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Re: [Qemu-devel] [PATCH v2] posix-aio-compat: fix latency issues
From: |
Jan Kiszka |
Subject: |
Re: [Qemu-devel] [PATCH v2] posix-aio-compat: fix latency issues |
Date: |
Mon, 22 Aug 2011 19:29:03 +0200 |
User-agent: |
Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv:1.8.1.12) Gecko/20080226 SUSE/2.0.0.12-1.1 Thunderbird/2.0.0.12 Mnenhy/0.7.5.666 |
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?
Jan
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- [Qemu-devel] [PATCH v2] posix-aio-compat: fix latency issues, Avi Kivity, 2011/08/14
- Re: [Qemu-devel] [PATCH v2] posix-aio-compat: fix latency issues, Kevin Wolf, 2011/08/22
- Re: [Qemu-devel] [PATCH v2] posix-aio-compat: fix latency issues,
Jan Kiszka <=
- Re: [Qemu-devel] [PATCH v2] posix-aio-compat: fix latency issues, Stefan Hajnoczi, 2011/08/23
- Re: [Qemu-devel] [PATCH v2] posix-aio-compat: fix latency issues, Anthony Liguori, 2011/08/23
- Re: [Qemu-devel] [PATCH v2] posix-aio-compat: fix latency issues, Jan Kiszka, 2011/08/23
- Re: [Qemu-devel] [PATCH v2] posix-aio-compat: fix latency issues, Anthony Liguori, 2011/08/23
- Re: [Qemu-devel] [PATCH v2] posix-aio-compat: fix latency issues, Jan Kiszka, 2011/08/23
- Re: [Qemu-devel] [PATCH v2] posix-aio-compat: fix latency issues, Avi Kivity, 2011/08/28