Subject: Re: [PATCH net-next] vhost_net: stop rx net polling when possible

From: Jason Wang <[email-masked]>
Date: Mon, 18 Aug 2014 11:11:15 +0800

On 08/17/2014 06:20 PM, Michael S. Tsirkin wrote:
> On Fri, Aug 15, 2014 at 11:40:08AM +0800, Jason Wang wrote:
>> After rx vq was enabled, we never stop polling its socket. This is sub optimal
>> when may lead unnecessary wake-ups after the rx net work has already been
>> queued. This could be optimized by stopping polling the rx net sock when
>> processing both rx and tx and restart it afterward. This could save unnecessary
>> wake-ups and even unnecessary spin locks acquiring with the help of commit
>> 9e641bdcfa4ef4d6e2fbaa59c1be0ad5d1551fd5 "net-tun: restructure tun_do_read for
>> better sleep/wakeup efficiency".
> OK so the point is to avoid expensive wake_up_process calls?
> It's a bit unfortunate that we are adding/removing things from wait
> queue which certainly does take extra spin-locks.

When nothing new were queued during vhost thread is running. This change
may add two more spin-locks which may not but optimal. But if several
packets were queued by tun during vhost thread is running, it may save
lots of unnecessary wake ups. So the patch helps the performance in the
heavy load case for sure. In light load case, it may hurt some
throughput but cpu and thru/cpu is still saved.

>
>
>
>> Test shows significant CPU% savings during almost all the cases:
>>
>> Guest rx stream:
>> size(B)/sessions/throughput/cpu/normalized thru/
>> 64/1/+0.7773%   -8.6224%     +10.2866%
>> 64/2/+0.6335%   -13.9109%    +16.8946%
>> 64/4/-0.8182%   -14.8336%    +16.4565%
>> 64/8/+0.4830%   -13.7675%    +16.5256%
>> 256/1/-7.0963%  -12.6880%    +6.4043%
>> 256/2/-1.3982%  -11.5424%    +11.4678%
>> 256/4/-0.0350%  -11.8323%    +13.3806%
>> 256/8/-1.5830%  -12.7693%    +12.8238%
>> 1024/1/-7.4895% -19.1449%       +14.4152%
>> 1024/2/-7.4575% -19.4018%       +14.8195%
>> 1024/4/-0.3881% -9.1183%        +9.6061%
>> 1024/8/+0.4713% -11.0155%       +12.9087%
>> 4096/1/+0.8786%  -8.4050%        +10.1355%
>> 4096/2/+0.0098%  -15.3094%       +18.0885%
>> 4096/4/+0.0445%  -10.8247%       +12.1886%
>> 4096/8/-2.1317%  -12.5111%       +11.8637%
>> 16384/1/-0.0008% -6.1891%        +6.5966%
>> 16384/2/-0.0117% -16.2716%       +19.4198%
>> 16384/4/+0.0001% -5.9197%        +6.2923%
>> 16384/8/+0.0173% -7.6681%        +8.3236%
>> 65535/1/+0.0011% -10.3594%       +11.5578%
>> 65535/2/-0.4108%  -14.4304%       +16.3838%
>> 65535/4/+0.0011%  -10.3594%       +11.5578%
>> 65535/8/-0.4108%  -14.4304%       +16.3838%
>>
>> Guest tx stream:
>> size(B)/sessions/throughput/cpu/normalized thru/
>> 64/1/-0.6228%     -2.1936%     +1.6060%
>> 64/2/+0.8646%     -3.5063%     +4.5297%
>> 64/4/+0.8733%     -3.2495%     +4.2613%
>> 64/8/+1.4290%     -3.5593%     +5.1724%
>> 256/1/+7.2098%    -3.1122%     +10.6535%
>> 256/2/-10.1408%   -6.8230%     -3.5607%
>> 256/4/-11.3531%   -6.7085%     -4.9785%
>> 256/8/-10.2723%   -6.5628%     -3.9701%
>> 1024/1/-18.9329%  -13.6162%    -6.1547%
>> 1024/2/-0.3728%   -1.3181%     +0.9580%
>> 1024/4/+0.0125%   -3.6338%     +3.7838%
>> 1024/8/-0.0030%   -2.7282%     +2.8017%
>> 4096/1/+16.9367%  -1.9435%     +19.2543%
>> 4096/2/+0.0121%   -6.1682%     +6.5866%
>> 4096/4/+0.0019%   -3.8510%     +4.0072%
>> 4096/8/-0.0222%   -4.1368%     +4.2922%
>> 16384/1/-0.0026%  -8.6892%     +9.5132%
>> 16384/2/-0.0012%  -10.1676%    +11.3171%
>> 16384/4/+0.0196%  -1.2551%     +1.2908%
>> 16384/8/+0.1303%  -3.2634%     +3.5082%
>> 65535/1/+0.0019%  -3.4694%     +3.5961%
>> 65535/2/-0.0003%  -0.7635%     +0.7690%
>> 65535/4/-0.0219%  -2.7875%     +2.8448%
>> 65535/8/+0.1137%  -2.7922%     +2.9894%
>>
>> TCP_RR:
>> size(B)/sessions/throughput/cpu/normalized thru/
>> 256/1/+1.9004%    -4.7985%     +7.0366%
>> 256/25/-4.7366%   -11.0809%    +7.1349%
>> 256/50/+3.9808%   -5.2037%     +9.6887%
>> 4096/1/+2.1619%   -0.7303%     +2.9134%
>> 4096/25/-13.1836% -14.7298%    +1.8134%
>> 4096/50/-11.1990% -15.4763%    +5.0605%
>>
>> Signed-off-by: Jason Wang <[email-masked]>
>
> Could you split RX/TX parts out please, and benchmark separately?
>
> They are really independent.

Ok.