When there are processes heavily creating small files while sync(2) is
running, it can easily happen that quite some new files are created
between WB_SYNC_NONE and WB_SYNC_ALL pass of sync(2). That can happen
especially if there are several busy filesystems (remember that sync
traverses filesystems sequentially and waits in WB_SYNC_ALL phase on one
fs before starting it on another fs). Because WB_SYNC_ALL pass is slow
(e.g. causes a transaction commit and cache flush for each inode in
ext3), resulting sync(2) times are rather large.
The following script reproduces the problem:
function run_writers
{
for (( i = 0; i < 10; i++ )); do
mkdir $1/dir$i
for (( j = 0; j < 40000; j++ )); do
dd if=/dev/zero of=$1/dir$i/$j bs=4k count=4 &>/dev/null
done &
done
}
for dir in "$@"; do
run_writers $dir
done
sleep 40
time sync
======
Fix the problem by disregarding inodes dirtied after sync(2) was called
in the WB_SYNC_ALL pass. To allow for this, sync_inodes_sb() now takes a
time stamp when sync has started which is used for setting up work for
flusher threads.
To give some numbers, when above script is run on two ext4 filesystems on
simple SATA drive, the average sync time from 10 runs is 267.549 seconds
with standard deviation 104.799426. With the patched kernel, the average
sync time from 10 runs is 2.995 seconds with standard deviation 0.096.
Signed-off-by: Jan Kara <jack@xxxxxxx>
---
fs/fs-writeback.c | 17 ++++++++---------
fs/sync.c | 15 +++++++++------
fs/xfs/xfs_super.c | 2 +-
include/linux/writeback.h | 2 +-
include/trace/events/writeback.h | 6 +++---
5 files changed, 22 insertions(+), 20 deletions(-)
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 9f4935b..70837da 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -39,7 +39,7 @@
struct wb_writeback_work {
long nr_pages;
struct super_block *sb;
- unsigned long *older_than_this;
+ unsigned long older_than_this;
enum writeback_sync_modes sync_mode;
unsigned int tagged_writepages:1;
unsigned int for_kupdate:1;
@@ -248,8 +248,7 @@ static int move_expired_inodes(struct list_head *delaying_queue,
while (!list_empty(delaying_queue)) {
inode = wb_inode(delaying_queue->prev);
- if (work->older_than_this &&
- inode_dirtied_after(inode, *work->older_than_this))
+ if (inode_dirtied_after(inode, work->older_than_this))
break;
list_move(&inode->i_wb_list, &tmp);
moved++;
@@ -791,12 +790,11 @@ static long wb_writeback(struct bdi_writeback *wb,
{
unsigned long wb_start = jiffies;
long nr_pages = work->nr_pages;
- unsigned long oldest_jif;
struct inode *inode;
long progress;
- oldest_jif = jiffies;
- work->older_than_this = &oldest_jif;
+ if (!work->older_than_this)
+ work->older_than_this = jiffies;
spin_lock(&wb->list_lock);
for (;;) {
@@ -830,10 +828,10 @@ static long wb_writeback(struct bdi_writeback *wb,
* safe.
*/
if (work->for_kupdate) {
- oldest_jif = jiffies -
+ work->older_than_this = jiffies -
msecs_to_jiffies(dirty_expire_interval * 10);
} else if (work->for_background)
- oldest_jif = jiffies;
+ work->older_than_this = jiffies;
trace_writeback_start(wb->bdi, work);
if (list_empty(&wb->b_io))
@@ -1350,13 +1348,14 @@ EXPORT_SYMBOL(try_to_writeback_inodes_sb);
* This function writes and waits on any dirty inode belonging to this
* super_block.
*/
-void sync_inodes_sb(struct super_block *sb)
+void sync_inodes_sb(struct super_block *sb, unsigned long older_than_this)
{
DECLARE_COMPLETION_ONSTACK(done);
struct wb_writeback_work work = {
.sb = sb,
.sync_mode = WB_SYNC_ALL,
.nr_pages = LONG_MAX,
+ .older_than_this = older_than_this,
.range_cyclic = 0,
.done = &done,
.reason = WB_REASON_SYNC,
diff --git a/fs/sync.c b/fs/sync.c
index 905f3f6..ff96f99 100644
--- a/fs/sync.c
+++ b/fs/sync.c
@@ -27,10 +27,11 @@
* wait == 1 case since in that case write_inode() functions do
* sync_dirty_buffer() and thus effectively write one block at a time.
