Subject: Re: [PATCH RFC v4 net-next 0/5] virtio_net: enabling tx interrupts

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 1 Dec 2014 12:42:23 +0200

On Mon, Dec 01, 2014 at 06:17:03PM +0800, Jason Wang wrote:
> Hello:
> 
> We used to orphan packets before transmission for virtio-net. This breaks
> socket accounting and can lead serveral functions won't work, e.g:
> 
> - Byte Queue Limit depends on tx completion nofication to work.
> - Packet Generator depends on tx completion nofication for the last
>   transmitted packet to complete.
> - TCP Small Queue depends on proper accounting of sk_wmem_alloc to work.
> 
> This series tries to solve the issue by enabling tx interrupts. To minize
> the performance impacts of this, several optimizations were used:
> 
> - In guest side, virtqueue_enable_cb_delayed() was used to delay the tx
>   interrupt untile 3/4 pending packets were sent.
> - In host side, interrupt coalescing were used to reduce tx interrupts.
> 
> Performance test results[1] (tx-frames 16 tx-usecs 16) shows:
> 
> - For guest receiving. No obvious regression on throughput were
>   noticed. More cpu utilization were noticed in few cases.
> - For guest transmission. Very huge improvement on througput for small
>   packet transmission were noticed. This is expected since TSQ and other
>   optimization for small packet transmission work after tx interrupt. But
>   will use more cpu for large packets.
> - For TCP_RR, regression (10% on transaction rate and cpu utilization) were
>   found. Tx interrupt won't help but cause overhead in this case. Using
>   more aggressive coalescing parameters may help to reduce the regression.

OK, you do have posted coalescing patches - does it help any?

I'm not sure the regression is due to interrupts.
It would make sense for CPU but why would it
hurt transaction rate?

It's possible that we are deferring kicks too much due to BQL.

As an experiment: do we get any of it back if we do
-        if (kick || netif_xmit_stopped(txq))
-                virtqueue_kick(sq->vq);
+        virtqueue_kick(sq->vq);
?

If yes, we can just kick e.g. periodically, e.g. after queueing each
X bytes.

