Re: [KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based notifierinterface

From: Davide Libenzi
Date: Wed Jun 17 2009 - 15:56:31 EST

On Wed, 17 Jun 2009, Gregory Haskins wrote:

> Davide Libenzi wrote:
> > How much the (possible, but not certain) kernel thread context switch time
> > weighs in the overall KVM IRQ service time?
> >
> Generally each one is costing me about 7us on average. For something
> like high-speed networking, we have a path that has about 30us of
> base-line overhead. So one additional ctx-switch puts me at base+7 ( =
> ~37us), two puts me in base+2*7 (= ~44us). So in that context (no pun
> intended ;), it hurts quite a bit. I'll be the first to admit that not
> everyone (most?) will care about latency, though. But FWIW, I do.

And how a frame reception is handled in Linux nowadays?

> True, but thats the notifiee's burden, not eventfd's. And its always
> going to be opt-in. Even today, someone is free to either try to sleep
> (which will oops on the might_sleep()), ...

No, today you just can't sleep. As you can't sleep in any
callback-registered wakeups, like epoll, for example.

- Davide

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