background on the ext3 batching performance issue

[Ric Wheeler <ric@emc.com>]

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.

regards,

ric