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

From: Daniel Borkmann <[email-masked]>
Date: Wed, 04 Sep 2013 13:59:27 +0200

On 09/04/2013 01:27 PM, 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 ?

Probably they internally switch to a lower resolution clock event source if
there's no hardware support available:

   The [source event] management layer provides interfaces for hrtimers to
   implement high resolution timers [...] [and it] supports these more advanced
   functions only when appropriate clock event sources have been registered,
   otherwise the traditional periodic tick based behaviour is retained. [1]

[1] https://www.kernel.org/doc/ols/2006/ols2006v1-pages-333-346.pdf

> 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) ||
>
>
> --
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