Re: [PATCH 2/6] writeback: reduce calls to global_page_state in balance_dirty_pages()

[Wu Fengguang <fengguang.wu@intel.com>]

> > This patch slightly changes behavior by replacing clip_bdi_dirty_limit()
> > with the explicit check (nr_reclaimable + nr_writeback >= dirty_thresh)
> > to avoid exceeding the dirty limit. Since the bdi dirty limit is mostly
> > accurate we don't need to do routinely clip. A simple dirty limit check
> > would be enough.
> > 
> > The check is necessary because, in principle we should throttle
> > everything calling balance_dirty_pages() when we're over the total
> > limit, as said by Peter.
> > 
> > We now set and clear dirty_exceeded not only based on bdi dirty limits,
> > but also on the global dirty limits. This is a bit counterintuitive, but
> > the global limits are the ultimate goal and shall be always imposed.
>   Thinking about this again - what you did is rather big change for systems
> with more active BDIs. For example if I have two disks sda and sdb and
> write for some time to sda, then dirty limit for sdb gets scaled down.
> So when we start writing to sbd we'll heavily throttle the threads until
> the dirty limit for sdb ramps up regardless of how far are we to reach the
> global limit...

The global threshold check is added in place of clip_bdi_dirty_limit()
for safety and not intended as a behavior change. If ever leading to
big behavior change and regression, that it would be indicating some
too permissive per-bdi threshold calculation.

Did you see the global dirty threshold get exceeded when writing to 2+
devices? Occasional small exceeding should be OK though. I tried the
following debug patch and see no warnings when doing two concurrent cp
over local disk and NFS.

Index: linux-next/mm/page-writeback.c
===================================================================
--- linux-next.orig/mm/page-writeback.c	2010-07-27 11:26:18.063817669 +0800
+++ linux-next/mm/page-writeback.c	2010-07-27 11:26:53.335855847 +0800
@@ -513,6 +513,11 @@
 		if (!dirty_exceeded)
 			break;
 
+		if (nr_reclaimable + nr_writeback >= dirty_thresh)
+			printk ("XXX: dirty exceeded: %lu + %lu = %lu ++ %lu\n",
+				nr_reclaimable, nr_writeback, dirty_thresh,
+				nr_reclaimable + nr_writeback - dirty_thresh);
+
 		/*
 		 * Throttle it only when the background writeback cannot
 		 * catch-up. This avoids (excessively) small writeouts

Thanks,
Fengguang