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authorAndi Kleen <ak@linux.intel.com>2012-01-12 17:20:35 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2012-01-12 20:13:12 -0800
commit65dd2aa90aa17a26703c28652408192856aa0396 (patch)
tree4ea4635f786fc295d16a459925da67701253df97 /lib
parent87192a2a49c475cf322cb143e0fa63b0102d8567 (diff)
downloadlinaro-lsk-65dd2aa90aa17a26703c28652408192856aa0396.tar.gz
dio: optimize cache misses in the submission path
Some investigation of a transaction processing workload showed that a major consumer of cycles in __blockdev_direct_IO is the cache miss while accessing the block size. This is because it has to walk the chain from block_dev to gendisk to queue. The block size is needed early on to check alignment and sizes. It's only done if the check for the inode block size fails. But the costly block device state is unconditionally fetched. - Reorganize the code to only fetch block dev state when actually needed. Then do a prefetch on the block dev early on in the direct IO path. This is worth it, because there is substantial code run before we actually touch the block dev now. - I also added some unlikelies to make it clear the compiler that block device fetch code is not normally executed. This gave a small, but measurable improvement on a large database benchmark (about 0.3%) [akpm@linux-foundation.org: coding-style fixes] [sfr@canb.auug.org.au: using prefetch requires including prefetch.h] Signed-off-by: Andi Kleen <ak@linux.intel.com> Cc: Jeff Moyer <jmoyer@redhat.com> Cc: Jens Axboe <axboe@kernel.dk> Cc: Christoph Hellwig <hch@lst.de> Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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