Josef Bacik wrote:
On Thursday 28 February 2008 10:05:11 am Josef Bacik wrote:
On Thursday 28 February 2008 7:09:17 am Ric Wheeler wrote:
At the LSF workshop, I mentioned that we have tripped across an
embarrassing performance issue in the jbd transaction code which is
clearly not tuned for low latency devices.
The short summary is that we can do say 800 10k files/sec in a
write/fsync/close loop with a single thread, but drop down to under 250
files/sec with 2 or more threads.
This is pretty easy to reproduce with any small file write synchronous
workload (i.e., fsync() each file before close). We used my fs_mark
tool to reproduce.
The core of the issue is the call in the jbd transaction code call out
to schedule_timeout_uninterruptible(1) which causes us to sleep for 4ms:
pid = current->pid;
if (handle->h_sync && journal->j_last_sync_writer != pid) {
journal->j_last_sync_writer = pid;
do {
old_handle_count = transaction->t_handle_count;
schedule_timeout_uninterruptible(1);
} while (old_handle_count !=
transaction->t_handle_count); }
This is quite topical to the concern we had with low latency devices in
general, but specifically things like SSD's.
Your testcase does in fact show a weakness in this optimization, but look
at the more likely case, where you have multiple writers on the same
filesystem rather than one guy doing write/fsync. If we wait we could
potentially add quite a few more buffers to this transaction before
flushing it, rather than flushing a buffer or two at a time. What would
you propose as a solution?
Forgive me, I said that badly, now that I've had my morning coffee let me try
again. You are ping-ponging the j_last_sync_writer back and forth between the
two threads, so you don't get the speedup you would get with one thread where
we would just bypass the next sleep since we know we've got one thread doing
write/sync. So this brings up the question, should we try and figure out if we
have the situation where we have multiple threads doing write/sync and
therefore exploiting the weakness in this optimization, and if we should, how
would we do this properly? The only thing I can think to do is to track sync
writers on a transaction, and if its more than one bypass this little snippet.
In fact I think I'll go ahead and do that and see what fs_mark comes up with.
Thank you,
Josef
One more thought - what we really want here is to have a sense of the
latency of the device. In the S-ATA disk case, this optimization works
well for batching since we "spend" an extra 4ms worst case in the chance
of combining multiple, slow 18ms operations.
With the clariion box we tested, the optimization fails badly since the
cost is only 1.3 ms so we optimize by waiting 3-4 times longer than it
would take to do the operation immediately.
This problem has also seemed to me to be the same problem that IO
schedulers do with plugging - we want to dynamically figure out when to
plug and unplug here without hard coding in device specific tunings.
If we bypass the snippet for multi-threaded writers, we would probably
slow down this workload on normal S-ATA/ATA drives (or even higher
performance non-RAID disks).
ric
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