Re: [PATCH 3/6] writeback: sync expired inodes first in background writeback

[Dave Chinner <david@fromorbit.com>]

On Tue, Apr 19, 2011 at 11:00:06AM +0800, Wu Fengguang wrote:
> A background flush work may run for ever. So it's reasonable for it to
> mimic the kupdate behavior of syncing old/expired inodes first.
> 
> The policy is
> - enqueue all newly expired inodes at each queue_io() time
> - enqueue all dirty inodes if there are no more expired inodes to sync
> 
> This will help reduce the number of dirty pages encountered by page
> reclaim, eg. the pageout() calls. Normally older inodes contain older
> dirty pages, which are more close to the end of the LRU lists. So
> syncing older inodes first helps reducing the dirty pages reached by
> the page reclaim code.

Once again I think this is the wrong place to be changing writeback
policy decisions. for_background writeback only goes through
wb_writeback() and writeback_inodes_wb() (same as for_kupdate
writeback), so a decision to change from expired inodes to fresh
inodes, IMO, should be made in wb_writeback.

That is, for_background and for_kupdate writeback start with the
same policy (older_than_this set) to writeback expired inodes first,
then when background writeback runs out of expired inodes, it should
switch to all remaining inodes by clearing older_than_this instead
of refreshing it for the next loop.

This keeps all the policy decisions in the one place, all using the
same (existing) mechanism, and all relatively simple to understand,
and easy to tracepoint for debugging.  Changing writeback policy
deep in the writeback stack is not a good idea as it will make
extending writeback policies in future (e.g. for cgroup awareness)
very messy.

> @@ -585,7 +597,8 @@ void writeback_inodes_wb(struct bdi_writ
>  	if (!wbc->wb_start)
>  		wbc->wb_start = jiffies; /* livelock avoidance */
>  	spin_lock(&inode_wb_list_lock);
> -	if (!wbc->for_kupdate || list_empty(&wb->b_io))
> +
> +	if (list_empty(&wb->b_io))
>  		queue_io(wb, wbc);
>  
>  	while (!list_empty(&wb->b_io)) {
> @@ -612,7 +625,7 @@ static void __writeback_inodes_sb(struct
>  	WARN_ON(!rwsem_is_locked(&sb->s_umount));
>  
>  	spin_lock(&inode_wb_list_lock);
> -	if (!wbc->for_kupdate || list_empty(&wb->b_io))
> +	if (list_empty(&wb->b_io))
>  		queue_io(wb, wbc);
>  	writeback_sb_inodes(sb, wb, wbc, true);
>  	spin_unlock(&inode_wb_list_lock);

That changes the order in which we queue inodes for writeback.
Instead of calling every time to move b_more_io inodes onto the b_io
list and expiring more aged inodes, we only ever do it when the list
is empty. That is, it seems to me that this will tend to give
b_more_io inodes a smaller share of writeback because they are being
moved back to the b_io list less frequently where there are lots of
other inodes being dirtied. Have you tested the impact of this
change on mixed workload performance? Indeed, can you starve
writeback of a large file simply by creating lots of small files in
another thread?

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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