Re: [patch] mm: Convert global dirty_exceeded flag to per-node node_dirty_exceeded

[Andrew Morton <akpm@osdl.org>]

Ravikiran G Thirumalai <kiran@scalex86.org> wrote:
>
> This is a repost.  I did not get any comments when I last posted this
> about a month back. So I guess this patch is all good :)
> 

I thought it was fairly ghastly, sorry ;)

> Convert global dirty_exceeded flag to per-node node_dirty_exceeded.
> 
> dirty_exceeded ping pongs between nodes in order to force all cpus in
> the system to increase the frequency of calls to balance_dirty_pages.
> 
> Currently dirty_exceeded is used by balance_dirty_pages_ratelimited to
> force all CPUs in the system call balance_dirty_pages often, in order to
> reduce the amount of dirty pages in the entire system (based on
> dirty_thresh and one CPU exceeding thee ratelimits).  As dirty_exceeded
> is a global variable, it will ping-pong between nodes of a NUMA system
> which is not good.

Did you not test this obvious little optimisation?

--- devel/mm/page-writeback.c~mm-dirty_exceeded-speedup	2006-01-16 18:11:36.000000000 -0800
+++ devel-akpm/mm/page-writeback.c	2006-01-16 18:11:56.000000000 -0800
@@ -212,7 +212,8 @@ static void balance_dirty_pages(struct a
 		if (nr_reclaimable + wbs.nr_writeback <= dirty_thresh)
 			break;
 
-		dirty_exceeded = 1;
+		if (!dirty_exceeded)
+			dirty_exceeded = 1;
 
 		/* Note: nr_reclaimable denotes nr_dirty + nr_unstable.
 		 * Unstable writes are a feature of certain networked
@@ -234,7 +235,7 @@ static void balance_dirty_pages(struct a
 		blk_congestion_wait(WRITE, HZ/10);
 	}
 
-	if (nr_reclaimable + wbs.nr_writeback <= dirty_thresh)
+	if (nr_reclaimable + wbs.nr_writeback <= dirty_thresh && dirty_exceeded)
 		dirty_exceeded = 0;
 
 	if (writeback_in_progress(bdi))
_

-
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