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SubjectRe: 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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