects by default when allocating. And it does it 
> immediately in slab_alloc() in order to retrieve the pointer to the next 
> object. So there is no point of hinting there right now.
> 

Please note SLUB touches by reading object.

prefetchw() gives a hint to cpu saying this cache line is going to be *modified*, even
if first access is a read. Some architectures can save some bus transactions, acquiring
the cache line in an exclusive way instead of shared one.


> If we go to the pointer arrays then the situation is similar to SLAB 
> where the object is not touched by the allocator. Then the hint would be 
> useful again.

It is usefull right now for ((SLAB_DESTROY_BY_RCU | SLAB_POISON) or ctor caches.

Probably not that important because many objects are very large anyway, and a prefetchw()
of the begining of object is partial.



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