Re: [PATCH 5/5] writeback: introduce writeback_control.more_io to indicate more io

[David Chinner <dgc@sgi.com>]

On Tue, Oct 02, 2007 at 04:41:48PM +0800, Fengguang Wu wrote:
>  		wbc.pages_skipped = 0;
> @@ -560,8 +561,9 @@ static void background_writeout(unsigned
>  		min_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
>  		if (wbc.nr_to_write > 0 || wbc.pages_skipped > 0) {
>  			/* Wrote less than expected */
> -			congestion_wait(WRITE, HZ/10);
> -			if (!wbc.encountered_congestion)
> +			if (wbc.encountered_congestion || wbc.more_io)
> +				congestion_wait(WRITE, HZ/10);
> +			else
>  				break;
>  		}

Why do you call congestion_wait() if there is more I/O to issue?  If
we have a fast filesystem, this might cause the device queues to
fill, then drain on congestion_wait(), then fill again, etc. i.e. we
will have trouble keeping the queues full, right?

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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