On 7/17/2012 12:26 AM, Dave Chinner wrote:
> I bet it's single threaded, which means it is:
The data given seems to strongly suggest a single thread.
> Which means throughput is limited by IO latency, not bandwidth.
> If it takes 10us to do the write(2), issue and process the IO
> completion, and it takes 10us for the hardware to do the IO, you're
> limited to 50,000 IOPS, or 200MB/s. Given that the best being seen
> is around 35MB/s, you're looking at around 10,000 IOPS of 100us
> round trip time. At 5MB/s, it's 1200 IOPS or around 800us round
> That's why you get different performance from the different raid
> controllers - some process cache hits a lot faster than others.
> IOWs, welcome to Understanding RAID Controller Caching Behaviours
> 101 :)
It would be somewhat interesting to see Stefan's latency and throughput
numbers for 4/8/16 threads. Maybe the sysbench "--num-threads=" option
is the ticket. The docs state this is for testing scheduler
performance, and it's not clear whether this actually does threaded IO.
If not, time for a new IO benchmark.