From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Sandeen Subject: Re: [PATCH] ext4: don't scan/accumulate more pages than mballoc will allocate Date: Wed, 07 Apr 2010 21:31:11 -0500 Message-ID: <4BBD3FEF.30501@redhat.com> References: <4BB9F6C3.5000001@redhat.com> <1270692619-16629-1-git-send-email-tytso@mit.edu> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Ext4 Developers List To: "Theodore Ts'o" Return-path: Received: from mx1.redhat.com ([209.132.183.28]:26411 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750775Ab0DHCbR (ORCPT ); Wed, 7 Apr 2010 22:31:17 -0400 In-Reply-To: <1270692619-16629-1-git-send-email-tytso@mit.edu> Sender: linux-ext4-owner@vger.kernel.org List-ID: Theodore Ts'o wrote: > From: From: Eric Sandeen > > There was a bug reported on RHEL5 that a 10G dd on a 12G box > had a very, very slow sync after that. > > At issue was the loop in write_cache_pages scanning all the way > to the end of the 10G file, even though the subsequent call > to mpage_da_submit_io would only actually write a smallish amt; then > we went back to the write_cache_pages loop ... wasting tons of time > in calling __mpage_da_writepage for thousands of pages we would > just revisit (many times) later. > > Upstream it's not such a big issue for sys_sync because we get > to the loop with a much smaller nr_to_write, which limits the loop. > > However, talking with Aneesh he realized that fsync upstream still > gets here with a very large nr_to_write and we face the same problem. > > This patch makes mpage_add_bh_to_extent stop the loop after we've > accumulated 2048 pages, by setting mpd->io_done = 1; which ultimately > causes the write_cache_pages loop to break. > > Repeating the test with a dirty_ratio of 80 (to leave something for > fsync to do), I don't see huge IO performance gains, but the reduction > in cpu usage is striking: 80% usage with stock, and 2% with the > below patch. Instrumenting the loop in write_cache_pages clearly > shows that we are wasting time here. > > Eventually we need to change mpage_da_map_pages() also submit its I/O > to the block layer, subsuming mpage_da_submit_io(), and then change it > call ext4_get_blocks() multiple times. > > Signed-off-by: Eric Sandeen > Signed-off-by: "Theodore Ts'o" > --- > > This is the slightly revised version of Eric's patch that I've added to > the ext4 patch queue. -- Ted Seems fine, thanks. -Eric > fs/ext4/inode.c | 9 +++++++++ > 1 files changed, 9 insertions(+), 0 deletions(-) > > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c > index 5c6ca10..2c12926 100644 > --- a/fs/ext4/inode.c > +++ b/fs/ext4/inode.c > @@ -2349,6 +2349,15 @@ static void mpage_add_bh_to_extent(struct mpage_da_data *mpd, > sector_t next; > int nrblocks = mpd->b_size >> mpd->inode->i_blkbits; > > + /* > + * XXX Don't go larger than mballoc is willing to allocate > + * This is a stopgap solution. We eventually need to fold > + * mpage_da_submit_io() into this function and then call > + * ext4_get_blocks() multiple times in a loop > + */ > + if (nrblocks >= 8*1024*1024/mpd->inode->i_sb->s_blocksize) > + goto flush_it; > + > /* check if thereserved journal credits might overflow */ > if (!(EXT4_I(mpd->inode)->i_flags & EXT4_EXTENTS_FL)) { > if (nrblocks >= EXT4_MAX_TRANS_DATA) {