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

[Toke Høiland-Jørgensen]

Sebastian Moeller <moeller0 at gmx.de> writes:

>> On May 2, 2018, at 17:11, Toke Høiland-Jørgensen <toke at 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?

But sure, in principle you are right; I have no idea how to measure the
probability, though. We could conceivably add another statistic, but,
well, not sure it's worth it... I am certainly not going to do it ;)

-Toke