[Cake] cake exploration

Dave Taht dave.taht at gmail.com
Sat Apr 11 14:47:53 EDT 2015

14) strict priority queues. Some CBR techniques, notably IPTV, want 0
packet loss, but run at a rate determined by the provider to be below
what the subscriber will use. Sharing that "fairly" will lead to loss
of packets to those applications.

I do not like strict priority queues. I would prefer, for example,
that the CBR application be marked with ECN, and ignored, vs the the
high probability someone will abuse a strict priority queue.

On Sat, Apr 11, 2015 at 11:45 AM, Dave Taht <dave.taht at gmail.com> wrote:
> 12) Better starting interval and target for codel´s maintence vars in
> relationship to existing flows
> Right now sch_fq, sch_pie give priority to flows in their first IW
> phases. This makes them vulnerable to DDOS attacks with tons of new
> flows.
> sch_fq_codel mitigates this somewhat by starting to hash flows into
> the same buckets.
> sch_cake´s more perfect hashing gives IW more of a boost.
> A thought was to do a combined ewma of all active flows and to hand
> their current codel settings to new flows as they arrive, with less of
> a boost.
> This MIGHT work better when you have short RTTs generally on local
> networks. Other thoughts appreciated.
> There is another related problem in the resumption portion of the
> algorithm as the decay of the existing state variables is arbitrary
> and way too long in some cases. I think I had solved this by coming up
> with an estimate for the amount of decay needed other than count - 2,
> doing a calculation from the last time a flow had packets to the next,
> but can´t remember how I did it! It is easy if you have a last time
> per queue and use a normal sqrt with a divide... but my brain crashes
> at the reciprocal cache math we have instead....
> I am not allergic to a divide. I am not allergic to using a shift for
> the target and calculating the interval only relative to bandwidth, as
> mentioned elsewhere. At 64k worth of bandwidth we just end up with a
> huge interval, no big deal. But plan to ride along with the two
> separately for now.
> 13)  It might be possible to write a faster codel - and easier to read
> by using a case statement on the 2 core variables in it. The current
> code does not show the 3 way state machine as well as that could, and
> for all I know there is something intelligent we could do with the 4th
> state.
> On Sat, Apr 11, 2015 at 11:44 AM, Dave Taht <dave.taht at gmail.com> wrote:
>> Stuff on my backlog of researchy stuff.
>> 1) cake_drop_monitor - I wanted a way to throw drop AND mark
>> notifications up to userspace,
>> including the packet´s time of entry and the time of drop, as well as
>> the IP headers
>> and next hop destination macaddr.
>> There are many use cases for this:
>> A)  - testing the functionality of the algorithm and being able to
>> collect and analyze drops as  they happen.
>> NET_DROP_MONITOR did not cut it but I have not looked at it in a year.
>> It drives me crazy to be dropping packets all over the system and to
>> not be able to track down where they happened.
>> This is the primary reason why I had switched back to 64 bit timestamps, btw.
>> B) Having the drop notifications might be useful in tuning or steering
>> traffic to different routes.
>> C) It is way easier to do a graph of the drop pattern with this info
>> thrown to userspace.
>> 2) Dearly wanted to actually be doing the timestamping AND hashing in
>> the native skb
>> struct on entry to the system itself, not the qdisc. Measuring the
>> latency from ingress from the
>> wire to egress would result in much better cpu overload behavior. I am
>> totally aware of
>> how much mainline linux would not take this option, but things have
>> evolved over there, so
>> leveraging the rxhash and skb->timestamp fields seems a possibility...
>> I think this would let us get along better with netem also, but would
>> have to go look again.
>> Call that cake-rxhash. :)
>> 3) In my benchmark of the latest cake3, ecn traffic was not as good as
>> expected, but that might have been an anomoly of the test. Need to
>> test ecn thoroughly this time, almost in preference to looking at drop
>> behavior. Toke probably has ecn off by default right now. On, after
>> this test run?
>> 4) Testing higher rates and looking at cwnd for codel is important.
>> The dropoff toke noted in his paper is real. Also there is possibly
>> some ideal ratio between number of flows and bandwidth that makes more
>> sense than a fixed number of flows. Also I keep harping on the darn
>> resumption algo... but need to test with lousier tcps like windows.
>> 5) Byte Mode-ish handling
>> Dropping a single 64 byte packet does little good. You will find in
>> the 50 flow tests that a ton of traffic is acks, not being dropped,
>> and pie does better in this case than does fq, as it shoots
>> wildly at everything, but usually misses the fat packets, where DRR
>> will merrily store up an entire
>> MTU worth of useless acks when only one is needed.
>> So just trying to drop more little packets might be helpful in some cases.
>> 6) Ack thinning. I gave what is conventionally called "stretch acks" a
>> new name, as stretch acks
>> have a deserved reputation as sucking. Well, they dont suck anymore in
>> linux, and what I was
>> mostly thinking was to drop no more than 2 in a row...
>> One thing this would help with is in packing wifi aggregates - which
>> have hard limits on the number of packets in a TXOP (42), and a byte
>> limit on wireless n of 64k. Sending 41 acks from
>> one flow, when you could send the last 2, seems like a big win on
>> packing a TXOP.
>> (this is something eric proposed, and given the drop rates we now see
>> from wifi and the wild and wooly internet I am inclined to agree that
>> it is worth fiddling with)
>> (I am not huge on it, though)
>> 7) Macaddr hashing on the nexthop instead of the 5tuple. When used on
>> an internal, switched network, it  would be better to try and maximize
>> the port usage rather than the 5 tuple in some cases.
>> I have never got around to writing a mac hash I liked, my goal
>> originally was to write one that found a minimal perfect hash solution
>> eventually as mac addrs tend to be pretty stable on a network and
>> rarely change.
>> Warning: minimal perfect hash attempts are a wet paint thing! I really
>> want a FPGA solver for them.... dont go play with the code out there,
>> you will lose days to it... you have been warned.
>> http://cmph.sourceforge.net/concepts.html
>> I would like there to be a generic mac hashing thing in tc, actually.
>> 8) Parallel FIB lookup
>> IF you assume that you have tons of queues routing packets from
>> ingress to egress, on tons of cpus, you can actually do the FIB lookup
>> in parallel also. There is some old stuff on virtualqueue
>> and virtual clock fqing which makes for tighter
>> 9) Need a codel *library* that works at the mac80211 layer. I think
>> codel*.h sufficies but am not sure. And for that matter, codel itself
>> seems like it would need a calculated target and a few other thing to
>> work right on wifi.
>> As for the hashing...
>> Personally I do not think that the 8 way set associative has is what
>> wifi needs for cake, I tend to think we need to "pack" aggregates with
>> as many different flows as possible, and randomize how we packet
>> them... I think.... maybe....
>> 10) I really dont like BQL with multi-queued hardware queues. More
>> backpressure is needed in that case than we get.
>> 11) GRO peeling
>> Offloads suck
>> --
>> Dave Täht
>> Let's make wifi fast, less jittery and reliable again!
>> https://plus.google.com/u/0/107942175615993706558/posts/TVX3o84jjmb
> --
> Dave Täht
> Let's make wifi fast, less jittery and reliable again!
> https://plus.google.com/u/0/107942175615993706558/posts/TVX3o84jjmb

Dave Täht
Let's make wifi fast, less jittery and reliable again!


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