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−25
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xfs: reduce lock hold times in buffer writeback
When we have a lot of metadata to flush from the AIL, the buffer list can get very long. The current submission code tries to batch submission to optimise IO order of the metadata (i.e. ascending block order) to maximise block layer merging or IO to adjacent metadata blocks. Unfortunately, the method used can result in long lock times occurring as buffers locked early on in the buffer list might not be dispatched until the end of the IO licst processing. This is because sorting does not occur util after the buffer list has been processed and the buffers that are going to be submitted are locked. Hence when the buffer list is several thousand buffers long, the lock hold times before IO dispatch can be significant. To fix this, sort the buffer list before we start trying to lock and submit buffers. This means we can now submit buffers immediately after they are locked, allowing merging to occur immediately on the plug and dispatch to occur as quickly as possible. This means there is minimal delay between locking the buffer and IO submission occuring, hence reducing the worst case lock hold times seen during delayed write buffer IO submission signficantly. Signed-off-by:Dave Chinner <dchinner@redhat.com> Reviewed-by: