Hi Toke,
> On May 2, 2018, at 17:30, Toke Høiland-Jørgensen <[email protected]> wrote: > > Sebastian Moeller <[email protected]> writes: > >>> On May 2, 2018, at 17:11, Toke Høiland-Jørgensen <[email protected]> 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... > > 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 ;) Again, I agree without proof that this is more than a theoretical issue, let's ignore this for now (especially since this is going to interfere with ATM encapsulation, but on ATM the bandwidth should always merit a segmentataion of supers (sorry, ISPs cake is not designed as the customer facing shaper on the DSLAM ;) )) Best Regards Sebastian > > -Toke *) I guess the kernel's routing codsts pales in comparison with the actual shaping cost, so this might be a bad idea. _______________________________________________ Cake mailing list [email protected] https://lists.bufferbloat.net/listinfo/cake
