Re: [Cerowrt-devel] bulk packet transmission

[Dave Taht <dave.taht@gmail.com>]

In the end, I weighed in on this thread on netdev:

http://www.spinics.net/lists/netdev/msg300590.html

On Fri, Oct 10, 2014 at 5:52 PM,  <dpreed@reed.com> wrote:
> The best approach to dealing with "locking overhead" is to stop thinking
> that if locks are good, more locking (finer grained locking) is better.  OS
> designers (and Linux designers in particular) are still putting in way too
> much locking.  I deal with this in my day job (we support systems with very
> large numbers of cpus and because of the "fine grained" locking obsession,
> the parallelized capacity is limited).   If you do a thoughtful design of

I'd certainly like to see you load up the new code under your workloads.

> your network code, you don't need lots of locking - because TCP/IP streams
> don't have to interact much - they are quite independent.   But instead OS
> designers spend all their time thinking about doing "one thing at a time".

Well, it's an engineering trait to focus on doing one thing at a time. I'd
like it if more CS folk had some EE influence and vice versa. Certainly
thinking about the system as a whole, as you must, in circuit design,
helps.

I really regret the shift towards specialization that has happened. When
I was a kid, programmers could design a circuit and soldier it up. And in
many cases, had to. Thankfully the maker movement seems to be bringing
these two fields back together again, and I look forward to the day where
I can look j random programmer in the eye and ask "What would you do
with a billion transitors", and get back a reasonable answer.

> There are some really good ideas out there (e.g. RCU) but you have to think
> about the big picture of networking to understand how to use them.

an RCU conversion is actually part of the xmit_more stuff. The end
results of all this work are being presented this week at linux
plumbers (the site with the preso with the pretty graphs is down
right now)

When people complain about slow progress in the network stack
or how it's overly complex somewhere, and how much easier
it would be to do something clean, and/or move everything in userspace,

I tend to point them at this skbuff structure, and explain how each
and every field
is needed in some circumstance:

http://lxr.free-electrons.com/source/include/linux/skbuff.h#L417

I am ALL in favor of moving packet processing to userspace,
but so far, aside from toy prototypes, I haven't seen anything
genuinely useful that covers the extreme range of link layer
technologies and speeds and devices that linux does.

I do think that netmap has some potential as does stuff
layered on the dpdk, but haven't played with either. And certainly
I'd like to see network hardware gain completion rings, be able
to deliver packets out of order, and thus be made fq_codel
capable.

>
>
> On Thursday, October 9, 2014 3:48pm, "Dave Taht" <dave.taht@gmail.com> said:
>
>> I have some hope that the skb->xmit_more API could be used to make
>> aggregating packets in wifi on an AP saner. (my vision for it was that
>> the overlying qdisc would set xmit_more while it still had packets
>> queued up for a given station and then stop and switch to the next.
>> But the rest of the infrastructure ended up pretty closely tied to
>> BQL....)
>>
>> Jesper just wrote a nice piece about it also.
>>
>> http://netoptimizer.blogspot.com/2014/10/unlocked-10gbps-tx-wirespeed-smallest.html
>>
>> It was nice to fool around at 10GigE for a while! And netperf-wrapper
>> scales to this speed also! :wow:
>>
>> I do worry that once sch_fq and fq_codel support is added that there
>> will be side effects. I would really like - now that there are al
>> these people profiling things at this level to see profiles including
>> those qdiscs.
>>
>> /me goes grumbling back to thinking about wifi.
>>
>> On Thu, Oct 9, 2014 at 12:40 PM, David Lang <david@lang.hm> wrote:
>> > lwn.net has an article about a set of new patches that avoid some
>> > locking
>> > overhead by transmitting multiple packets at once.
>> >
>> > It doesn't work for things with multiple queues (like fq_codel) in it's
>> > current iteration, but it sounds like something that should be looked at
>> > and
>> > watched for latency related issues.
>> >
>> > http://lwn.net/Articles/615238/
>> >
>> > David Lang
>> > _______________________________________________
>> > Cerowrt-devel mailing list
>> > Cerowrt-devel@lists.bufferbloat.net
>> > https://lists.bufferbloat.net/listinfo/cerowrt-devel
>>
>>
>>
>> --
>> Dave Täht
>>
>> https://www.bufferbloat.net/projects/make-wifi-fast
>> _______________________________________________
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>> Cerowrt-devel@lists.bufferbloat.net
>> https://lists.bufferbloat.net/listinfo/cerowrt-devel
>>



-- 
Dave Täht

https://www.bufferbloat.net/projects/make-wifi-fast