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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 <avi@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Read the documentation in qemu-doc.html. Fabrice Bellard.