[PATCH] xfs: log IO completion workqueue is a high priority queue

[PATCH] xfs: log IO completion workqueue is a high priority queue

[Dave Chinner]

From: Dave Chinner <dchinner@redhat.com>

The workqueue implementation in 2.6.36-rcX has changed, resulting in the
workqueues no longer having dedicated threads for work processing. This has
caused severe livelocks under heavy parallel create workloads because the
log IO completions have been getting held up behind metadata IO completions.
Hence log commits would stall, memory allocation would stall because pages
could not be cleaned, and lock contention on the AIL during inode IO completion
processing was being seen to slow everything down even further.

By making the log Io completion workqueue a high priority workqueue, they are
queued ahead of all data/metadata IO completions and processed before the
data/metadata completions. Hence the log never gets stalled, and operations
needed to clean memory can continue as quickly as possible. This avoids the
livelock conditions and allos the system to keep running under heavy load as
per normal.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
 fs/xfs/linux-2.6/xfs_buf.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/fs/xfs/linux-2.6/xfs_buf.c b/fs/xfs/linux-2.6/xfs_buf.c
index 6130828..2a6645f 100644
--- a/fs/xfs/linux-2.6/xfs_buf.c
+++ b/fs/xfs/linux-2.6/xfs_buf.c
@@ -1956,7 +1956,8 @@ xfs_buf_init(void)
 	if (!xfs_buf_zone)
 		goto out;
 
-	xfslogd_workqueue = create_workqueue("xfslogd");
+	xfslogd_workqueue = alloc_workqueue("xfslogd",
+					WQ_RESCUER | WQ_HIGHPRI, 1);
 	if (!xfslogd_workqueue)
 		goto out_free_buf_zone;
 
-- 
1.7.1

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Re: [PATCH] xfs: log IO completion workqueue is a high priority queue

[Christoph Hellwig]

On Wed, Sep 08, 2010 at 07:00:22PM +1000, Dave Chinner wrote:
> queued ahead of all data/metadata IO completions and processed before the
> data/metadata completions. Hence the log never gets stalled, and operations
> needed to clean memory can continue as quickly as possible. This avoids the
> livelock conditions and allos the system to keep running under heavy load as
> per normal.

>From a quick review of how the WQ_HIGHPRI flag is handled this look good
to me.  I'm still rather suspicious of the new workqueue code, and the
complete lack of documentation for it, though.


Reviewed-by: Christoph Hellwig <hch@lst.de>

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