netdev - Re: [GIT PULL] AlacrityVM guest drivers for 2.6.33

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Date:	Wed, 23 Dec 2009 07:17:22 -0800
From:	Chris Wright <chrisw@...s-sol.org>
To:	Anthony Liguori <anthony@...emonkey.ws>
Cc:	Chris Wright <chrisw@...s-sol.org>,
	Andi Kleen <andi@...stfloor.org>,
	Gregory Haskins <gregory.haskins@...il.com>,
	Avi Kivity <avi@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	kvm@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
	torvalds@...ux-foundation.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	netdev@...r.kernel.org,
	"alacrityvm-devel@...ts.sourceforge.net" 
	<alacrityvm-devel@...ts.sourceforge.net>
Subject: Re: [GIT PULL] AlacrityVM guest drivers for 2.6.33

* Anthony Liguori (anthony@...emonkey.ws) wrote:
> On 12/22/2009 06:02 PM, Chris Wright wrote:
>> * Anthony Liguori (anthony@...emonkey.ws) wrote:
>>> The
>>> virtio-net setup probably made extensive use of pinning and other tricks
>>> to make things faster than a normal user would see them.  It ends up
>>> creating a perfect combination of batching which is pretty much just
>>> cooking the mitigation schemes to do extremely well for one benchmark.
>>
>> Just pinning, the rest is stock virtio features like mergeable rx buffers,
>> GRO, GSO (tx mitigation is actually disabled).
>
> Technically, tx mitigation isn't disabled.  The heuristic is changed  
> such that instead of relying on a fixed timer, tx notification is  
> disabled until you can switch to another thread and process packets.
>
> The effect is that depending on time slice length and system load, you  
> adaptively enable tx mitigation.  It's heavily dependent on the  
> particulars of the system and the overall load.
>
> For instance, this mitigation scheme looks great at high throughputs but  
> looks very bad at mid-to-low throughputs compared to timer based  
> mitigation (at least, when comparing CPU cost).

Yep, you're right.

thanks,
-chris
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