On 08/08/2011 03:49 PM, Frediano Ziglio wrote:
2011/8/8 Avi Kivity<[email protected]>:
> In certain circumstances, posix-aio-compat can incur a lot of latency:
> - threads are created by vcpu threads, so if vcpu affinity is set,
> aio threads inherit vcpu affinity. This can cause many aio threads
> to compete for one cpu.
> - we can create up to max_threads (64) aio threads in one go; since a
> pthread_create can take around 30μs, we have up to 2ms of cpu time
> under a global lock.
>
> Fix by:
> - moving thread creation to the main thread, so we inherit the main
> thread's affinity instead of the vcpu thread's affinity.
> - if a thread is currently being created, and we need to create yet
> another thread, let thread being born create the new thread, reducing
> the amount of time we spend under the main thread.
> - drop the local lock while creating a thread (we may still hold the
> global mutex, though)
>
> Note this doesn't eliminate latency completely; scheduler artifacts or
> lack of host cpu resources can still cause it. We may want pre-allocated
> threads when this cannot be tolerated.
>
> Thanks to Uli Obergfell of Red Hat for his excellent analysis and
suggestions.
>
> Signed-off-by: Avi Kivity<[email protected]>
Why not calling pthread_attr_setaffinity_np (where available) before
thread creation or shed_setaffinity at thread start instead of telling
another thread to create a thread for us just to get affinity cleared?
The entire qemu process may be affined to a subset of the host cpus; we
don't want to break that.
For example:
taskset 0xf0 qemu ....
(qemu) info cpus
<pin individual vcpu threads to host cpus>
--
error compiling committee.c: too many arguments to function
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