Re: [Cake] [PATCH net-next v7 6/7] sch_cake: Add overhead compensation support to the rate shaper

["Toke Høiland-Jørgensen" <toke@toke.dk>]

Sebastian Moeller <moeller0@gmx.de> writes:

> Hi Toke,
>
>
>
>> On May 2, 2018, at 17:30, Toke Høiland-Jørgensen <toke@toke.dk> wrote:
>> 
>> Sebastian Moeller <moeller0@gmx.de> writes:
>> 
>>>> On May 2, 2018, at 17:11, Toke Høiland-Jørgensen <toke@toke.dk> wrote:
>>>> 
>>>> +		/* The last segment may be shorter; we ignore this, which means
>>>> +		 * that we will over-estimate the size of the whole GSO segment
>>>> +		 * by the difference in size. This is conservative, so we live
>>>> +		 * with that to avoid the complexity of dealing with it.
>>>> +		 */
>>>> +		len = shinfo->gso_size + hdr_len;
>>>> +	}
>>> 
>>> 
>>> Hi Toke,
>>> 
>>> so I am on the fence with this one, as the extreme case is having a
>>> super packet consisting out of 1 full-MTU packet plus a tiny leftover
>>> in that case we pay a 50% bandwidth sacrifice which seems a bit high.
>>> Nowm I have no real feling how likely this full MTU plus 64 byte
>>> packet issue is in real life, but in the past I often saw maximum
>>> packetsizes of around 3K bytes on my router indicating that having a
>>> sup packet consisting just out of two segments might not be that rare.
>>> So is there an easy way for me to measure the probability of seeing
>>> that issue?
>>> 
>>> I am all for sacrificing some bandwidth for better latency under load,
>>> but few users will be happy with a 50% loss of bandwidth...
>> 
>> Well, in most cases such GSO segments will be split anyway (we split if
>> <= 1 Gbps). So this inaccuracy will only hit someone who enables the
>> shaper *and sets it to a rate rate > 1Gbps*. Which is not a deployment
>> mode we have seen a lot of, I think?
>
> Oh, I agree with that rationale; I was still under the impression that
> we want to go back to a (configurable) serialization delay based
> segmentation threshold and then this might become an issue (especially
> on puny routers will profit from the reduced routing cost* of
> GSO/GRO). Also I fear that 1Gbps service will become an issue rather
> sooner than later, even though I would assume that then dual segment
> super-packets should really be rare...

Sure, let's go back and revisit if and when we do that :)

-Toke