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Re: [Qemu-devel] [PATCH] main-loop: drop the BQL if the I/O appears to b
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From: |
Paolo Bonzini |
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Subject: |
Re: [Qemu-devel] [PATCH] main-loop: drop the BQL if the I/O appears to be spinning |
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Date: |
Fri, 05 Apr 2013 17:05:10 +0200 |
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User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130311 Thunderbird/17.0.4 |
Il 05/04/2013 15:46, Anthony Liguori ha scritto:
> The char-flow refactoring introduced a busy-wait that depended on
> an action from the VCPU thread. However, the VCPU thread could
> never take that action because the busy-wait starved the VCPU thread
> of the BQL because it never dropped the mutex while running select.
>
> Paolo doesn't want to drop this optimization for fear that we will
> stop detecting these busy waits. I'm afraid to keep this optimization
> even with the busy-wait fixed because I think a similar problem can
> occur just with heavy I/O thread load manifesting itself as VCPU pauses.
>
> As a compromise, introduce an artificial timeout after a thousand
> iterations but print a rate limited warning when this happens. This
> let's us still detect when this condition occurs without it being
> a fatal error.
Good idea, thanks!
Paolo