netdev - Re: [PATCH v2 net-next] pkt_sched: fq: Fair Queue packet scheduler

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Date:	Wed, 04 Sep 2013 17:50:26 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Yuchung Cheng <ycheng@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	"Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH v2 net-next] pkt_sched: fq: Fair Queue packet scheduler

On Wed, 2013-09-04 at 04:27 -0700, Eric Dumazet wrote:
> On Wed, 2013-09-04 at 03:30 -0700, Eric Dumazet wrote:
> > On Wed, 2013-09-04 at 14:30 +0800, Jason Wang wrote:
> > 
> > > > And tcpdump would certainly help ;)
> > > 
> > > See attachment.
> > > 
> > 
> > Nothing obvious on tcpdump (only that lot of frames are missing)
> > 
> > 1) Are you capturing part of the payload only (like tcpdump -s 128)
> > 
> > 2) What is the setup.
> > 
> > 3) tc -s -d qdisc
> 
> If you use FQ in the guest, then it could be that high resolution timers
> have high latency ?
> 
> So FQ arms short timers, but effective duration could be much longer.
> 
> Here I get a smooth latency of up to ~3 us
> 
> lpq83:~# ./netperf -H lpq84 ; ./tc -s -d qd ; dmesg | tail -n1
> MIGRATED TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to lpq84.prod.google.com () port 0 AF_INET
> Recv   Send    Send                          
> Socket Socket  Message  Elapsed              
> Size   Size    Size     Time     Throughput  
> bytes  bytes   bytes    secs.    10^6bits/sec  
> 
>  87380  16384  16384    10.00    9410.82   
> qdisc fq 8005: dev eth0 root refcnt 32 limit 10000p flow_limit 100p buckets 1024 quantum 3028 initial_quantum 15140 
>  Sent 50545633991 bytes 33385894 pkt (dropped 0, overlimits 0 requeues 19) 
>  rate 9258Mbit 764335pps backlog 0b 0p requeues 19 
>   117 flow, 115 inactive, 0 throttled
>   0 gc, 0 highprio, 0 retrans, 96861 throttled, 0 flows_plimit
> [  572.551664] latency = 3035 ns
> 
> 
> What do you get with this debugging patch ?
> 
> diff --git a/net/sched/sch_fq.c b/net/sched/sch_fq.c
> index 32ad015..c1312a0 100644
> --- a/net/sched/sch_fq.c
> +++ b/net/sched/sch_fq.c
> @@ -103,6 +103,7 @@ struct fq_sched_data {
>  	u64		stat_internal_packets;
>  	u64		stat_tcp_retrans;
>  	u64		stat_throttled;
> +	s64		slatency;
>  	u64		stat_flows_plimit;
>  	u64		stat_pkts_too_long;
>  	u64		stat_allocation_errors;
> @@ -393,6 +394,7 @@ static int fq_enqueue(struct sk_buff *skb, struct Qdisc *sch)
>  static void fq_check_throttled(struct fq_sched_data *q, u64 now)
>  {
>  	struct rb_node *p;
> +	bool first = true;
>  
>  	if (q->time_next_delayed_flow > now)
>  		return;
> @@ -405,6 +407,13 @@ static void fq_check_throttled(struct fq_sched_data *q, u64 now)
>  			q->time_next_delayed_flow = f->time_next_packet;
>  			break;
>  		}
> +		if (first) {
> +			s64 delay = now - f->time_next_packet;
> +
> +			first = false;
> +			delay -= q->slatency >> 3;
> +			q->slatency += delay;
> +		}
>  		rb_erase(p, &q->delayed);
>  		q->throttled_flows--;
>  		fq_flow_add_tail(&q->old_flows, f);
> @@ -711,6 +720,7 @@ static int fq_dump(struct Qdisc *sch, struct sk_buff *skb)
>  	if (opts == NULL)
>  		goto nla_put_failure;
>  
> +	pr_err("latency = %lld ns\n", q->slatency >> 3);
>  	if (nla_put_u32(skb, TCA_FQ_PLIMIT, sch->limit) ||
>  	    nla_put_u32(skb, TCA_FQ_FLOW_PLIMIT, q->flow_plimit) ||
>  	    nla_put_u32(skb, TCA_FQ_QUANTUM, q->quantum) ||
> 


BTW what is your HZ value ?

We have a problem in TCP stack, because srtt is in HZ units.

Before we change to us units, I guess tcp_update_pacing_rate() should be
changed a bit if HZ=250




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