Re: [Cake] GSO peel behaviour tweaks

[Kevin Darbyshire-Bryant <kevin@darbyshire-bryant.me.uk>]

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On 24/11/15 10:48, Dave Taht wrote:
> I don't know what this used to look like but it is essentially wrong
> in both (all?) versions.
>
> -               q->peel_threshold = (q->rate_flags & CAKE_FLAG_ATM) ?
> -                       0 : min(65535U, q->rate_bps >> 12);
> +               q->peel_threshold = (q->rate_flags & CAKE_FLAG_ATM) ||
> +                       q->rate_overhead ? 0 : min(65535U, q->rate_bps >> 12);
>
> What we want to do is closer to:
>
> A) start peeling once we start accruing or incurring delay in excess
> of, say, 250usec.  At 1Mbit, this is basically peel always. At a gbit,
> it's peel with roughly two 10 full-size packet offloads in play. There
> are nuances vs a vs ack GRO stuff (served with a 300 quantum in
> fq_codel), and in the 10-100Mbit range...
>
> A1) So doing nothing at a rate unlimited is wrong
> A2) Taking the current len * flows as a way to calculate it is wrong
> A3) I don't know if this was ever "right". It doesn't need to be
> perfect, but this is far from right...
>
> While I am unfond of the rate estimator's overhead, it perhaps could
> be used to calculate the peel threshold in a saner way...
>
> B) always peel when we are trying to do accurate on-wire accounting.
>
> As for the other patch...
>
> In general random pointer lookups into memory (like the skb->gro
> pointer) cost more than math as the other two params here are possibly
> part of a local cache hit already... and I have no idea what the ratio
> is between gso packets and how often you'd hit the comparison... but
> see point A2 above...
>
> -       if (unlikely((len * max_t(u32, b->bulk_flow_count, 1U) >
> -                     q->peel_threshold && skb_is_gso(skb)))) {
>
> +       if (unlikely(skb_is_gso(skb) &&
> +               (len * max_t(u32, b->bulk_flow_count, 1U) >
> +                     q->peel_threshold))) {
>
>
>
>
>
> Dave Täht
> Let's go make home routers and wifi faster! With better software!
> https://www.gofundme.com/savewifi
>
>
> On Tue, Nov 24, 2015 at 10:12 AM, Kevin Darbyshire-Bryant
> <kevin@darbyshire-bryant.me.uk> wrote:
>> I've just pushed 2 commits related to GSO peeling behaviour to master.
>>
>> 1st tweak is at worst benign and at best removes a multiply compare for
>> every packet enqueued.  I'd like to think the optimiser in the compiler
>> would have done what I've done explicitly (in essence check this is a
>> gso packet 1st before thinking about peeling it) but when I checked on
>> x86_64 there was a definite difference in produced code.
>>
>> 2nd tweak is *not* benign.  In essence this forces peeling if either ATM
>> framing or packet overhead is specified.  Previously only ATM framing
>> forced peeling.  I think this is more correct but unfortunately will be
>> slower.
>>
>> Commits can be reverted - feel free :-)
>>
>>
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>>

Both changes reverted


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