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

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Message-ID: <49D35D12.6080209@redhat.com>
Date:	Wed, 01 Apr 2009 15:24:50 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Gregory Haskins <ghaskins@...ell.com>
CC:	linux-kernel@...r.kernel.org, agraf@...e.de, pmullaney@...ell.com,
	pmorreale@...ell.com, anthony@...emonkey.ws, rusty@...tcorp.com.au,
	netdev@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC PATCH 01/17] shm-signal: shared-memory signals

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.

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