[PATCH 03/18] xfs: Do background CIL flushes via a workqueue

[Mark Tinguely]

On 04/13/12 07:10, Dave Chinner wrote:
> From: Dave Chinner<dchinner at redhat.com>
>
> Doing background CIL flushes adds significant latency to whatever
> async transaction that triggers it. To avoid blocking async
> transactions on things like waiting for log buffer IO to complete,
> move the CIL push off into a workqueue.  By moving the push work
> into a workqueue, we remove all the latency that the commit adds
> from the foreground transaction commit path. This also means that
> single threaded workloads won't do the CIL push procssing, leaving
> them more CPU to do more async transactions.
>
> To do this, we need to keep track of the sequence number we have
> pushed work for. This avoids having many transaction commits
> attempting to schedule work for the same sequence, and ensures that
> we only ever have one push (background or forced) in progress at a
> time. It also means that we don't need to take the CIL lock in write
> mode to check for potential background push races, which reduces
> lock contention.
>
> To avoid potential issues with "smart" IO schedulers, don't use the
> workqueue for log force triggered flushes. Instead, do them directly
> so that the log IO is done directly by the process issuing the log
> force and so doesn't get stuck on IO elevator queue idling
> incorrectly delaying the log IO from the workqueue.
>
> Signed-off-by: Dave Chinner<dchinner at redhat.com>
> ---

Makes sense.

Reviewed-by: Mark Tinguely <tinguely at sgi.com>