Re: background on the ext3 batching performance issue

[Ric Wheeler <ric@emc.com>]

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