[Rpm] receive window bug fix

[rjmcmahon]

I think better tooling can help and I am always interested in 
suggestions on what to add to iperf 2 for better coverages.

I've thought it good for iperf 2 to support some sort of graph which 
drives socket read/write/delays vs a simplistic pattern of AFAP. It for 
sure stresses things differently, even in drivers. I've seen huge delays 
in some 10G drivers where some UDP packets seem to get stuck in queues 
and where the e2e latency is driven by the socket write rates vs the 
network delays. This is most obvious using burst patterns where the last 
packet of a latency burst is coupled to the first packet of the 
subsequent burst. The coupling between the syscalls to network 
performance is nonobvious and sometimes hard to believe.

We've been adding more "traffic profile" knobs for socket testing and 
have much of the latency metrics incorporated. Most don't use these. 
They seem to be hard to generalize. Cloudflare seems to have crafted 
specific tests after obtaining knowledge of causality.

Bob

PS. As a side note, I'm now being asked how to generate "AI loads" into 
switch fabrics, though there it probably won't be based upon socket 
syscalls but maybe using io_urings - not sure.

> This is good work!  I love reading their posts on scale like this.
> 
> It’s wild to me that the Linux kernel has (apparently) never
> implemented shrinking the receive window, or handling the case of
> userspace starting a large transfer and then just not ever reading
> it…  the latter is less surprising, I guess, because that’s an
> application bug that you probably would catch separately, and would be
> focused on fixing in the application layer…
> 
> -Aaron
> 
> On Sat, Jun 3, 2023 at 1:04 AM Dave Taht via Rpm
> <rpm at lists.bufferbloat.net> wrote:
> 
>> these folk do good work, and I loved the graphs
>> 
>> 
> https://blog.cloudflare.com/unbounded-memory-usage-by-tcp-for-receive-buffers-and-how-we-fixed-it/
>> 
>> --
>> Podcast:
>> 
> https://www.linkedin.com/feed/update/urn:li:activity:7058793910227111937/
>> Dave Täht CSO, LibreQos
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> - Sent from my iPhone.
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