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* [Cake] Query regarding COBALT implementation in CAKE
@ 2019-01-22  8:29 Shefali Gupta
  2019-01-22  9:04 ` Jonathan Morton
  0 siblings, 1 reply; 3+ messages in thread
From: Shefali Gupta @ 2019-01-22  8:29 UTC (permalink / raw)
  To: Cake List

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Hello Jonathan and Dave,

We were trying to look into Linux Kernel CAKE code to understand COBALT
implementation.

We are unable to understand how over_target variable is being used to
change the dropping state, on the line number 535 of CAKE code.

We are referring to this CAKE code:
https://elixir.bootlin.com/linux/latest/source/net/sched/sch_cake.c

Thanks and regards,
Shefali Gupta
Jendaipou Palmei

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^ permalink raw reply	[flat|nested] 3+ messages in thread

* Re: [Cake] Query regarding COBALT implementation in CAKE
  2019-01-22  8:29 [Cake] Query regarding COBALT implementation in CAKE Shefali Gupta
@ 2019-01-22  9:04 ` Jonathan Morton
  2019-01-22  9:12   ` Jonathan Morton
  0 siblings, 1 reply; 3+ messages in thread
From: Jonathan Morton @ 2019-01-22  9:04 UTC (permalink / raw)
  To: Shefali Gupta; +Cc: Cake List

> On 22 Jan, 2019, at 10:29 am, Shefali Gupta <shefaligups11@gmail.com> wrote:
> 
> We are unable to understand how over_target variable is being used to change the dropping state, on the line number 535 of CAKE code.

A little earlier in the code, you can see how over_target is calculated.  It's simply a boolean decision:

	over_target = sojourn > p->target &&
		      sojourn > p->mtu_time * bulk_flows * 2 &&
		      sojourn > p->mtu_time * 4;

There are three decision terms, all of which compare the sojourn time of the current packet to some threshold, and all of which must be true for over_target to be true, and thus for the Codel logic to enter dropping state.  In principle, it could be replaced by "sojourn > min(p->target, p_mtu_time * min(4, bulk_flows * 2))", but that would be harder to lay out readably in the Linux kernel's 80-column style.

The middle term is not relevant for your implementation, because you have a single-queue version of COBALT; you could consider bulk_flows as being fixed at 1, so that term is always true when the last term is true.

The last term depends on p->mtu_time which is initialised whenever Cake's shaper is reconfigured.  It refers to the time required to stuff an MTU-sized packet down the pipe.  Since your implementation is not closely integrated with a shaper and therefore you don't have an easy way to calculate mtu_time, you may eliminate this term, and instead manually configure the target to satisfy the same criterion.

You can therefore simplify if(over_target) to if(sojourn > p->target) for your purposes.

 - Jonathan Morton


^ permalink raw reply	[flat|nested] 3+ messages in thread

* Re: [Cake] Query regarding COBALT implementation in CAKE
  2019-01-22  9:04 ` Jonathan Morton
@ 2019-01-22  9:12   ` Jonathan Morton
  0 siblings, 0 replies; 3+ messages in thread
From: Jonathan Morton @ 2019-01-22  9:12 UTC (permalink / raw)
  To: Shefali Gupta; +Cc: Cake List

> On 22 Jan, 2019, at 11:04 am, Jonathan Morton <chromatix99@gmail.com> wrote:
> 
> In principle, it could be replaced by "sojourn > min(p->target, p_mtu_time * min(4, bulk_flows * 2))"

Correction: sojourn > max(p->target, p_mtu_time * max(4, bulk_flows * 2))

 - Jonathan Morton


^ permalink raw reply	[flat|nested] 3+ messages in thread

end of thread, other threads:[~2019-01-22  9:12 UTC | newest]

Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
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2019-01-22  9:04 ` Jonathan Morton
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