Re: [PATCH 3/5] writeback: dirty rate control

Wu Fengguang
2011-08-10T14:11:01Z

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On Wed, Aug 10, 2011 at 12:56:56AM +0800, Peter Zijlstra wrote:
> On Sat, 2011-08-06 at 16:44 +0800, Wu Fengguang wrote:
> > bdi->dirty_ratelimit = (bw * 3 + ref_bw) / 4;
>
> I can't actually find this low-pass filter in the code.. could be I'm
> blind from staring at it too long though..

Sorry, it's implemented in another patch (attached). I've also removed
it from _this_ changelog.

Here you can find all the other patches in addition to the core bits.

http://git.kernel.org/?p=linux/kernel/git/wfg/writeback.git;a=shortlog;h=refs/heads/dirty-throttling-v8%2B

Thanks,
Fengguang

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Subject: writeback: make dirty_ratelimit stable/smooth
Date: Thu Aug 04 22:05:05 CST 2011

Half the dirty_ratelimit update step size to avoid overshooting, and
further slow down the updates when the tracking error is smaller than
(base_rate / 8).

It's desirable to have a _constant_ dirty_ratelimit given a stable
workload. Because each jolt of dirty_ratelimit will directly show up
in all the bdi tasks' dirty rate.

The cost will be slightly increased dirty position error, which is
pretty acceptable.

Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
mm/page-writeback.c | 24 +++++++++++++++++++++---
1 file changed, 21 insertions(+), 3 deletions(-)

--- linux-next.orig/mm/page-writeback.c 2011-08-10 21:35:11.000000000 +0800
+++ linux-next/mm/page-writeback.c 2011-08-10 21:35:31.000000000 +0800
@@ -741,6 +741,7 @@ static void bdi_update_dirty_ratelimit(s
unsigned long dirty_rate;
unsigned long pos_rate;
unsigned long balanced_rate;
+ unsigned long delta;
unsigned long long pos_ratio;

/*
@@ -755,7 +756,6 @@ static void bdi_update_dirty_ratelimit(s
* pos_rate reflects each dd's dirty rate enforced for the past 200ms.
*/
pos_rate = base_rate * pos_ratio >> BANDWIDTH_CALC_SHIFT;
- pos_rate++; /* this avoids bdi->dirty_ratelimit get stuck in 0 */

/*
* balanced_rate = pos_rate * write_bw / dirty_rate
@@ -777,14 +777,32 @@ static void bdi_update_dirty_ratelimit(s
* makes it more stable, but also is essential for preventing it being
* driven away by possible systematic errors in balanced_rate.
*/
+ delta = 0;
if (base_rate > pos_rate) {
if (base_rate > balanced_rate)
- base_rate = max(balanced_rate, pos_rate);
+ delta = base_rate - max(balanced_rate, pos_rate);
} else {
if (base_rate < balanced_rate)
- base_rate = min(balanced_rate, pos_rate);
+ delta = min(balanced_rate, pos_rate) - base_rate;
}

+ /*
+ * Don't pursue 100% rate matching. It's impossible since the balanced
+ * rate itself is constantly fluctuating. So decrease the track speed
+ * when it gets close to the target. Eliminates unnecessary jolting.
+ */
+ delta >>= base_rate / (8 * delta + 1);
+ /*
+ * Limit the step size to avoid overshooting. It also implicitly
+ * prevents dirty_ratelimit from dropping to 0.
+ */
+ delta >>= 2;
+
+ if (base_rate < pos_rate)
+ base_rate += delta;
+ else
+ base_rate -= delta;
+
bdi->dirty_ratelimit = base_rate;

trace_dirty_ratelimit(bdi, dirty_rate, pos_rate, balanced_rate);