writeback: speed up writeback of big dirty files
Fengguang Wu [Tue, 5 Feb 2008 06:29:36 +0000 (22:29 -0800)]
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 invokation 5s later.

In sync_sb_inodes() we only set more_io on super_blocks we actually
visited.  This avoids 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 <snitzer@gmail.com>
Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
Cc: Michael Rubin <mrubin@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/fs-writeback.c
include/linux/writeback.h
mm/page-writeback.c

index 3fe782d..0b30640 100644 (file)
@@ -284,7 +284,17 @@ __sync_single_inode(struct inode *inode, struct writeback_control *wbc)
                                 * 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
@@ -468,8 +478,12 @@ sync_sb_inodes(struct super_block *sb, struct writeback_control *wbc)
                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 */
 }
index b2cd826..b7b3362 100644 (file)
@@ -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 */
 };
 
 /*
index a4ca162..5e00f17 100644 (file)
@@ -567,6 +567,7 @@ static void background_writeout(unsigned long _min_pages)
                        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;
@@ -574,8 +575,9 @@ static void background_writeout(unsigned long _min_pages)
                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;
                }
        }
@@ -640,11 +642,12 @@ 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 */