commit: 3deaa7190a8da38453c4fabd9dec7f66d17fff67 From: Shaohua Li <[email protected]> Date: Fri, 3 Feb 2012 15:37:17 -0800 Subject: 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 <[email protected]> Signed-off-by: Wu Fengguang <[email protected]> Reported-by: Herbert Poetzl <[email protected]> Tested-by: Eric Dumazet <[email protected]> Cc: Christoph Hellwig <[email protected]> Cc: Jens Axboe <[email protected]> Cc: Vivek Goyal <[email protected]> Cc: <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]> --- mm/filemap.c | 8 ++++---- 1 files changed, 4 insertions(+), 4 deletions(-) diff --git a/mm/filemap.c b/mm/filemap.c index 97f49ed..b662757 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -1400,15 +1400,12 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov, unsigned long seg = 0; size_t count; loff_t *ppos = &iocb->ki_pos; - struct blk_plug plug; count = 0; retval = generic_segment_checks(iov, &nr_segs, &count, VERIFY_WRITE); if (retval) return retval; - blk_start_plug(&plug); - /* coalesce the iovecs and go direct-to-BIO for O_DIRECT */ if (filp->f_flags & O_DIRECT) { loff_t size; @@ -1424,8 +1421,12 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov, retval = filemap_write_and_wait_range(mapping, pos, pos + iov_length(iov, nr_segs) - 1); if (!retval) { + struct blk_plug plug; + + blk_start_plug(&plug); retval = mapping->a_ops->direct_IO(READ, iocb, iov, pos, nr_segs); + blk_finish_plug(&plug); } if (retval > 0) { *ppos = pos + retval; @@ -1481,7 +1482,6 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov, break; } out: - blk_finish_plug(&plug); return retval; } EXPORT_SYMBOL(generic_file_aio_read); -- 1.7.3.4 -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