> Changes from RFC V3:
> - Don't free tx packets in ndo_start_xmit()
> - Add interrupt coalescing support for virtio-net
> Changes from RFC v2:
> - clean up code, address issues raised by Jason
> Changes from RFC v1:
> - address comments by Jason Wang, use delayed cb everywhere
> - rebased Jason's patch on top of mine and include it (with some tweaks)
> 
> Please reivew. Comments were more than welcomed.
> 
> [1] Performance Test result:
> 
> Environment:
> - Two Intel(R) Xeon(R) CPU E5620 @ 2.40GHz machines connected back to back
>   with 82599ES cards.
> - Both host and guest were net-next.git plus the patch
> - Coalescing parameters for the card:
>   Adaptive RX: off  TX: off
>   rx-usecs: 1
>   rx-frames: 0
>   tx-usecs: 0
>   tx-frames: 0
> - Vhost_net was enabled and zerocopy was disabled
> - Tests was done by netperf-2.6
> - Guest has 2 vcpus with single queue virtio-net
> 
> Results:
> - Numbers of square brackets are whose significance is grater than 95%
> 
> Guest RX:
> 
> size/sessions/+throughput/+cpu/+per_cpu_throughput/
> 64/1/+2.0326/[+6.2807%]/-3.9970%/
> 64/2/-0.2104%/[+3.2012%]/[-3.3058%]/
> 64/4/+1.5956%/+2.2451%/-0.6353%/
> 64/8/+1.1732%/+3.5123%/-2.2598%/
> 256/1/+3.7619%/[+5.8117%]/-1.9372%/
> 256/2/-0.0661%/[+3.2511%]/-3.2127%/
> 256/4/+1.1435%/[-8.1842%]/[+10.1591%]/
> 256/8/[+2.2447%]/[+6.2044%]/[-3.7283%]/
> 1024/1/+9.1479%/[+12.0997%]/[-2.6332%]/
> 1024/2/[-17.3341%]/[+0.0000%]/[-17.3341%]/
> 1024/4/[-0.6284%]/-1.0376%/+0.4135%/
> 1024/8/+1.1444%/-1.6069%/+2.7961%/
> 4096/1/+0.0401%/-0.5993%/+0.6433%/
> 4096/2/[-0.5894%]/-2.2071%/+1.6542%/
> 4096/4/[-0.5560%]/-1.4969%/+0.9553%/
> 4096/8/-0.3362%/+2.7086%/-2.9645%/
> 16384/1/-0.0285%/+0.7247%/-0.7478%/
> 16384/2/-0.5286%/+0.3287%/-0.8545%/
> 16384/4/-0.3297%/-2.0543%/+1.7608%/
> 16384/8/+1.0932%/+4.0253%/-2.8187%/
> 65535/1/+0.0003%/-0.1502%/+0.1508%/
> 65535/2/[-0.6065%]/+0.2309%/-0.8355%/
> 65535/4/[-0.6861%]/[+3.9451%]/[-4.4554%]/
> 65535/8/+1.8359%/+3.1590%/-1.2825%/
> 
> Guest RX:
> size/sessions/+throughput/+cpu/+per_cpu_throughput/
> 64/1/[+65.0961%]/[-8.6807%]/[+80.7900%]/
> 64/2/[+6.0288%]/[-2.2823%]/[+8.5052%]/
> 64/4/[+5.9038%]/[-2.1834%]/[+8.2677%]/
> 64/8/[+5.4154%]/[-2.1804%]/[+7.7651%]/
> 256/1/[+184.6462%]/[+4.8906%]/[+171.3742%]/
> 256/2/[+46.0731%]/[-8.9626%]/[+60.4539%]/
> 256/4/[+45.8547%]/[-8.3027%]/[+59.0612%]/
> 256/8/[+45.3486%]/[-8.4024%]/[+58.6817%]/
> 1024/1/[+432.5372%]/[+3.9566%]/[+412.2689%]/
> 1024/2/[-1.4207%]/[-23.6426%]/[+29.1025%]/
> 1024/4/-0.1003%/[-13.6416%]/[+15.6804%]/
> 1024/8/[+0.2200%]/[+2.0634%]/[-1.8061%]/
> 4096/1/[+18.4835%]/[-46.1508%]/[+120.0283%]/
> 4096/2/+0.1770%/[-26.2780%]/[+35.8848%]/
> 4096/4/-0.1012%/-0.7353%/+0.6388%/
> 4096/8/-0.6091%/+1.4159%/-1.9968%/
> 16384/1/-0.0424%/[+11.9373%]/[-10.7021%]/
> 16384/2/+0.0482%/+2.4685%/-2.3620%/
> 16384/4/+0.0840%/[+5.3587%]/[-5.0064%]/
> 16384/8/+0.0048%/[+5.0176%]/[-4.7733%]/
> 65535/1/-0.0095%/[+10.9408%]/[-9.8705%]/
> 65535/2/+0.1515%/[+8.1709%]/[-7.4137%]/
> 65535/4/+0.0203%/[+5.4316%]/[-5.1325%]/
> 65535/8/+0.1427%/[+6.2753%]/[-5.7705%]/
> 
> size/sessions/+trans.rate/+cpu/+per_cpu_trans.rate/
> 64/1/+0.2346%/[+11.5080%]/[-10.1099%]/
> 64/25/[-10.7893%]/-0.5791%/[-10.2697%]/
> 64/50/[-11.5997%]/-0.3429%/[-11.2956%]/
> 256/1/+0.7219%/[+13.2374%]/[-11.0524%]/
> 256/25/-6.9567%/+0.8887%/[-7.7763%]/
> 256/50/[-4.8814%]/-0.0338%/[-4.8492%]/
> 4096/1/-1.6061%/-0.7561%/-0.8565%/
> 4096/25/[+2.2120%]/[+1.0839%]/+1.1161%/
> 4096/50/[+5.6180%]/[+3.2116%]/[+2.3315%]/
> 
> Jason Wang (4):
>   virtio_net: enable tx interrupt
>   virtio-net: optimize free_old_xmit_skbs stats
>   virtio-net: add basic interrupt coalescing support
>   vhost_net: interrupt coalescing support
> 
> Michael S. Tsirkin (1):
>   virtio_net: bql
> 
>  drivers/net/virtio_net.c        | 211 ++++++++++++++++++++++++++++++++--------
>  drivers/vhost/net.c             | 200 +++++++++++++++++++++++++++++++++++--
>  include/uapi/linux/vhost.h      |  12 +++
>  include/uapi/linux/virtio_net.h |  12 +++
>  4 files changed, 383 insertions(+), 52 deletions(-)
> 
> -- 
> 1.8.3.1