Messages in this thread | | | Date | Thu, 23 Oct 2008 18:35:20 +0200 | From | Eric Dumazet <> | Subject | Re: SLUB defrag pull request? |
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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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