netdev - Re: [Bloat] Trying to *really* understand Linux pacing

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Message-Id: <B4827A61-6324-40BB-9BDE-3A87DEABB65C@ifi.uio.no>
Date: Wed, 7 Feb 2024 13:08:40 +0100
From: Michael Welzl <michawe@....uio.no>
To: Dave Taht <dave.taht@...il.com>
Cc: Linux Kernel Network Developers <netdev@...r.kernel.org>,
 bloat@...ts.bufferbloat.net
Subject: Re: [Bloat] Trying to *really* understand Linux pacing

Whoa… and now I’m even more afraid   :-)

My sincere apologies to anyone whose code I may have mis-represented!  I just tried to get it…  sorry if there are silly mistakes in there!


> On 7 Feb 2024, at 13:05, Dave Taht <dave.taht@...il.com> wrote:
> 
> Dear Michael:
> 
> Thank you for digging deeply into packet pacing, TSQ, etc. I think
> there are some interesting new possibilities in probing (especially
> during slow start) that could make the core idea even more effective
> than it is. I also tend to think that attempting it in various cloudy
> environments and virtualization schemes, and with certain drivers, the
> side effects are not as well understood as I would like. For example,
> AWS's nitro lacks BQL as does virtio-net.
> 
> I think the netdev community, now cc'd, would be interested in your
> document and explorations so far, below. I hope for more
> enlightenment.
> 
> On Wed, Feb 7, 2024 at 6:57 AM Michael Welzl via Bloat
> <bloat@...ts.bufferbloat.net> wrote:
>> 
>> Dear de-bloaters of buffers,
>> Esteemed experts of low delay and pacing!
>> 
>> I have no longer been satisfied with high-level descriptions of how pacing works in Linux, and how it interacts with TSQ (I’ve seen some, in various papers, over the years) - but I wanted to REALLY understand it. So, I have dug through the code.
>> 
>> I documented this experience here:
>> https://docs.google.com/document/d/1-uXnPDcVBKmg5krkG5wYBgaA2yLSFK_kZa7xGDWc7XU/edit?usp=sharing
>> but it has some holes and may have mistakes.
>> 
>> Actually, my main problem is that I don’t really know what goes on when I configure a larger IW… things seem to get quite “off” there. Why? Anyone up for solving that riddle?  ;-)
>> (see the tests I documented towards the end of the document)
>> 
>> Generally, if someone who has their hands on files such as tcp_output.c all the time could take a look, and perhaps “fill” my holes, or improve anything that might be wrong, that would be fantastic!   I think that anyone should be allowed to comment and make suggestions in this doc.
>> 
>> MANY thanks to whoever finds the time to take a look !
>> 
>> Cheers,
>> Michael
>> 
>> _______________________________________________
>> Bloat mailing list
>> Bloat@...ts.bufferbloat.net
>> https://lists.bufferbloat.net/listinfo/bloat
> 
> 
> 
> -- 
> 40 years of net history, a couple songs:
> https://www.youtube.com/watch?v=D9RGX6QFm5E
> Dave Täht CSO, LibreQos