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Subjectfio mmap randread 64k more than 40% regression with 2.6.33-rc1
Comparing with kernel 2.6.32, fio mmap randread 64k has more than 40% regression with

The test scenario: 1 JBOD has 12 disks and every disk has 2 partitions. Create
8 1-GB files per partition and start 8 processes to do rand read on the 8 files
per partitions. There are 8*24 processes totally. randread block size is 64K.

We found the regression on 2 machines. One machine has 8GB memory and the other has

Bisect is very unstable. The related patches are many instead of just one.

1) commit 8e550632cccae34e265cb066691945515eaa7fb5
Author: Corrado Zoccolo <>
Date: Thu Nov 26 10:02:58 2009 +0100

cfq-iosched: fix corner cases in idling logic

This patch introduces about less than 20% regression. I just reverted below section
and this part regression disappear. It shows this regression is stable and not impacted
by other patches.

@@ -1253,9 +1254,9 @@ static void cfq_arm_slice_timer(struct cfq_data *cfqd)

- * still requests with the driver, don't idle
+ * still active requests from this queue, don't idle
- if (rq_in_driver(cfqd))
+ if (cfqq->dispatched)

2) How about other 20%~30% regressions? It's complicated. My bisect plus
Li Shaohua's investigation located 3 patches,

tiobench also has regression and Li Shaohua located the same patches. See link

Shaohua worked about patches to fix the tiobench regression. However, his patch
doesn't work for fio randread 64k regression.
I retried bisect manually and eventually located below patch,

commit 718eee0579b802aabe3bafacf09d0a9b0830f1dd
Author: Corrado Zoccolo <>
Date: Mon Oct 26 22:45:29 2009 +0100

cfq-iosched: fairness for sync no-idle queues

The patch is a little big. After many try, I found below section is the key.
@@ -2218,13 +2352,10 @@ cfq_update_idle_window(struct cfq_data *cfqd, struct cfq_queue *cfqq,
enable_idle = old_idle = cfq_cfqq_idle_window(cfqq);

if (!atomic_read(&cic->ioc->nr_tasks) || !cfqd->cfq_slice_idle ||
- (!cfqd->cfq_latency && cfqd->hw_tag && CFQQ_SEEKY(cfqq)))
+ (sample_valid(cfqq->seek_samples) && CFQQ_SEEKY(cfqq)))
enable_idle = 0;

That section deletes the condition checking of !cfqd->cfq_latency, so
enable_idle=0 with more possibility.

I wrote a testing patch which just overlooks the original 3 patches related
to tiobench regression, and a patch which adds back the checking of !cfqd->cfq_latency.
Then, all regression of fio randread 64k disappears.

Then, instead of working around the original 3 patches, I applied Shaohua's 2 patches
and added the checking of !cfqd->cfq_latency while also reverting the patch mentioned in 1).
But the result still has more than 20% regression. So Shaohua's patches couldn't improve
fio rand read 64k regression.

fio_mmap_randread_4k has about 10% improvement instead of regression. I checked
that my patch plus the debugging patch have no impact on this improvement.

randwrite 64k has about 25% regression. My method also restores its performance.

I worked out a patch to add the checking of !cfqd->cfq_latency back in
function cfq_update_idle_window.

In addition, as for item 1), could we just revert the section in cfq_arm_slice_timer?

As Shaohua's patches don't work for this regression, we might continue to find
better methods. I will check it next week.
With kernel 2.6.33-rc1, fio rand read 64k has more than 40% regression. Located
below patch.

commit 718eee0579b802aabe3bafacf09d0a9b0830f1dd
Author: Corrado Zoccolo <>
Date: Mon Oct 26 22:45:29 2009 +0100

cfq-iosched: fairness for sync no-idle queues

It introduces for more than 20% regression. The reason is function cfq_update_idle_window
forgets to check cfqd->cfq_latency, so enable_idle=0 with more possibility.

Below patch against 2.6.33-rc1 adds the checking back.

Signed-off-by: Zhang Yanmin <>

diff -Nraup linux-2.6.33_rc1/block/cfq-iosched.c linux-2.6.33_rc1_rand64k/block/cfq-iosched.c
--- linux-2.6.33_rc1/block/cfq-iosched.c 2009-12-23 14:12:03.000000000 +0800
+++ linux-2.6.33_rc1_rand64k/block/cfq-iosched.c 2009-12-31 16:26:32.000000000 +0800
@@ -3064,8 +3064,8 @@ cfq_update_idle_window(struct cfq_data *

if (!atomic_read(&cic->ioc->nr_tasks) || !cfqd->cfq_slice_idle ||
- (!cfq_cfqq_deep(cfqq) && sample_valid(cfqq->seek_samples)
- && CFQQ_SEEKY(cfqq)))
+ (!cfqd->cfq_latency && !cfq_cfqq_deep(cfqq) &&
+ sample_valid(cfqq->seek_samples) && CFQQ_SEEKY(cfqq)))
enable_idle = 0;
else if (sample_valid(cic->ttime_samples)) {
if (cic->ttime_mean > cfqd->cfq_slice_idle)

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