linux-kernel - Re: SLUB defrag pull request?

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Date:	Thu, 23 Oct 2008 11:47:48 -0500 (CDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Eric Dumazet <dada1@...mosbay.com>
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?

On Thu, 23 Oct 2008, Eric Dumazet wrote:

>> 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.

Most architectures actually can do that. Its probably worth to run some 
tests with that. Conversion of a cacheline from shared to exclusive can 
cost something.

>> 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.

Correct.

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

Right.

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