linux-kernel - Re: [PATCH 12/12] mm: per BDI congestion feedback

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Date:	Thu, 5 Apr 2007 16:24:25 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	root@...gramming.kicks-ass.net
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	miklos@...redi.hu, neilb@...e.de, dgc@....com,
	tomoki.sekiyama.qu@...achi.com, a.p.zijlstra@...llo.nl,
	nikita@...sterfs.com
Subject: Re: [PATCH 12/12] mm: per BDI congestion feedback

On Thu, 05 Apr 2007 19:42:21 +0200
root@...gramming.kicks-ass.net wrote:

> Now that we have per BDI dirty throttling is makes sense to also have oer BDI
> congestion feedback; why wait on another device if the current one is not
> congested.

Similar comments apply.  congestion_wait() should be called
throttle_at_a_rate_proportional_to_the_speed_of_presently_uncongested_queues().

If a process is throttled in the page allocator waiting for pages to become
reclaimable, that process absolutely does not care whether those pages were
previously dirty against /dev/sda or against /dev/sdb.  It wants to be woken
up for writeout completion against any queue.


-		wbc.encountered_congestion = 0;
+		wbc.encountered_congestion = NULL;
 		wbc.nr_to_write = MAX_WRITEBACK_PAGES;
 		wbc.pages_skipped = 0;
 		writeback_inodes(&wbc);
 		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)
+				congestion_wait(wbc.encountered_congestion,
+						WRITE, HZ/10);
+			else

Well that confused me.  You'd be needing to rename
wbc.encountered_congestion to congested_bdi or something.

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