netdev - Re: Network performance with small packets

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Date:	Thu, 10 Mar 2011 11:16:11 -0600
From:	Tom Lendacky <tahm@...ux.vnet.ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Shirley Ma <mashirle@...ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Krishna Kumar2 <krkumar2@...ibm.com>,
	David Miller <davem@...emloft.net>, kvm@...r.kernel.org,
	netdev@...r.kernel.org, steved@...ibm.com
Subject: Re: Network performance with small packets - continued

On Thursday, March 10, 2011 09:34:22 am Michael S. Tsirkin wrote:
> On Thu, Mar 10, 2011 at 09:23:42AM -0600, Tom Lendacky wrote:
> > On Thursday, March 10, 2011 12:54:58 am Michael S. Tsirkin wrote:
> > > On Wed, Mar 09, 2011 at 05:25:11PM -0600, Tom Lendacky wrote:
> > > > As for which CPU the interrupt gets pinned to, that doesn't matter -
> > > > see below.
> > > 
> > > So what hurts us the most is that the IRQ jumps between the VCPUs?
> > 
> > Yes, it appears that allowing the IRQ to run on more than one vCPU hurts.
> > Without the publish last used index patch, vhost keeps injecting an irq
> > for every received packet until the guest eventually turns off
> > notifications.
> 
> Are you sure you see that? If yes publish used should help a lot.

I definitely see that.  I ran lockstat in the guest and saw the contention on 
the lock when the irq was able to run on either vCPU.  Once the irq was pinned 
the contention disappeared.  The publish used index patch should eliminate the 
extra irq injections and then the pinning or use of irqbalance shouldn't be 
required.  I'm getting a kernel oops during boot with the publish last used 
patches that I pulled from the mailing list - I had to make some changes in 
order to get them to apply and compile and might not have done the right 
things.  Can you re-spin that patchset against kvm.git?

> 
> > Because the irq injections end up overlapping we get contention on the
> > irq_desc_lock_class lock. Here are some results using the "baseline"
> > setup with irqbalance running.
> > 
> >   Txn Rate: 107,714.53 Txn/Sec, Pkt Rate: 214,006 Pkts/Sec
> >   Exits: 121,050.45 Exits/Sec
> >   TxCPU: 9.61%  RxCPU: 99.45%
> >   Virtio1-input  Interrupts/Sec (CPU0/CPU1): 13,975/0
> >   Virtio1-output Interrupts/Sec (CPU0/CPU1): 0/0
> > 
> > About a 24% increase over baseline.  Irqbalance essentially pinned the
> > virtio irq to CPU0 preventing the irq lock contention and resulting in
> > nice gains.
> 
> OK, so we probably want some form of delayed free for TX
> on top, and that should get us nice results already.
> 
> > > --
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