Re: [Qemu-devel] [PATCH] main-loop: drop the BQL if the I/O appears to be spinning

[Paolo Bonzini <pbonzini@redhat.com>]

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