Andreas Dilger wrote:
On Aug 18, 2008 21:31 -0700, Andrew Morton wrote:
On Wed, 6 Aug 2008 15:15:36 -0400 Josef Bacik <jbacik@xxxxxxxxxx> wrote:
Using the following fs_mark command to test the speeds
./fs_mark -d /mnt/ext3-test -s 4096 -n 2000 -D 64 -t 2
I got the following results (with write cacheing turned off)
type threads with patch without patch
sata 2 26.4 27.8
sata 4 44.6 44.4
sata 8 70.4 72.8
sata 16 75.2 89.6
sata 32 92.7 96.0
ram 1 2399.1 2398.8
ram 2 257.3 3603.0
ram 4 395.6 4827.9
ram 8 659.0 4721.1
ram 16 1326.4 4373.3
ram 32 1964.2 3816.3
I used a ramdisk to emulate a "fast" disk since I don't happen to have a
clariion sitting around. I didn't test single thread in the sata case as it
should be relatively the same between the two. Thanks,
This is all a bit mysterious. That delay doesn't have much at all to
do with commit times. The code is looping around giving other
userspace processes an opportunity to get scheduled and to run an fsync
and to join the current transaction rather than having to start a new
one.
(that code was quite effective when I first added it, but in more
recent testing, which was some time ago, it doesn't appear to provide
any improvement. This needs to be understood)
I don't think it is mysterious at all. With a HZ=100 system 1 jiffie
is 10ms, which was comparable to the seek time of a disk, so sleeping
for 1 jiffie to avoid doing 2 transactions was a win. With a flash
device (simulated by RAM here) seek time is 1ms so waiting 10ms
isn't going to be useful if there are only 2 threads and both have
already joined the transaction.
The code was originally tuned to S-ATA & ATA disk response times which
are closer to 12-15ms. Sleeping for 10ms (100HZ kernel) or 4ms (250HZ)
did not overly penalize the low thread count case and worked well for
higher thread counts (and ext3 special cases the single threaded writer
so no sleep happens).
This is still a really, really good thing to do, but we need to sleep
less when the device characteristics are radically different. For
example, a fibre channel attached disk array drops that 12-15 ms down to
1.5 ms (not to mention RAM disks!).
Also, I'd expect that the average commit time is much longer that one
jiffy on most disks, and perhaps even on fast disks and maybe even on
ramdisk. So perhaps what's happened here is that you've increased the
sleep period and more tasks are joining particular transactions.
Or you've shortened the sleep time (which wasn't really doing anything
useful) and this causes tasks to spend less time asleep.
I think both are true. By making the sleep time dynamic it removes
the "useless" sleep time, but can also increase the sleep time if
there are many threads and the commit cost is better amortized over
more operations.
Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.
It would be great to be able to use this batching technique for faster
devices, but we currently sleep 3-4 times longer waiting to batch for an
array than it takes to complete the transaction.
Thanks!
Ric
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