[PATCH] ext4: reduce scheduling latency with delayed allocation

From: Michal Schmidt
Date: Mon Mar 01 2010 - 07:34:50 EST

mpage_da_submit_io() may process tens of thousands of pages at a time.
Unless full preemption is enabled, it causes scheduling latencies in the order
of tens of milliseconds.

It can be reproduced simply by writing a big file on ext4 repeatedly with
dd if=/dev/zero of=/tmp/dummy bs=10M count=50

The patch fixes it by allowing to reschedule in the loop.

cyclictest can be used to measure the latency. I tested with:
$ cyclictest -t1 -p 80 -n -i 5000 -m -l 20000

The results from an UP AMD Turion 2GHz with voluntary preemption:

Without the patch:
T: 0 ( 2535) P:80 I:5000 C: 20000 Min: 12 Act: 23 Avg: 3166 Max: 70524
(i.e. Average latency was more than 3 ms. Max observed latency was 71 ms.)

With the patch:
T: 0 ( 2588) P:80 I:5000 C: 20000 Min: 13 Act: 33 Avg: 49 Max: 11009
(i.e. Average latency was only 49 us. Max observed latency was 11 ms.)

Signed-off-by: Michal Schmidt <mschmidt@xxxxxxxxxx>
fs/ext4/inode.c | 1 +
1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index e119524..687a993 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2007,6 +2007,7 @@ static int mpage_da_submit_io(struct mpage_da_data *mpd)
ret = err;
+ cond_resched();
return ret;

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