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
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