Re: [PATCH net-next 2/2] net: exit busy loop when another process is runnable

[Jason Wang]

On 08/21/2014 04:11 PM, Michael S. Tsirkin wrote:
> On Thu, Aug 21, 2014 at 04:05:10PM +0800, Jason Wang wrote:
>> > Rx busy loop does not scale well in the case when several parallel
>> > sessions is active. This is because we keep looping even if there's
>> > another process is runnable. For example, if that process is about to
>> > send packet, keep busy polling in current process will brings extra
>> > delay and damage the performance.
>> > 
>> > This patch solves this issue by exiting the busy loop when there's
>> > another process is runnable in current cpu. Simple test that pin two
>> > netperf sessions in the same cpu in receiving side shows obvious
>> > improvement:
>> > 
>> > Before:
>> > netperf -H 192.168.100.2 -T 0,0 -t TCP_RR -P 0 & \
>> > netperf -H 192.168.100.2 -T 1,0 -t TCP_RR -P 0
>> > 16384  87380  1        1       10.00    15513.74
>> > 16384  87380
>> > 16384  87380  1        1       10.00    15092.78
>> > 16384  87380
>> > 
>> > After:
>> > netperf -H 192.168.100.2 -T 0,0 -t TCP_RR -P 0 & \
>> > netperf -H 192.168.100.2 -T 1,0 -t TCP_RR -P 0
>> > 16384  87380  1        1       10.00    23334.53
>> > 16384  87380
>> > 16384  87380  1        1       10.00    23327.58
>> > 16384  87380
>> > 
>> > Benchmark was done through two 8 cores Xeon machine back to back connected
>> > with mlx4 through netperf TCP_RR test (busy_read were set to 50):
>> > 
>> > sessions/bytes/before/after/+improvement%/busy_read=0/
>> > 1/1/30062.10/30034.72/+0%/20228.96/
>> > 16/1/214719.83/307669.01/+43%/268997.71/
>> > 32/1/231252.81/345845.16/+49%/336157.442/
>> > 64/512/212467.39/373464.93/+75%/397449.375/
>> > 
>> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
>> > ---
>> >  include/net/busy_poll.h | 3 ++-
>> >  1 file changed, 2 insertions(+), 1 deletion(-)
>> > 
>> > diff --git a/include/net/busy_poll.h b/include/net/busy_poll.h
>> > index 1d67fb6..8a33fb2 100644
>> > --- a/include/net/busy_poll.h
>> > +++ b/include/net/busy_poll.h
>> > @@ -109,7 +109,8 @@ static inline bool sk_busy_loop(struct sock *sk, int nonblock)
>> >  		cpu_relax();
>> >  
>> >  	} while (!nonblock && skb_queue_empty(&sk->sk_receive_queue) &&
>> > -		 !need_resched() && !busy_loop_timeout(end_time));
>> > +		 !need_resched() && !busy_loop_timeout(end_time) &&
>> > +		 nr_running_this_cpu() < 2);
> <= 1 would be a bit clearer? We want at most one process here.
>

Ok, will change it in next version.
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