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SubjectRe: Do not overload dispatch queue (Was: Re: IO scheduler based IO controller V10)
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On Sat, 2009-10-03 at 21:49 +0200, Mike Galbraith wrote:

> It's a huge winner for sure, and there's no way to quantify. I'm just
> afraid the other shoe will drop from what I see/hear. I should have
> kept my trap shut and waited really, but the impression was strong.

Seems there was one "other shoe" at least. For concurrent read vs
write, we're losing ~10% throughput that we weren't losing prior to that
last commit. I got it back, and the concurrent git throughput back as
well with the tweak below, _seemingly_ without significant sacrifice.

cfq-iosched: adjust async delay.

8e29675: "implement slower async initiate and queue ramp up" introduced a
throughput regression for concurrent reader vs writer. Adjusting async delay
to use cfq_slice_async, unless someone adjusts async to have more bandwidth
allocation than sync, restored throughput.

Signed-off-by: Mike Galbraith <efault@gmx.de>

---
block/cfq-iosched.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)
Index: linux-2.6/block/cfq-iosched.c
===================================================================
--- linux-2.6.orig/block/cfq-iosched.c
+++ linux-2.6/block/cfq-iosched.c
@@ -1343,17 +1343,19 @@ static int cfq_dispatch_requests(struct
*/
if (!cfq_cfqq_sync(cfqq) && cfqd->cfq_desktop) {
unsigned long last_sync = jiffies - cfqd->last_end_sync_rq;
+ unsigned long slice = max(cfq_slice_sync, cfq_slice_async);
unsigned int depth;

+ slice = min(slice, cfq_slice_async);
/*
* must wait a bit longer
*/
- if (last_sync < cfq_slice_sync) {
- cfq_schedule_dispatch(cfqd, cfq_slice_sync - last_sync);
+ if (last_sync < slice) {
+ cfq_schedule_dispatch(cfqd, slice - last_sync);
return 0;
}

- depth = last_sync / cfq_slice_sync;
+ depth = last_sync / slice;
if (depth < max_dispatch)
max_dispatch = depth;
}
--numbers--

dd vs konsole -e exit
1.70 1.94 1.32 1.89 1.87 1.7 fairness=1 overload_delay=1
1.55 1.79 1.38 1.53 1.57 1.5 desktop=1 +last_end_sync
1.09 0.87 1.11 0.96 1.11 1.02 block-for-linus
1.10 1.13 0.98 1.11 1.13 1.09 block-for-linus + tweak

concurrent git test
Avg
108.12 106.33 106.34 97.00 106.52 104.8 1.000 virgin
89.81 88.82 91.56 96.57 89.38 91.2 .870 desktop=1 +last_end_sync
92.61 94.60 92.35 93.17 94.05 93.3 .890 blk-for-linus
89.33 88.82 89.99 88.54 89.09 89.1 .850 blk-for-linus + tweak

read vs write test

desktop=0 Avg
elapsed 98.23 91.97 91.77 93.9 sec 1.000
30s-dd-read 48.5 49.6 49.1 49.0 mb/s 1.000
30s-dd-write 23.1 27.3 31.3 27.2 1.000
dd-read-total 49.4 50.1 49.6 49.7 1.000
dd-write-total 34.5 34.9 34.9 34.7 1.000

desktop=1 pop 8e296755 Avg
elapsed 93.30 92.77 90.11 92.0 .979
30s-dd-read 50.5 50.4 51.8 50.9 1.038
30s-dd-write 22.7 26.4 27.7 25.6 .941
dd-read-total 51.2 50.1 51.6 50.9 1.024
dd-write-total 34.2 34.5 35.6 34.7 1.000

desktop=1 push 8e296755 Avg
elapsed 104.51 104.52 101.20 103.4 1.101
30s-dd-read 43.0 43.6 44.5 43.7 .891
30s-dd-write 21.4 23.9 28.9 24.7 .908
dd-read-total 42.9 43.0 43.5 43.1 .867
dd-write-total 30.4 30.3 31.5 30.7 .884

desktop=1 push 8e296755 + tweak Avg
elapsed 92.10 94.34 93.68 93.3 .993
30s-dd-read 49.7 49.3 48.8 49.2 1.004
30s-dd-write 23.7 27.1 23.1 24.6 .904
dd-read-total 50.2 50.1 48.7 49.6 .997
dd-write-total 34.7 33.9 34.0 34.2 .985

#!/bin/sh

# dd if=/dev/zero of=deleteme bs=1M count=3000

echo 2 > /proc/sys/vm/drop_caches
dd if=/dev/zero of=deleteme2 bs=1M count=3000 &
dd if=deleteme of=/dev/null bs=1M count=3000 &
sleep 30
killall -q -USR1 dd &
wait
rm -f deleteme2
sync





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