Comment 141 for bug 620074

I think the appearance of this bug is conditioned on cpu speed and drive speed.

I have make some more tests. Currently I am using the following command.

for i in 1 2 ; do \
 dd if=/dev/zero of=test-$i bs=1M count=4K oflag=direct & echo test-$i; \
done

Once with oflag=direct and once without.

With ext3 the problem occurs immediately in both cases. With ext4 the problem occurs without oflag=direct occurs immediately. With with oflag=direct I can use even firefox, but sometime the desktop in unusable.

In direct mode new application does not start and disk intensive operations take a long time, but I can move the windows and change the desktops without problems and io-wait at 60%. With dd in non direct mode, I can start new application (it takes still a lot of time), but everything is freezing from time to time and io-wait is immediately at 100%.

I have captures some statistic by adding a printk with the duration time in the function __make_request (blk-core.c). The time is taken directly before and after the spin_lock_irq(q->queue_lock); and finally before the unlock.

There is a dramatic difference between the request per seconds in direct and non-direct mode.
W: wait time before entering lock state
D: duration time of the make_request
T: total time = W + D

ext3 - direct
requests: 209.694080/s
total: W: 0.000645 / D: 0.014584 / T: 0.015229
W: avg: 0.000000307 / min: 0.000000000 / max: 0.000007606
D: avg: 0.000006948 / min: 0.000000255 / max: 0.000085018
T: avg: 0.000007255 / min: 0.000000365 / max: 0.000085018
4294967296 Bytes (4,3 GB) kopiert, 203,66 s, 21,1 MB/s
4294967296 Bytes (4,3 GB) kopiert, 203,582 s, 21,1 MB/s

ext3
requests: 4662.272968/s
total: W: 0.013624 / D: 15.256149 / T: 15.269773
W: avg: 0.000000291 / min: 0.000000000 / max: 0.000275893
D: avg: 0.000325819 / min: 0.000000000 / max: 1.092940760
T: avg: 0.000326110 / min: 0.000000000 / max: 1.092940920
4294967296 Bytes (4,3 GB) kopiert, 203,559 s, 21,1 MB/s
4294967296 Bytes (4,3 GB) kopiert, 214,995 s, 20,0 MB/s

ext4 - direct
requests: 114.510132/s
total: W: 0.000356 / D: 0.017658 / T: 0.018014
W: avg: 0.000000311 / min: 0.000000110 / max: 0.000000630
D: avg: 0.000015408 / min: 0.000000220 / max: 0.000127249
T: avg: 0.000015719 / min: 0.000000330 / max: 0.000127689
4294967296 Bytes (4,3 GB) kopiert, 154,491 s, 27,8 MB/s
4294967296 Bytes (4,3 GB) kopiert, 157,853 s, 27,2 MB/s

ext4
requests: 7009.744726/s
total: W: 0.018928 / D: 6.110891 / T: 6.129819
W: avg: 0.000000270 / min: 0.000000000 / max: 0.000032916
D: avg: 0.000087046 / min: 0.000000000 / max: 0.603327176
T: avg: 0.000087316 / min: 0.000000000 / max: 0.603327516
4294967296 Bytes (4,3 GB) kopiert, 146,303 s, 29,4 MB/s
4294967296 Bytes (4,3 GB) kopiert, 149,361 s, 28,8 MB/s