Date:	Mon, 12 Apr 2010 10:35:31 -0700
From:	Sridhar Samudrala <sri@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Tom Lendacky <toml@...ibm.com>, netdev <netdev@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH] vhost: Make it more scalable by creating a vhost
	thread per device.

On Sun, 2010-04-11 at 18:47 +0300, Michael S. Tsirkin wrote:
> On Thu, Apr 08, 2010 at 05:05:42PM -0700, Sridhar Samudrala wrote:
> > On Mon, 2010-04-05 at 10:35 -0700, Sridhar Samudrala wrote:
> > > On Sun, 2010-04-04 at 14:14 +0300, Michael S. Tsirkin wrote:
> > > > On Fri, Apr 02, 2010 at 10:31:20AM -0700, Sridhar Samudrala wrote:
> > > > > Make vhost scalable by creating a separate vhost thread per vhost
> > > > > device. This provides better scaling across multiple guests and with
> > > > > multiple interfaces in a guest.
> > > > 
> > > > Thanks for looking into this. An alternative approach is
> > > > to simply replace create_singlethread_workqueue with
> > > > create_workqueue which would get us a thread per host CPU.
> > > > 
> > > > It seems that in theory this should be the optimal approach
> > > > wrt CPU locality, however, in practice a single thread
> > > > seems to get better numbers. I have a TODO to investigate this.
> > > > Could you try looking into this?
> > > 
> > > Yes. I tried using create_workqueue(), but the results were not good
> > > atleast when the number of guest interfaces is less than the number
> > > of CPUs. I didn't try more than 8 guests.
> > > Creating a separate thread per guest interface seems to be more
> > > scalable based on the testing i have done so far.
> > > 
> > > I will try some more tests and get some numbers to compare the following
> > > 3 options.
> > > - single vhost thread
> > > - vhost thread per cpu
> > > - vhost thread per guest virtio interface
> > 
> > Here are the results with netperf TCP_STREAM 64K guest to host on a
> > 8-cpu Nehalem system. It shows cumulative bandwidth in Mbps and host 
> > CPU utilization.
> > 
> > Current default single vhost thread
> > -----------------------------------
> > 1 guest:  12500  37%    
> > 2 guests: 12800  46%
> > 3 guests: 12600  47%
> > 4 guests: 12200  47%
> > 5 guests: 12000  47%
> > 6 guests: 11700  47%
> > 7 guests: 11340  47%
> > 8 guests: 11200  48%
> > 
> > vhost thread per cpu
> > --------------------
> > 1 guest:   4900 25%
> > 2 guests: 10800 49%
> > 3 guests: 17100 67%
> > 4 guests: 20400 84%
> > 5 guests: 21000 90%
> > 6 guests: 22500 92%
> > 7 guests: 23500 96%
> > 8 guests: 24500 99%
> > 
> > vhost thread per guest interface
> > --------------------------------
> > 1 guest:  12500 37%
> > 2 guests: 21000 72%
> > 3 guests: 21600 79%
> > 4 guests: 21600 85%
> > 5 guests: 22500 89%
> > 6 guests: 22800 94%
> > 7 guests: 24500 98%
> > 8 guests: 26400 99%
> > 
> > Thanks
> > Sridhar
> 
> 
> Consider using Ingo's perf tool to get error bars, but looks good
> overall. 

What do you mean by getting error bars?

> One thing I note though is that we seem to be able to
> consume up to 99% CPU now. So I think with this approach
> we can no longer claim that we are just like some other parts of
> networking stack, doing work outside any cgroup, and we should
> make the vhost thread inherit the cgroup and cpu mask
> from the process calling SET_OWNER.

Yes. I am not sure what is the right interface to do this, but this
should also allow binding qemu to a set of cpus and automatically having
vhost thread inherit the same cpu mask.

Thanks
Sridhar

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