Hi,
I've been doing some tests with the global thread pool [1], and I've observed one important thing:
Since this new thread pool has very low contention (apparently), it exposes other problems when the number of threads grows. What I've seen is that some workloads use all available threads on bricks to do I/O, causing avgload to grow rapidly and saturating the machine (or it seems so), which really makes everything slower. Reducing the maximum number of threads improves performance actually. Other workloads, though, do little I/O (probably most is locking or smallfile operations). In this case limiting the number of threads to a small value causes a performance reduction. To increase performance we need more threads.
So this is making me thing that maybe we should implement some sort of I/O queue with a maximum I/O depth for each brick (or disk if bricks share same disk). This way we can limit the amount of requests physically accessing the underlying FS concurrently, without actually limiting the number of threads that can be doing other things on each brick. I think this could improve performance.
Maybe this approach could also be useful in client side, but I think it's not so critical there.
What do you think ?
Xavi
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