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

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Date:	Wed, 23 Dec 2009 10:31:29 -0800
From:	Chris Wright <chrisw@...s-sol.org>
To:	Anthony Liguori <anthony@...emonkey.ws>
Cc:	Bartlomiej Zolnierkiewicz <bzolnier@...il.com>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	Gregory Haskins <gregory.haskins@...il.com>,
	Avi Kivity <avi@...hat.com>, 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:
> The "poor" packet latency of virtio-net is a result of the fact that we  
> do software timer based TX mitigation.  We do this such that we can  
> decrease the number of exits per-packet and increase throughput.  We set  
> a timer for 250ms and per-packet latency will be at least that much.

Actually that's 150us ;-)  It's the AlacrityVM numbers that show 250us
(note micro, not milli) for latency.  That makes sense, shave off 150us
for the timer and you're left w/ 100us, which is not substantially
slower than what we see (for that bare metal latency we see ~60us)
when we switched tx mitigation schemes from timer based to thread
scheduling.  Quite similar to the 56.8us that vbus/venet shows.

thanks,
-chris
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