From: Wu Fengguang Date: Tue, 6 Dec 2011 19:17:17 +0000 (-0600) Subject: writeback: avoid tiny dirty poll intervals X-Git-Tag: v3.3-rc1~112^2~3 X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=5b9b357435a51ff14835c06d8b00765a4c68f313;p=pandora-kernel.git writeback: avoid tiny dirty poll intervals The LKP tests see big 56% regression for the case fio_mmap_randwrite_64k. Shaohua manages to root cause it to be the much smaller dirty pause times and hence much more frequent invocations to the IO-less balance_dirty_pages(). Since fio_mmap_randwrite_64k effectively contains both reads and writes, the more frequent pauses triggered more idling in the cfq IO scheduler. The solution is to increase pause time all the way up to the max 200ms in this case, which is found to restore most performance. This will help reduce CPU overheads in other cases, too. Note that I don't expect many performance critical workloads to run this access pattern: the mmap read-on-write is rather inefficient and could be avoided by doing normal writes syscalls. CC: Jan Kara CC: Peter Zijlstra Reported-by: Li Shaohua Tested-by: Li Shaohua Signed-off-by: Wu Fengguang --- Reading git-diff-tree failed