erasure-code: buffer alignment #2558
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SIMD optimized erasure code computation needs aligned memory. Buffers aligned to a page boundary are wasted on it though. The buffers used for the erasure code computation are typical smaller than a page. An alignment of 32 bytes is chosen to satisfy the needs of AVX/AVX2. Could be made arch specific to reduce the alignment to 16 bytes for arm/aarch64 NEON. Signed-off-by: Janne Grunau <j@jannau.net>
Requiring page aligned buffers and realigning the input if necessary creates measurable oberhead. ceph_erasure_code_benchmark is ~30% faster with this change for technique=reed_sol_van,k=2,m=1. Also prevents a misaligned buffer when bufferlist::c_str(bufferlist) has to allocate a new buffer to provide continuous one. See bug #9408 Signed-off-by: Janne Grunau <j@jannau.net>
The benchmark is supposed to measure the encoding speed and not the overhead of buffer realignments. Signed-off-by: Janne Grunau <j@jannau.net>
| { | ||
| std::list<ptr>::iterator p = _buffers.begin(); | ||
| while (p != _buffers.end()) { | ||
| // keep anything that's already page sized+aligned |
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leftover from the rebuild_page_aligned function from which it was copied
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I think it would be tidier to make the alignment an argument to rebuild_aligned(n) is_aligned(n) and create_aligned(n). Unless that causes problems, in which case let's not hold this up... |
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For the record performance compared : http://tracker.ceph.com/issues/9601 |
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I've fixed the copy-n-paste error in the comment and reworked the set to allow variable alignment. I don't see an immediate need for a variable alignment though. Unifying it with page_aligned code is probably worth it though, |
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jannau will open a new pull request with updated code |
ghost
closed this
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re-using the same pull request instead |
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