Comment 114 for bug 620074

One observation is that ext4 seems quite latency prone in waiting for write access to the journal. IIRC, that matches earlier results where ext3 was much quicker in that area. No idea what causes this, as I'm not familiar with the ext4 internals.

Another observation is that I neglected to include the buffer waiting in the async promotion, it only worked for page locking. I'll add an updated patch below after this posting.

And finally, lots of time is spent waiting for a new write request in the block layer. So you are maxing all 128 requests out in this test case. You can try and increase that to 512 for testing purposes, you can do that ala:

# echo 512 > /sys/block/sda/queue/nr_requests

That will get your async wait numbers down, but it may not reduce your latencies. Fact is that 128 writes is already a lot, and with more requests in the queue, you will have higher completion times for each individual request.