From: Fengguang Wu Subject: [PATCH] writeback: speed up writeback of big dirty files Date: Thu, 17 Jan 2008 13:28:18 +0800 Message-ID: References: <230154.6549.qm@web32604.mail.mud.yahoo.com> <170fa0d20801160827p73ae28d0u702120a4f9f48936@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Mike Snitzer , Martin Knoblauch , Peter Zijlstra , jplatte@naasa.net, Ingo Molnar , linux-kernel@vger.kernel.org, "linux-ext4@vger.kernel.org" To: Linus Torvalds Return-path: Content-Disposition: inline In-Reply-To: <170fa0d20801160827p73ae28d0u702120a4f9f48936@mail.gmail.com> Message-Id: Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org > On Jan 16, 2008 9:15 AM, Martin Knoblauch wrote: > Fengguang's latest writeback patch applies cleanly, builds, boots on 2.6.24-rc8. Linus, if possible, I'd suggest this patch be merged for 2.6.24. It's a safer version of the reverted patch. It was tested on ext2/ext3/jfs/xfs/reiserfs and won't 100% iowait even without the other bug fixing patches. Fengguang --- writeback: speed up writeback of big dirty files After making dirty a 100M file, the normal behavior is to start the writeback for all data after 30s delays. But sometimes the following happens instead: - after 30s: ~4M - after 5s: ~4M - after 5s: all remaining 92M Some analyze shows that the internal io dispatch queues goes like this: s_io s_more_io ------------------------- 1) 100M,1K 0 2) 1K 96M 3) 0 96M 1) initial state with a 100M file and a 1K file 2) 4M written, nr_to_write <= 0, so write more 3) 1K written, nr_to_write > 0, no more writes(BUG) nr_to_write > 0 in (3) fools the upper layer to think that data have all been written out. The big dirty file is actually still sitting in s_more_io. We cannot simply splice s_more_io back to s_io as soon as s_io becomes empty, and let the loop in generic_sync_sb_inodes() continue: this may starve newly expired inodes in s_dirty. It is also not an option to draw inodes from both s_more_io and s_dirty, an let the loop go on: this might lead to live locks, and might also starve other superblocks in sync time(well kupdate may still starve some superblocks, that's another bug). We have to return when a full scan of s_io completes. So nr_to_write > 0 does not necessarily mean that "all data are written". This patch introduces a flag writeback_control.more_io to indicate that more io should be done. With it the big dirty file no longer has to wait for the next kupdate invocation 5s later. In sync_sb_inodes() we only set more_io on super_blocks we actually visited. This aviods the interaction between two pdflush deamons. Also in __sync_single_inode() we don't blindly keep requeuing the io if the filesystem cannot progress. Failing to do so may lead to 100% iowait. Tested-by: Mike Snitzer Signed-off-by: Fengguang Wu --- fs/fs-writeback.c | 18 ++++++++++++++++-- include/linux/writeback.h | 1 + mm/page-writeback.c | 9 ++++++--- 3 files changed, 23 insertions(+), 5 deletions(-) --- linux.orig/fs/fs-writeback.c +++ linux/fs/fs-writeback.c @@ -284,7 +284,17 @@ __sync_single_inode(struct inode *inode, * soon as the queue becomes uncongested. */ inode->i_state |= I_DIRTY_PAGES; - requeue_io(inode); + if (wbc->nr_to_write <= 0) { + /* + * slice used up: queue for next turn + */ + requeue_io(inode); + } else { + /* + * somehow blocked: retry later + */ + redirty_tail(inode); + } } else { /* * Otherwise fully redirty the inode so that @@ -479,8 +489,12 @@ sync_sb_inodes(struct super_block *sb, s iput(inode); cond_resched(); spin_lock(&inode_lock); - if (wbc->nr_to_write <= 0) + if (wbc->nr_to_write <= 0) { + wbc->more_io = 1; break; + } + if (!list_empty(&sb->s_more_io)) + wbc->more_io = 1; } return; /* Leave any unwritten inodes on s_io */ } --- linux.orig/include/linux/writeback.h +++ linux/include/linux/writeback.h @@ -62,6 +62,7 @@ struct writeback_control { unsigned for_reclaim:1; /* Invoked from the page allocator */ unsigned for_writepages:1; /* This is a writepages() call */ unsigned range_cyclic:1; /* range_start is cyclic */ + unsigned more_io:1; /* more io to be dispatched */ }; /* --- linux.orig/mm/page-writeback.c +++ linux/mm/page-writeback.c @@ -558,6 +558,7 @@ static void background_writeout(unsigned global_page_state(NR_UNSTABLE_NFS) < background_thresh && min_pages <= 0) break; + wbc.more_io = 0; wbc.encountered_congestion = 0; wbc.nr_to_write = MAX_WRITEBACK_PAGES; wbc.pages_skipped = 0; @@ -565,8 +566,9 @@ static void background_writeout(unsigned min_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write; if (wbc.nr_to_write > 0 || wbc.pages_skipped > 0) { /* Wrote less than expected */ - congestion_wait(WRITE, HZ/10); - if (!wbc.encountered_congestion) + if (wbc.encountered_congestion || wbc.more_io) + congestion_wait(WRITE, HZ/10); + else break; } } @@ -631,11 +633,12 @@ static void wb_kupdate(unsigned long arg global_page_state(NR_UNSTABLE_NFS) + (inodes_stat.nr_inodes - inodes_stat.nr_unused); while (nr_to_write > 0) { + wbc.more_io = 0; wbc.encountered_congestion = 0; wbc.nr_to_write = MAX_WRITEBACK_PAGES; writeback_inodes(&wbc); if (wbc.nr_to_write > 0) { - if (wbc.encountered_congestion) + if (wbc.encountered_congestion || wbc.more_io) congestion_wait(WRITE, HZ/10); else break; /* All the old data is written */