Date: Tue, 29 Oct 2013 12:58:46 -0700 From: Luigi Rizzo <rizzo@iet.unipi.it> To: Andre Oppermann <andre@freebsd.org> Cc: Randall Stewart <rrs@lakerest.net>, "freebsd-net@freebsd.org" <net@freebsd.org> Subject: Re: MQ Patch. Message-ID: <CA%2BhQ2%2BgTc87M0f5pvFeW_GCZDogrLkT_1S2bKHngNcDEBUeZYQ@mail.gmail.com> In-Reply-To: <526FFED9.1070704@freebsd.org> References: <40948D79-E890-4360-A3F2-BEC34A389C7E@lakerest.net> <526FFED9.1070704@freebsd.org>
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my short, top-post comment is that I'd rather see some more coordination with Andre, and especially some high level README or other form of documentation explaining the architecture you have in mind before this goes in. To expand my point of view (and please do not read me as negative, i am trying to be constructive and avoid future troubles and volunteer to help with the design and implementation): (i'll omit issues re. style and unrelated patches in the diff because they are premature) 1. Having multiple separate software queues attached to a physical queue makes sense only if we have a clear and documented plan for scheduling traffic from these queues into the hw one. Otherwise it ends up being just another confusing hack that makes it difficult to reason about device drivers. We already have something similar now (with the drbr queue on top used in some cases when the hw ring overflows), the ALTQ hooks, and without documentation this does not seem to improve the current situation. 2. QoS is not just priority scheduling or AQM a-la RED/CODEL/PI, but a coherent framework where you can classify/partition traffic into separate queues, apply one of several queue management (taildrop/RED/CODEL/whatever) and scheduling (which queue to serve next) policies in an efficient way. Linux mostly gets this right (they even support hierarchical schedulers). Dummynet has a reasonable architecture although not hierarchical and it operates at the IP level (or possibly at layer 2), which is probably too high (but not necessarily). We can also recycle the components, i.e. the classifier in ipfw and the scheduling algorithms. I am happy to help on this. ALTQ is too old and complex and inefficient and unmaintained to be considered. And i cannot comment on your code because you don't really explain what you want to do and how. Codel/PI are only queue management, not qos; and strict priority is just one (and probably the worse) policy one can have. One comment i can make, however, on the fact that 256 queues are way too few for a proper system. You need the number to be dynamic and much larger (e.g. using flowid as a key). So, to conclude: i fully support any plan to design something that lets us implement scheduling (and qos, if you want to call it this way) in a reasonable way, but what is in your patch now does not really seem to improve the current situation in any way. cheers luigi On Tue, Oct 29, 2013 at 11:30 AM, Andre Oppermann <andre@freebsd.org> wrote= : > On 29.10.2013 11:50, Randall Stewart wrote: > >> Hi: >> >> As discussed at vBSDcon with andre/emaste and gnn, I am sending >> this patch out to all of you ;-) >> > > I wasn't at vBSDcon but it's good that you're sending it (again). ;) > > > I have previously sent it to gnn, andre, jhb, rwatson, and several other >> of the usual suspects (as gnn put it) and received dead silence. >> > > Sorry 'bout that. Too many things going on recently. > > > What does this patch do? >> >> Well it add the ability to do multi-queue at the driver level. Basically >> any driver that uses the new interface gets under it N queues (default >> is 8) for each physical transmit ring it has. The driver picks up >> its queue 0 first, then queue 1 .. up to the max. >> > > To make I understand this correctly there are 8 soft-queues for each real > transmit ring, correct? And the driver will dequeue the lowest numbered > queue for as long as there are packets in it. Like a hierarchical strict > queuing discipline. > > This is prone to head of line blocking and starvation by higher priority > queues. May become a big problem under adverse traffic patterns. > > > This allows you to prioritize packets. Also in here is the start of some >> work I will be doing for AQM.. think either Pi or Codel ;-) >> >> Right now thats pretty simple and just (in a few drivers) as the ability >> to limit the amount of data on the ring=85 which can help reduce buffer >> bloat. That needs to be refined into a lot more. >> > > We actually have two queues, the soft-queue and the hardware ring which > both can be rather large leading to various issues as you mention. > > I've started work on an FF contract to rethink the whole IFQ* model and > to propose and benchmark different approaches. After that to convert all > drivers in the tree to the chosen model(s) and get rid of the legacy. In > general the choice of model will be done in the driver and no longer by > the ifnet layer. One or (most likely) more optimized models will be > provided by the kernel for drivers to chose from. The idea that most, > if not all drivers use these standard kernel provided models to avoid > code duplication. However as the pace of new features is quite high > we provide the full discretion for the driver to choose and experiment > with their own ways of dealing with it. This is under the assumption > that once a now model has been found it is later moved to the kernel > side and subsequently used by other drivers as well. > > > This work is donated by Adara Networks and has been discussed in several >> of the past vendor summits. >> >> I plan on committing this before the IETF unless I hear major objections= . >> > > There seems to be a couple of white space issues where first there is a t= ab > and then actual whitespace for the second one and others all over the > place. > > There seem to be a number of unrelated changes in sys/dev/cesa/cesa.c, > sys/dev/fdt/fdt_common.c, sys/dev/fdt/simplebus.c, sys/kern/subr_bus.c, > usr.sbin/ofwdump/ofwdump.c. > > It would be good to separate out the soft multi-queue changes from the ri= ng > depth changes and do each in at least one commit. > > There are two separate changes to sys/dev/oce/, one is renaming of the lo= ck > macros and the other the change to drbr. > > The changes to sys/kern/subr_bufring.c are not style compliant and we > normally > don't use Linux "wb()" barriers in FreeBSD native code. The atomics_* > should > be used instead. > > Why would we need a multi-consumer dequeue? > > The new bufring functions on a first glance do seem to be safe on > architectures > with a more relaxed memory ordering / cache coherency model than x86. > > The atomic dance in a number of drbr_* functions doesn't seem to make muc= h > sense > and a single spin-lock may result in atomic operations and bus lock cycle= s. > > There is a huge amount of includes pollution in sys/net/drbr.h which we a= re > currently trying to get rid of and to avoid for the future. > > > I like the general conceptual approach but the implementation feels bumpy > and > not (yet) ready for prime time. In any case I'd like to take forward > conceptual > parts for the FF sponsored IFQ* rework. > > -- > Andre > > > ______________________________**_________________ > freebsd-net@freebsd.org mailing list > http://lists.freebsd.org/**mailman/listinfo/freebsd-net<http://lists.free= bsd.org/mailman/listinfo/freebsd-net> > To unsubscribe, send any mail to "freebsd-net-unsubscribe@**freebsd.org<f= reebsd-net-unsubscribe@freebsd.org> > " > --=20 -----------------------------------------+------------------------------- Prof. Luigi RIZZO, rizzo@iet.unipi.it . Dip. di Ing. dell'Informazione http://www.iet.unipi.it/~luigi/ . Universita` di Pisa TEL +39-050-2211611 . via Diotisalvi 2 Mobile +39-338-6809875 . 56122 PISA (Italy) -----------------------------------------+-------------------------------
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