From: Shaohua Li Date: Fri, 3 Feb 2012 23:37:17 +0000 (-0800) Subject: readahead: fix pipeline break caused by block plug X-Git-Tag: v3.3-rc3~25^2~1 X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=3deaa7190a8da38453c4fabd9dec7f66d17fff67;p=pandora-kernel.git readahead: fix pipeline break caused by block plug Herbert Poetzl reported a performance regression since 2.6.39. The test is a simple dd read, but with big block size. The reason is: T1: ra (A, A+128k), (A+128k, A+256k) T2: lock_page for page A, submit the 256k T3: hit page A+128K, ra (A+256k, A+384). the range isn't submitted because of plug and there isn't any lock_page till we hit page A+256k because all pages from A to A+256k is in memory T4: hit page A+256k, ra (A+384, A+ 512). Because of plug, the range isn't submitted again. T5: lock_page A+256k, so (A+256k, A+512k) will be submitted. The task is waitting for (A+256k, A+512k) finish. There is no request to disk in T3 and T4, so readahead pipeline breaks. We really don't need block plug for generic_file_aio_read() for buffered I/O. The readahead already has plug and has fine grained control when I/O should be submitted. Deleting plug for buffered I/O fixes the regression. One side effect is plug makes the request size 256k, the size is 128k without it. This is because default ra size is 128k and not a reason we need plug here. Vivek said: : We submit some readahead IO to device request queue but because of nested : plug, queue never gets unplugged. When read logic reaches a page which is : not in page cache, it waits for page to be read from the disk : (lock_page_killable()) and that time we flush the plug list. : : So effectively read ahead logic is kind of broken in parts because of : nested plugging. Removing top level plug (generic_file_aio_read()) for : buffered reads, will allow unplugging queue earlier for readahead. Signed-off-by: Shaohua Li Signed-off-by: Wu Fengguang Reported-by: Herbert Poetzl Tested-by: Eric Dumazet Cc: Christoph Hellwig Cc: Jens Axboe Cc: Vivek Goyal Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- Reading git-diff-tree failed