Re: [Bloat] iperf3 and packet bursts

["Dave Täht" <dave@taht.net>]

Groovy. I note that I am really fond of the linux "fdtimer" notion for
tickers, we use that throughout the high speed stats gathering code in
flent.

I'd really like a voip or ping tool that used those, and I've always
worried about iperf's internal notion of a sampling interval.

On 9/20/16 3:00 PM, Aaron Wood wrote:
> We were using iperf3 at work to test a network path (we wanted the
> fixed-rate throttling that it can do).  And while TCP would fill the
> 50Mbps link from the ISP, UDP couldn't.  UDP couldn't get over 8Mbps of
> goodput, no matter what rate we specified on the command line.
> 
> We found a 100ms timer that's used to PWM the packet transmission to
> perform the throttling.  Fine for TCP, fine where the end-to-end
> physical links are the same rate.  But throw a 10:1 rate change through
> a switch into that, and suddenly you find out that the switch isn't that
> bloated.
> 
> I modified iperf3 to use a 1ms timer, and was able to get things much
> smoother.  I doubt it's as smooth as iperf3 gets on Linux when fq pacing
> is used, but it's a big improvement vs. the nice small buffers in switches.
> 
> I put together a writeup with graphs on my blog:
> http://burntchrome.blogspot.com/2016/09/iperf3-and-microbursts.html
> 
> I have a forked version of iperf3 on github:
> https://github.com/woody77/iperf
> 
> This uses the 1ms timer, and has a few other fixes, such as it resets
> the throttling calculations at the start of each stats interval.  That
> change stops iperf3 from transmitting at maximum rate after congestion
> has stopped it from achieving the target rate.  There will be another
> writeup on that, but I need to get some good sample data together for
> graphing.
> 
> -Aaron Wood
> 
> 
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