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Message-ID: <4B2FA655.6030205@gmail.com>
Date: Mon, 21 Dec 2009 11:46:13 -0500
From: Gregory Haskins <gregory.haskins@gmail.com>
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To: Anthony Liguori <anthony@codemonkey.ws>
CC: Avi Kivity <avi@redhat.com>, Ingo Molnar <mingo@elte.hu>,
       kvm@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>,
       torvalds@linux-foundation.org,
       "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
       netdev@vger.kernel.org,
       "alacrityvm-devel@lists.sourceforge.net" 
	<alacrityvm-devel@lists.sourceforge.net>
Subject: Re: [GIT PULL] AlacrityVM guest drivers for 2.6.33
References: <4B1D4F29.8020309@gmail.com> <20091218215107.GA14946@elte.hu> <4B2F9582.5000002@gmail.com> <4B2F978D.7010602@redhat.com> <4B2F9C85.7070202@gmail.com> <4B2FA42F.3070408@codemonkey.ws>
In-Reply-To: <4B2FA42F.3070408@codemonkey.ws>
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On 12/21/09 11:37 AM, Anthony Liguori wrote:
> On 12/21/2009 10:04 AM, Gregory Haskins wrote:
>> No, B and C definitely are, but A is lacking.  And the performance
>> suffers as a result in my testing (vhost-net still throws a ton of exi=
ts
>> as its limited by virtio-pci and only adds about 1Gb/s to virtio-u, fa=
r
>> behind venet even with things like zero-copy turned off).
>>   =20
>=20
> How does virtio-pci limit vhost-net?  The only time exits should occur
> are when the guest notifies the host that something has been placed on
> the ring.  Since vhost-net has no tx mitigation scheme right now, the
> result may be that it's taking an io exit on every single packet but
> this is orthogonal to virtio-pci.
>=20
> Since virtio-pci supports MSI-X, there should be no IO exits on
> host->guest notification other than EOI in the virtual APIC.

The very best you can hope to achieve is 1:1 EOI per signal (though
today virtio-pci is even worse than that).  As I indicated above, I can
eliminate more than 50% of even the EOIs in trivial examples, and even
more as we scale up the number of devices or the IO load (or both).

> This is a
> light weight exit today and will likely disappear entirely with newer
> hardware.

By that argument, this is all moot.  New hardware will likely obsolete
the need for venet or virtio-net anyway.  The goal of my work is to
provide an easy to use framework for maximizing the IO transport _in
lieu_ of hardware acceleration.  Software will always be leading here,
so we don't want to get into a pattern of waiting for new hardware to
cover poor software engineering.  Its simply not necessary in most
cases.  A little smart software design and a framework that allows it to
be easily exploited/reused is the best step forward, IMO.

Kind Regards,
-Greg


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