On Mon, Aug 22, 2011 at 6:29 PM, Jan Kiszka <jan.kiszka@xxxxxxxxxxx> wrote: > On 2011-08-14 06:04, Avi Kivity wrote: >> 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. > > At this chance: What is the state of getting rid of the remaining delta > between upstream's version and qemu-kvm? That would be nice. qemu-kvm.git uses a signalfd to handle I/O completion whereas qemu.git uses a signal, writes to a pipe from the signal handler, and uses qemu_notify_event() to break the vcpu. Once the force iothread patch is merged we should be able to move to qemu-kvm.git's signalfd approach. Stefan -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
