On 02/26/2010 02:47 AM, Avi Kivity wrote:
qcow2 is still not fully asynchronous. All the other format drivers
(except raw) are fully synchronous. If we had a threaded
infrastructure, we could convert them all in a day. As it is, you can
only use the other block format drivers in 'qemu-img convert'.
I've got a healthy amount of scepticism that it's that easy. But I'm
happy to consider patches :-)
Each such thread could run the same loop as the iothread. Any
pollable fd or timer would be associated with a thread, so things
continue as normal more or less. Unassociated objects continue with
the main iothread.
Is the point latency or increasing available CPU resources?
Yes.
If the device models are re-entrant, that reduces a ton of the demand
on the qemu_mutex which means that IO thread can run uncontended.
While we have evidence that the VCPU threads and IO threads are
competing with each other today, I don't think we have any evidence
to suggest that the IO thread is self-starving itself with long
running events.
I agree we have no evidence and that this is all speculation. But
consider a 64-vcpu guest, it has a 1:64 ratio of vcpu time
(initiations) to iothread time (completions). If each vcpu generates
5000 initiations per second, the iothread needs to handle 320,000
completions per second. At that rate you will see some internal
competition. That thread will also have a hard time shuffling data
since every completion's data will reside in the wrong cpu cache.
Ultimately, it depends on what you're optimizing for. If you've got a
64-vcpu guest on a 128-way box, then sure, we want to have 64 IO threads
because that will absolutely increase throughput.
But realistically, it's more likely that if you've got a 64-vcpu guest,
you're on a 1024-way box and you've got 64 guests running at once.
Having 64 IO threads per VM means you've got 4k threads floating. It's
still just as likely that one completion will get delayed by something
less important. Now with all of these threads on a box like this, you
get nasty NUMA interactions too.
The difference between the two models is that with threads, we rely on
pre-emption to enforce fairness and the Linux scheduler to perform
scheduling. With a single IO thread, we're determining execution order
and priority.
A lot of main loops have a notion of priority for timer and idle
callbacks. For something that is latency sensitive, you absolutely
could introduce the concept of priority for bottom halves. It would
ensure that a +1 priority bottom half would get scheduled before
handling any lower priority I/O/BHs.
Note, an alternative to multiple iothreads is to move completion
handling back to vcpus, provided we can steer the handler close to the
guest completion handler.
Looking at something like linux-aio, I think we might actually want to
do that. We can submit the request from the VCPU thread and we can
certainly program the signal to get delivered to that VCPU thread.
Maintaining affinity for the request is likely a benefit.
For host services though, it's much more difficult to isolate them
like this.
What do you mean by host services?
Things like VNC and live migration. Things that aren't directly related
to a guest's activity. One model I can imagine is to continue to
relegate these things to a single IO thread, but then move device driven
callbacks either back to the originating thread or to a dedicated device
callback thread. Host services generally have a much lower priority.
I'm not necessarily claiming that this will never be the right thing
to do, but I don't think we really have the evidence today to suggest
that we should focus on this in the short term.
Agreed. We will start to see evidence (one way or the other) as fully
loaded 64-vcpu guests are benchmarked. Another driver may be
real-time guests; if a timer can be deferred by some block device
initiation or completion, then we can say goodbye to any realtime
guarantees we want to make.
I'm wary of making decisions based on performance of a 64-vcpu guest.
It's an important workload to characterize because it's an extreme case
but I think 64 1-vcpu guests will continue to be significantly more
important than 1 64-vcpu guest.
Regards,
Anthony Liguori
