Re: [tcpm] A description of Linux pacing

[Michael Welzl <[e-mail]>]

This is useful indeed, many thanks for the pointer!  (*I* knew about it, but still).

This said, I found this reference, and others, to be too "high-level" for  my own purposes. I wanted to understand what I see at a finer detail, and I think that this is the kind of documentation that’s missing IMO.  (e.g., the initial burst limit of 10 (not IW, but hard-coded 10) packets, which took me by surprise, and will probably surprise others who were unaware; how micro bursts are used to implement a rate at a ~ 1ms granularity, with upper and lower bounds; the divergence from this for hosts that are less than 3ms away, …).

Cheers,
Michael



> On Mar 20, 2024, at 7:55 PM, Yuchung Cheng <ycheng=[e-mail]> wrote:
> 
> Neal and I have a short paper on Linux pacing, which integrates TSQ, and TSO autosizing. it has evolved over the years but the core design remains.
> https://netdevconf.info/1.2/papers/bbr-netdev-1.2.new.new.pdf
> 
> On Wed, Mar 20, 2024 at 12:42 AM Michael Welzl <[e-mail] <mailto:[e-mail]>> wrote:
>> Hi,
>> 
>> and thanks for your feedback!
>> 
>> I agree; I can write this up in time for the Vancouver IETF. As for documenting other implementations, I don’t know internals of any others… but I’m very willing to involve others in such an I-D - if any expert on a different OS is interested, get in touch!
>> 
>> Else, I could just make a start with this and see where it goes. It’s not much work, I can keep it short and simple.
>> 
>> Cheers,
>> Michael
>> 
>> 
>>> On 12 Mar 2024, at 23:08, Martin Duke <[e-mail] <mailto:[e-mail]>> wrote:
>>> 
>>> Speaking as an individual, it's a bummer we don't have a real RFC about pacing, this would be a decent Informational RFC for a way to do pacing, that is also a useful reference for how an important implementation does it.
>>> 
>>> Even more ambitious, one could document the approaches of several major implementations.
>>> 
>>> On Mon, Feb 19, 2024 at 6:06 AM Michael Welzl <[e-mail] <mailto:[e-mail]>> wrote:
>>>> Dear TCPM and ICCRG (I assume that there’s so much overlap with people in CCWG that it would just be spamming to send it there too?),
>>>> 
>>>> Over the last two weeks or so, I have put some effort into trying to understand how Linux pacing *really* works  (I had some descriptions that were somewhat high-level; I wanted to obtain a more precise lower-level understanding).
>>>> I wrote a document, just for myself, explaining what goes on in the code, as I can’t even try to follow the Linux kernel without taking notes.
>>>> 
>>>> After a first iteration, I shared it with the bufferbloat mailing list, in the hope of getting corrections.
>>>> I did!  Most notably (but not only), Neal Cardwell helped me a ton - and now the document should be quite thorough and hopefully correct.
>>>> 
>>>> While I only did this for myself and just asked the list for help, several people have in the meantime told me that this document is actually valuable for the community - and so I thought I should share it here, too.
>>>> It lives as a Google doc at this URL:
>>>> https://docs.google.com/document/d/1-uXnPDcVBKmg5krkG5wYBgaA2yLSFK_kZa7xGDWc7XU/edit?usp=sharing
>>>> 
>>>> Comments or fixes are very welcome!
>>>> Please feel free to forward this as you want.
>>>> 
>>>> I know that Google docs is not the format that we people here normally use  :-)   well, if someone thinks that it would indeed be useful to write this up as an I-D (probably skipping all the code details though), please let me know - I can do that, and I can also present it if there’s interest (but in Vancouver, not Brisbane).
>>>> 
>>>> Cheers,
>>>> Michael
>>>> 
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