[ovs-discuss] Potential OVS+OpenStack Kilo issue

James Page james.page at ubuntu.com
Thu Apr 16 21:57:28 UTC 2015


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Hi Joe

On 16/04/15 22:44, Joe Stringer wrote:
> I had a lengthy discussion with someone in IRC last night who
> observed some odd behaviour in OVS-2.3 userspace + module from
> Linux-3.16. It seems like if OVS >= 2.3 is paired with OVS module
> from Linux <= 3.16, then only one handler thread will handle
> traffic even if multiple such threads exist. This limits the
> performance of OVS particularly under high load.
> 
> Prior to OVS-2.3 we had a dispatcher thread which distributed the 
> misses to multiple handler threads, but in 2.3 we shifted the 
> equivalent logic into the kernel. I understand that the requisite 
> patch for this functionality was merged in Linux-3.17: 
> https://github.com/torvalds/linux/commit/5cd667b0a4567048bb555927d6ee5
64f4e5620a9
>
>  For the most part, I don't think this is an issue because major 
> distros haven't shipped OVS-2.3 yet. Hopefully by the time they do 
> they will have recent kernels or will recommend dkms module. For 
> instance, Ubuntu 15.04 looks like it will have Linux-3.19. Using
> the OVS module from this version should not exhibit this problem.

Well that's good news....

> My main concern is whether this issue has been raised in OpenStack,
> as it appears that the Kilo release (imminent) is expected to run
> against Ubuntu 14.04, but there is no updated kernel or newer OVS
> kernel module in Ubuntu Cloud Archive (launchpad summary of
> packages below). If Ubuntu 14.04.3 hardware enablement stack is
> required then this issue should not be present [as it would
> presumably use the 15.04 kernel]. If an openvswitch-datapath-dkms
> package for OVS-2.3 is distributed with the Ubuntu Cloud Archive,
> then users can be recommended to use the dkms module. If users run
> on a single-core machine, there'll be no problem ;-)

Obviously not such good news; the 3.19 kernel will be made available
on 14.04 - but not for a few weeks yet - and we intentionally dropped
the dkms package in 2.3.x of ovs in Ubuntu - and we're backporting
2.3.1 as part of the OpenStack kilo cloud archive.

> https://launchpad.net/~ubuntu-cloud-archive/+archive/ubuntu/kilo-stagi
ng
>
>  I reproduced this today using Ubuntu Cloud Archive paired with
> Ubuntu 14.10, but it is equally applicable to Ubuntu 14.04 LTS.
> 
> Steps to reproduce: - Install Ubuntu 14.04.2 - Add ubuntu cloud
> archive for OpenStack Kilo[url below] - Install openvswitch-switch 
> - Add bridge, interface - ovs-vsctl set Open_vSwitch .
> n-handler-threads 3 - ovs-appctl upcall/disable-megaflows (To
> produce lots of misses) - hping3 --flood - Observe that only one
> handler thread handles misses
> 
> http://lists.openstack.org/pipermail/openstack-dev/2015-January/054820
.html
>
>  Are there some OpenStack people out there that could clarify
> whether this is a problem and if it's being addressed?

Thanks for raising this potential issue; there is not a huge amount I
can do between now and release of the Kilo Cloud Archive (Final Freeze
is today), but if this does present a problem we can look into some
potential stable updates to help resolve any performance impacts.

Regards

James

- -- 
James Page
Ubuntu and Debian Developer
james.page at ubuntu.com
jamespage at debian.org
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