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; } pagevec_release(&pvec); + cond_resched(); } return ret; } -- 1.7.0 -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
