Re: Network performance with small packets

[Shirley Ma <mashirle@us.ibm.com>]

On Wed, 2011-02-02 at 23:20 +0200, Michael S. Tsirkin wrote:
> > I think I need to define the test matrix to collect data for TX xmit
> > from guest to host here for different tests.
> > 
> > Data to be collected:
> > ---------------------
> > 1. kvm_stat for VM, I/O exits
> > 2. cpu utilization for both guest and host
> > 3. cat /proc/interrupts on guest
> > 4. packets rate from vhost handle_tx per loop
> > 5. guest netif queue stop rate
> > 6. how many packets are waiting for free between vhost signaling and
> > guest callback
> > 7. performance results
> > 
> > Test
> > ----
> > 1. TCP_STREAM single stream test for 1K to 4K message size
> > 2. TCP_RR (64 instance test): 128 - 1K request/response size
> > 
> > Different hacks
> > ---------------
> > 1. Base line data ( with the patch to fix capacity check first,
> > free_old_xmit_skbs returns number of skbs)
> > 
> > 2. Drop packet data (will put some debugging in generic networking
> code)

Since I found that the netif queue stop/wake up is so expensive, I
created a dropping packets patch on guest side so I don't need to debug
generic networking code.

guest start_xmit()
	capacity = free_old_xmit_skb() + virtqueue_get_num_freed()
	if (capacity == 0)
		drop this packet;
		return;

In the patch, both guest TX interrupts and callback have been omitted.
Host vhost_signal in handle_tx can totally be removed as well. (A new
virtio_ring API is needed for exporting total of num_free descriptors
here -- virtioqueue_get_num_freed)

Initial TCP_STREAM performance results I got for guest to local host 
4.2Gb/s for 1K message size, (vs. 2.5Gb/s)
6.2Gb/s for 2K message size, and (vs. 3.8Gb/s)
9.8Gb/s for 4K message size. (vs.5.xGb/s)

Since large message size (64K) doesn't hit (capacity == 0) case, so the
performance only has a little better. (from 13.xGb/s to 14.x Gb/s)

kvm_stat output shows significant exits reduction for both VM and I/O,
no guest TX interrupts.

With dropping packets, TCP retrans has been increased here, so I can see
performance numbers are various.

This might be not a good solution, but it gave us some ideas on
expensive netif queue stop/wake up between guest and host notification.

I couldn't find a better solution on how to reduce netif queue stop/wake
up rate for small message size. But I think once we can address this,
the guest TX performance will burst for small message size.

I also compared this with return TX_BUSY approach when (capacity == 0),
it is not as good as dropping packets.

> > 3. Delay guest netif queue wake up until certain descriptors (1/2
> ring
> > size, 1/4 ring size...) are available once the queue has stopped.
> > 
> > 4. Accumulate more packets per vhost signal in handle_tx?
> > 
> > 5. 3 & 4 combinations
> > 
> > 6. Accumulate more packets per guest kick() (TCP_RR) by adding a
> timer? 
> > 
> > 7. Accumulate more packets per vhost handle_tx() by adding some
> delay?
> > 
> > > Haven't noticed that part, how does your patch make it
> > handle more packets?
> > 
> > Added a delay in handle_tx().
> > 
> > What else?
> > 
> > It would take sometimes to do this.
> > 
> > Shirley
> 
> 
> Need to think about this.
> 
>