Re: [Cake] total download rate with many flows

[George Amanakis <g_amanakis@yahoo.com>]

I totally understand what you are saying. However, I believe cake's 
egress and ingress modes currently behave as two extremes. One could 
argue that neither of them is the golden mean. With a patch in ingress 
mode (see below) and a single host using 32 flows to download I managed 
to increase throughput from ~7Mbps to ~10Mbps (configured limit 
12200kbps) while latency increased from ~10ms to ~50ms, which would 
still be acceptable. As a comparison egress mode in the same setup gives 
me throughput ~11.5Mbps and latency ~500ms.

I would like to hear your thoughts about this idea: the patch is 
incrementing q->time_next_packet for dropped packets differently than 
for passed-through ones. Please focus on the idea, not the actual 
implementation :) (also pasted in https://pastebin.com/SZ14WiYw)

=============8<=============

diff --git a/sch_cake.c b/sch_cake.c
index 82f264f..a3a4a88 100644
--- a/sch_cake.c
+++ b/sch_cake.c
@@ -769,6 +769,7 @@ static void cake_heapify_up(struct cake_sched_data 
*q, u16 i)
  }

  static void cake_advance_shaper(struct cake_sched_data *q, struct 
cake_tin_data *b, u32 len, u64 now);
+static void cake_advance_shaper2(struct cake_sched_data *q, struct 
cake_tin_data *b, u32 len, u64 now);

  #if LINUX_VERSION_CODE < KERNEL_VERSION(4, 8, 0)
  static unsigned int cake_drop(struct Qdisc *sch)
@@ -1274,7 +1275,7 @@ retry:
                 /* drop this packet, get another one */
                 if(q->rate_flags & CAKE_FLAG_INGRESS) {
                         len = cake_overhead(q, qdisc_pkt_len(skb));
-                       cake_advance_shaper(q, b, len, now);
+                       cake_advance_shaper2(q, b, len, now);
                         flow->deficit -= len;
                         b->tin_deficit -= len;
                 }
@@ -1286,8 +1287,6 @@ retry:
                 qdisc_qstats_drop(sch);
                 kfree_skb(skb);
  #endif
-               if(q->rate_flags & CAKE_FLAG_INGRESS)
-                       goto retry;
         }

         b->tin_ecn_mark += !!flow->cvars.ecn_marked;
@@ -1351,6 +1350,24 @@ static void cake_advance_shaper(struct 
cake_sched_data *q, struct cake_tin_data
         }
  }

+static void cake_advance_shaper2(struct cake_sched_data *q, struct 
cake_tin_data *b, u32 len, u64 now)
+{
+       /* charge packet bandwidth to this tin, lower tins,
+        * and to the global shaper.
+        */
+       if(q->rate_ns) {
+               s64 tdiff1 = b->tin_time_next_packet - now;
+               s64 tdiff2 = (len * (u64)b->tin_rate_ns) >> 
b->tin_rate_shft;
+               s64 tdiff3 = (len * (u64)q->rate_ns) >> q->rate_shft;
+
+               if(tdiff1 < 0)
+                       b->tin_time_next_packet += tdiff2;
+               else if(tdiff1 < tdiff2)
+                       b->tin_time_next_packet = now + tdiff2;
+
+               q->time_next_packet += (tdiff3*27)>>5;
+       }
+}
  static void cake_reset(struct Qdisc *sch)
  {
         u32 c;

=============8<=============

On 11/10/2017 4:50 PM, Jonathan Morton wrote:
>
> In fact, that's why I put a failsafe into ingress mode, so that it 
> would never stall completely.  It can happen, however, that throughput 
> is significantly reduced when the drop rate is high.
>
> If throughput is more important to you than induced latency, switch to 
> egress mode.
>
> Unfortunately it's not possible to guarantee both low latency and high 
> throughput when operating downstream of the bottleneck link.  ECN 
> gives you better results, though.
>
> - Jonathan Morton
>