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:

quoted

On Thursday, March 10, 2011 12:54:58 am Michael S. Tsirkin wrote:

quoted

On Wed, Mar 09, 2011 at 05:25:11PM -0600, Tom Lendacky wrote:

quoted

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?

quoted

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.

quoted

quoted

--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html