From: Michal Schmidt Subject: [PATCH] ext4: reduce scheduling latency with delayed allocation Date: Mon, 1 Mar 2010 13:34:35 +0100 Message-ID: <20100301133435.141c4bc5@leela> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: linux-kernel@vger.kernel.org, linux-ext4@vger.kernel.org To: "Theodore Ts'o" Return-path: Received: from mx1.redhat.com ([209.132.183.28]:7766 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751244Ab0CAMem (ORCPT ); Mon, 1 Mar 2010 07:34:42 -0500 Sender: linux-ext4-owner@vger.kernel.org List-ID: mpage_da_submit_io() may process tens of thousands of pages at a time. Unless full preemption is enabled, it causes scheduling latencies in the order of tens of milliseconds. It can be reproduced simply by writing a big file on ext4 repeatedly with dd if=/dev/zero of=/tmp/dummy bs=10M count=50 The patch fixes it by allowing to reschedule in the loop. cyclictest can be used to measure the latency. I tested with: $ cyclictest -t1 -p 80 -n -i 5000 -m -l 20000 The results from an UP AMD Turion 2GHz with voluntary preemption: Without the patch: T: 0 ( 2535) P:80 I:5000 C: 20000 Min: 12 Act: 23 Avg: 3166 Max: 70524 (i.e. Average latency was more than 3 ms. Max observed latency was 71 ms.) With the patch: T: 0 ( 2588) P:80 I:5000 C: 20000 Min: 13 Act: 33 Avg: 49 Max: 11009 (i.e. Average latency was only 49 us. Max observed latency was 11 ms.) Signed-off-by: Michal Schmidt --- fs/ext4/inode.c | 1 + 1 files changed, 1 insertions(+), 0 deletions(-) diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c index e119524..687a993 100644 --- a/fs/ext4/inode.c +++ b/fs/ext4/inode.c @@ -2007,6 +2007,7 @@ static int mpage_da_submit_io(struct mpage_da_data *mpd) ret = err; } pagevec_release(&pvec); + cond_resched(); } return ret; } -- 1.7.0