Re: [Cerowrt-devel] bulk packet transmission

["David P. Reed" <dpreed@reed.com>]

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I do know that. I would say that benchmarks rarely match real world problems of real systems- they come from sources like academia and technical marketing depts. My job for the last few years has been looking at stems with dozens of processors across 2 and 4 sockets and multiple 10 GigE adapters.

There are few benchmarks that look like real workloads. And even smaller systems do very poorly compared to what is possible.  Linux is slowly getting better but not so much in the network area at scale.  That would take a plan and a rethinking. Beyond incremental tweaks. My opinion ... ymmv.

On Oct 10, 2014, David Lang <david@lang.hm> wrote:
>I've been watching Linux kernel development for a long time and they
>add locks
>only when benchmarks show that a lock is causing a bottleneck. They
>don't just
>add them because they can.
>
>They do also spend a lot of time working to avoid locks.
>
>One thing that you are missing is that you are thinking of the TCP/IP
>system as
>a single thread of execution, but there's far more going on than that,
>especially when you have multiple NICs and cores and have lots of
>interrupts
>going on.
>
>Each TCP/IP stream is not a separate queue of packets in the kernel,
>instead
>the details of what threads exist is just a table of information. The
>packets
>are all put in a small number of queues to be sent out, and the
>low-level driver
>picks the next packet to send from these queues without caring about
>what TCP/IP
>stream it's from.
>
>David Lang
>
>On Fri, 10 Oct 2014, 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
>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".
>>
>> 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.
>I'm not
>> impressed with the folks who do the Linux networking stacks.
>>
>>
>> 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
>>> _______________________________________________
>>> Cerowrt-devel mailing list
>>> Cerowrt-devel@lists.bufferbloat.net
>>> https://lists.bufferbloat.net/listinfo/cerowrt-devel
>>>

-- Sent from my Android device with K-@ Mail. Please excuse my brevity.

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