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Re: [Xen-users] Bad TCP accept performance

To: Carl Byström <cgbystrom@xxxxxxxxx>
Subject: Re: [Xen-users] Bad TCP accept performance
From: Pasi Kärkkäinen <pasik@xxxxxx>
Date: Mon, 23 May 2011 23:22:57 +0300
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On Mon, May 23, 2011 at 06:31:01PM +0200, Carl Byström wrote:
>    I've been running some simple tests trying to find out why the TCP
>    accept() rate has been so low on my Xen guest.
>    The rate at which I can accept new TCP connections is about five times
>    better on a bare metal machine compared to my guest.
>    Been using netperf with the TCP_CRR test to simulate this behavior.
>    I originally posted this question at Server Fault
> ([1]http://serverfault.com/questions/272483/why-is-tcp-accept-performance-so-bad-under-xen)
>    along with lots of more details how I have performed these tests.
>    After a suggestion from a user there, I decided to try this list. Judging
>    from the number of views the questions did receive at Server Fault and
>    being top-3 voted at Hacker News, I presume this issue is something a lot
>    of users care about.
>    One user at HN also reported that this apparently is a known issue and is
>    due to small packet performance, affecting both Xen and KVM.
>    After collecting feedback from SF and HN users, my question is: what can
>    you do to improve small packet performance in Xen?
>    Is this a fundamentally difficult problem to solve with Xen or is there a
>    "quick fix"?
>    Thanks!


- Did you try giving dom0 and the VM dedicated cpu cores? Did that help?
  (See http://wiki.xen.org/xenwiki/XenBestPractices)

- Can you use Xen PCI passthru to dedicate a physical NIC to the VM ?

- Can you post your benchmark numbers.. we need more info so we know
  what kind of numbers are we talking about. Also post the specs
  of your hardware and also full software/kernel/xen versions.

-- Pasi

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