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
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
