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Re: [PATCH 20/34] xfs: remove all the inodes on a buffer from the AIL in

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 20/34] xfs: remove all the inodes on a buffer from the AIL in bulk
From: Alex Elder <aelder@xxxxxxx>
Date: Tue, 21 Dec 2010 20:20:46 -0600
Cc: xfs@xxxxxxxxxxx
In-reply-to: <1292916570-25015-21-git-send-email-david@xxxxxxxxxxxxx>
References: <1292916570-25015-1-git-send-email-david@xxxxxxxxxxxxx> <1292916570-25015-21-git-send-email-david@xxxxxxxxxxxxx>
Reply-to: aelder@xxxxxxx
On Tue, 2010-12-21 at 18:29 +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
> When inode buffer IO completes, usually all of the inodes are removed from the
> AIL. This involves processing them one at a time and taking the AIL lock once
> for every inode. When all CPUs are processing inode IO completions, this 
> causes
> excessive amount sof contention on the AIL lock.
> Instead, change the way we process inode IO completion in the buffer
> IO done callback. Allow the inode IO done callback to walk the list
> of IO done callbacks and pull all the inodes off the buffer in one
> go and then process them as a batch.
> Once all the inodes for removal are collected, take the AIL lock
> once and do a bulk removal operation to minimise traffic on the AIL
> lock.
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> Reviewed-by: Christoph Hellwig <hch@xxxxxx>

One question, below.            -Alex

. . .

> @@ -861,28 +910,37 @@ xfs_iflush_done(
>        * the lock since it's cheaper, and then we recheck while
>        * holding the lock before removing the inode from the AIL.
>        */
> -     if (iip->ili_logged && lip->li_lsn == iip->ili_flush_lsn) {
> +     if (need_ail) {
> +             struct xfs_log_item *log_items[need_ail];

What's the worst-case value of need_ail we might see here?

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