From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: Re: [RESEND] [PATCH] readahead:add blk_run_backing_dev Date: Tue, 22 Sep 2009 13:58:38 -0700 Message-ID: <20090922135838.33ebe36b.akpm@linux-foundation.org> References: <6.0.0.20.2.20090529142527.071669e0@172.19.0.2> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, Wu Fengguang To: Hisashi Hifumi Return-path: In-Reply-To: <6.0.0.20.2.20090529142527.071669e0@172.19.0.2> Sender: owner-linux-mm@kvack.org List-Id: linux-fsdevel.vger.kernel.org On Fri, 29 May 2009 14:35:55 +0900 Hisashi Hifumi wrote: > I added blk_run_backing_dev on page_cache_async_readahead > so readahead I/O is unpluged to improve throughput on > especially RAID environment. I still haven't sent this upstream. It's unclear to me that we've decided that it merits merging? From: Hisashi Hifumi I added blk_run_backing_dev on page_cache_async_readahead so readahead I/O is unpluged to improve throughput on especially RAID environment. The normal case is, if page N become uptodate at time T(N), then T(N) <= T(N+1) holds. With RAID (and NFS to some degree), there is no strict ordering, the data arrival time depends on runtime status of individual disks, which breaks that formula. So in do_generic_file_read(), just after submitting the async readahead IO request, the current page may well be uptodate, so the page won't be locked, and the block device won't be implicitly unplugged: if (PageReadahead(page)) page_cache_async_readahead() if (!PageUptodate(page)) goto page_not_up_to_date; //... page_not_up_to_date: lock_page_killable(page); Therefore explicit unplugging can help. Following is the test result with dd. #dd if=testdir/testfile of=/dev/null bs=16384 -2.6.30-rc6 1048576+0 records in 1048576+0 records out 17179869184 bytes (17 GB) copied, 224.182 seconds, 76.6 MB/s -2.6.30-rc6-patched 1048576+0 records in 1048576+0 records out 17179869184 bytes (17 GB) copied, 206.465 seconds, 83.2 MB/s (7Disks RAID-0 Array) -2.6.30-rc6 1054976+0 records in 1054976+0 records out 17284726784 bytes (17 GB) copied, 212.233 seconds, 81.4 MB/s -2.6.30-rc6-patched 1054976+0 records out 17284726784 bytes (17 GB) copied, 198.878 seconds, 86.9 MB/s (7Disks RAID-5 Array) The patch was found to improve performance with the SCST scsi target driver. See http://sourceforge.net/mailarchive/forum.php?thread_name=a0272b440906030714g67eabc5k8f847fb1e538cc62%40mail.gmail.com&forum_name=scst-devel [akpm@linux-foundation.org: unbust comment layout] [akpm@linux-foundation.org: "fix" CONFIG_BLOCK=n] Signed-off-by: Hisashi Hifumi Acked-by: Wu Fengguang Cc: Jens Axboe Cc: KOSAKI Motohiro Tested-by: Ronald Cc: Bart Van Assche Cc: Vladislav Bolkhovitin Cc: Randy Dunlap Signed-off-by: Andrew Morton --- mm/readahead.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff -puN mm/readahead.c~readahead-add-blk_run_backing_dev mm/readahead.c --- a/mm/readahead.c~readahead-add-blk_run_backing_dev +++ a/mm/readahead.c @@ -547,5 +547,17 @@ page_cache_async_readahead(struct addres /* do read-ahead */ ondemand_readahead(mapping, ra, filp, true, offset, req_size); + +#ifdef CONFIG_BLOCK + /* + * Normally the current page is !uptodate and lock_page() will be + * immediately called to implicitly unplug the device. However this + * is not always true for RAID conifgurations, where data arrives + * not strictly in their submission order. In this case we need to + * explicitly kick off the IO. + */ + if (PageUptodate(page)) + blk_run_backing_dev(mapping->backing_dev_info, NULL); +#endif } EXPORT_SYMBOL_GPL(page_cache_async_readahead); _ -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org