SLUB: Lockless freelists for objects V3
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- v1 Latest21 patches 0 repliesNew
V2->V3 from the cover letter
- Provide statistics
- Fallback logic to page lock if cmpxchg16b is not available.
- Better counter support
- More cleanups and clarifications
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Cover letter [slubllv3 00/21] SLUB: Lockless freelists for objects V3
V2->V3
- Provide statistics
- Fallback logic to page lock if cmpxchg16b is not available.
- Better counter support
- More cleanups and clarifications
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-rc3 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 fashion. One can envision page state changes that involved flags and mappings or maybe do list operations locklessly (but with the current scheme we would need to update two other words elsewhere at the same time too, so another scheme would be needed).