On Tue, Jan 31, 2012 at 03:59:40PM +0800, Shaohua Li wrote: > 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. > > Signed-off-by: Shaohua Li <shaohua.li@xxxxxxxxx> > Tested-by: Herbert Poetzl <herbert@xxxxxxxxxxxx> > Tested-by: Eric Dumazet <eric.dumazet@xxxxxxxxx> > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> Please also CC -stable on this. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>