*/
-static int __sync_filesystem(struct super_block *sb, int wait)
+static int __sync_filesystem(struct super_block *sb, int wait,
+ unsigned long start)
{
if (wait)
- sync_inodes_sb(sb);
+ sync_inodes_sb(sb, start);
else
writeback_inodes_sb(sb, WB_REASON_SYNC);
@@ -47,6 +48,7 @@ static int __sync_filesystem(struct super_block *sb, int wait)
int sync_filesystem(struct super_block *sb)
{
int ret;
+ unsigned long start = jiffies;
/*
* We need to be protected against the filesystem going from
@@ -60,17 +62,17 @@ int sync_filesystem(struct super_block *sb)
if (sb->s_flags & MS_RDONLY)
return 0;
- ret = __sync_filesystem(sb, 0);
+ ret = __sync_filesystem(sb, 0, start);
if (ret < 0)
return ret;
- return __sync_filesystem(sb, 1);
+ return __sync_filesystem(sb, 1, start);
}
EXPORT_SYMBOL_GPL(sync_filesystem);
static void sync_inodes_one_sb(struct super_block *sb, void *arg)
{
if (!(sb->s_flags & MS_RDONLY))
- sync_inodes_sb(sb);
+ sync_inodes_sb(sb, *((unsigned long *)arg));
}
static void sync_fs_one_sb(struct super_block *sb, void *arg)
@@ -102,9 +104,10 @@ static void fdatawait_one_bdev(struct block_device *bdev, void *arg)
SYSCALL_DEFINE0(sync)
{
int nowait = 0, wait = 1;
+ unsigned long start = jiffies;
wakeup_flusher_threads(0, WB_REASON_SYNC);
- iterate_supers(sync_inodes_one_sb, NULL);
+ iterate_supers(sync_inodes_one_sb, &start);
iterate_supers(sync_fs_one_sb, &nowait);
iterate_supers(sync_fs_one_sb, &wait);
iterate_bdevs(fdatawrite_one_bdev, NULL);
diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index 15188cc..8968f50 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -918,7 +918,7 @@ xfs_flush_inodes(
struct super_block *sb = mp->m_super;
if (down_read_trylock(&sb->s_umount)) {
- sync_inodes_sb(sb);
+ sync_inodes_sb(sb, jiffies);
up_read(&sb->s_umount);
}
}
diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index 021b8a3..fc0e432 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -97,7 +97,7 @@ void writeback_inodes_sb_nr(struct super_block *, unsigned long nr,
int try_to_writeback_inodes_sb(struct super_block *, enum wb_reason reason);
int try_to_writeback_inodes_sb_nr(struct super_block *, unsigned long nr,
enum wb_reason reason);
-void sync_inodes_sb(struct super_block *);
+void sync_inodes_sb(struct super_block *sb, unsigned long older_than_this);
void wakeup_flusher_threads(long nr_pages, enum wb_reason reason);
void inode_wait_for_writeback(struct inode *inode);
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index 464ea82..c7bbbe7 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -287,11 +287,11 @@ TRACE_EVENT(writeback_queue_io,
__field(int, reason)
),
TP_fast_assign(
- unsigned long *older_than_this = work->older_than_this;
+ unsigned long older_than_this = work->older_than_this;
strncpy(__entry->name, dev_name(wb->bdi->dev), 32);
- __entry->older = older_than_this ? *older_than_this : 0;
+ __entry->older = older_than_this;
__entry->age = older_than_this ?
- (jiffies - *older_than_this) * 1000 / HZ : -1;
+ (jiffies - older_than_this) * 1000 / HZ : -1;
__entry->moved = moved;
__entry->reason = work->reason;
),
--
1.8.1.4
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
