SLUB: Implement mostly lockless slowpaths
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Cover letter [slubll1 00/19] SLUB: Implement mostly lockless slowpaths
Well here is another result of my obsession with SLAB allocators. There must be some way to get an allocator done that is faster without queueing and I hope that we are now there (maybe only almost...).
This patchset implement wider lockless operations in slub affecting most of the slowpaths. In particular the patch decreases the overhead in the performance critical section of __slab_free.
One test that I ran was "hackbench 200 process 200" on 2.6.29-rc1 under KVM
Run SLAB SLUB SLUB LL
1st 35.2 35.9 31.9
2nd 34.6 30.8 27.9
3rd 33.8 29.9 28.8
Note that the SLUB version in 2.6.29-rc1 already has an optimized allocation and free path using this_cpu_cmpxchg_double(). SLUB LL takes it to new heights by also using cmpxchg_double() in the slowpaths (especially in the kfree() case where we cannot queue).
The patch uses a cmpxchg_double (also introduced here) to do an atomic change on the state of a slab page that includes the following pieces of information:
1. Freelist pointer
2. Number of objects inuse
3. Frozen state of a slab
Disabling of interrupts (which is a significant latency in the allocator paths) is avoided in the __slab_free case.
There are some concerns with this patch. The use of cmpxchg_double on fields of the page struct requires alignment of the fields to double word boundaries. That can only be accomplished by adding some padding to struct page which blows it up to 64 byte (on x86_64). Comments in the source describe these things in more detail.
The cmpxchg_double() operation introduced here could also be used to update other doublewords in the page struct in a lockless fashone. One can envision page state changes that involved flags and mappings or do list operations locklessly.
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