Subject: Re: [RFC PATCH 01/17] shm-signal: shared-memory signals

From: Avi Kivity <[email-masked]>
Date: Wed, 01 Apr 2009 15:24:50 +0300

Gregory Haskins wrote:
> Note that this is exactly what I do (though it is device specific). 
> venet-tap has a ioq_notifier registered on its "rx" ring (which is the
> tx-ring for the guest) that simply calls ioq_notify_disable() (which
> calls shm_signal_disable() under the covers) and it wakes its
> rx-thread.  This all happens in the context of the hypercall, which then
> returns and allows the vcpu to re-enter guest mode immediately.
>   
I think this is suboptimal.  The ring is likely to be cache hot on the 
current cpu, waking a thread will introduce scheduling latency + IPI 
+cache-to-cache transfers.

On a benchmark setup, host resources are likely to exceed guest 
requirements, so you can throw cpu at the problem and no one notices.  
But I think the bits/cycle figure will decrease, even if bits/sec increases.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.