Re: iozone write 50% regression in kernel 2.6.24-rc1

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Fri, 2007-11-09 at 17:47 +0800, Zhang, Yanmin wrote:
> Comparing with 2.6.23, iozone sequential write/rewrite (512M) has 50% regression
> in kernel 2.6.24-rc1. 2.6.24-rc2 has the same regression.
> 
> My machine has 8 processor cores and 8GB memory.
> 
> By bisect, I located patch
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=04fbfdc14e5f48463820d6b9807daa5e9c92c51f.
> 
> 
> Another behavior: with kernel 2.6.23, if I run iozone for many times after rebooting machine,
> the result looks stable. But with 2.6.24-rc1, the first run of iozone got a very small result and
> following run has 4Xorig_result.

So the second run is 4x as fast as the first run?

> What I reported is the regression of 2nd/3rd run, because first run has bigger regression.

So the 2nd and 3rd run are stable at 50% slower than .23?

> I also tried to change /proc/sys/vm/dirty_ratio,dirty_backgroud_ratio and didn't get improvement.

Could you try:

---
Subject: mm: speed up writeback ramp-up on clean systems

We allow violation of bdi limits if there is a lot of room on the
system. Once we hit half the total limit we start enforcing bdi limits
and bdi ramp-up should happen. Doing it this way avoids many small
writeouts on an otherwise idle system and should also speed up the
ramp-up.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Reviewed-by: Fengguang Wu <wfg@mail.ustc.edu.cn> 
---
 mm/page-writeback.c |   19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

Index: linux-2.6/mm/page-writeback.c
===================================================================
--- linux-2.6.orig/mm/page-writeback.c	2007-09-28 10:08:33.937415368 +0200
+++ linux-2.6/mm/page-writeback.c	2007-09-28 10:54:26.018247516 +0200
@@ -355,8 +355,8 @@ get_dirty_limits(long *pbackground, long
  */
 static void balance_dirty_pages(struct address_space *mapping)
 {
-	long bdi_nr_reclaimable;
-	long bdi_nr_writeback;
+	long nr_reclaimable, bdi_nr_reclaimable;
+	long nr_writeback, bdi_nr_writeback;
 	long background_thresh;
 	long dirty_thresh;
 	long bdi_thresh;
@@ -376,11 +376,26 @@ static void balance_dirty_pages(struct a
 
 		get_dirty_limits(&background_thresh, &dirty_thresh,
 				&bdi_thresh, bdi);
+
+		nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
+					global_page_state(NR_UNSTABLE_NFS);
+		nr_writeback = global_page_state(NR_WRITEBACK);
+
 		bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
 		bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
+
 		if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh)
 			break;
 
+		/*
+		 * Throttle it only when the background writeback cannot
+		 * catch-up. This avoids (excessively) small writeouts
+		 * when the bdi limits are ramping up.
+		 */
+		if (nr_reclaimable + nr_writeback <
+				(background_thresh + dirty_thresh) / 2)
+			break;
+
 		if (!bdi->dirty_exceeded)
 			bdi->dirty_exceeded = 1;

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