netdev - Re: [RFC PATCH 00/17] virtual-bus

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Date:	Thu, 02 Apr 2009 08:30:47 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Herbert Xu <herbert@...dor.apana.org.au>, anthony@...emonkey.ws,
	andi@...stfloor.org, linux-kernel@...r.kernel.org, agraf@...e.de,
	pmullaney@...ell.com, pmorreale@...ell.com, rusty@...tcorp.com.au,
	netdev@...r.kernel.org, kvm@...r.kernel.org,
	Mark McLoughlin <markmc@...hat.com>
Subject: Re: [RFC PATCH 00/17] virtual-bus

Avi Kivity wrote:
> Gregory Haskins wrote:
>
>  
>
>>> Why does a kernel solution not need to know when a packet is
>>> transmitted?
>>>
>>>     
>>
>> You do not need to know when the packet is copied (which I currently
>> do).  You only need it for zero-copy (of which I would like to support,
>> but as I understand it there are problems with the reliability of proper
>> callback (i.e. skb->destructor).
>>
>> Its "fire and forget" :)
>>   
>
> It's more of a "schedule and forget" which I think brings you the
> win.  The host disables notifications and schedules the actual tx work
> (rx from the host's perspective).  So now the guest and host continue
> producing and consuming packets in parallel.  So long as the guest is
> faster (due to the host being throttled?), notifications continue to
> be disabled.
Yep, when the "producer::consumer" ratio is > 1, we mitigate signaling. 
When its < 1, we signal roughly once per packet.

>
> If you changed your rx_isr() to process the packets immediately
> instead of scheduling, I think throughput would drop dramatically.
Right, that is the point. :) This is that "soft asic" thing I was
talking about yesterday.

-Greg



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