[Cerowrt-devel] ingress rate limiting falling short

[Sebastian Moeller]

Hi Aaron,

about the 5% loss with the wndr, please remember that the shaper works typically on raw Ethernet rates, while flent reports TCP good put I believe. So roughly 2 to 6 percent difference can be explained with a combination of the following overheads: PTM/ATM, ethernet, VLAN(s), PPPoE, IPv4/IPv6, TCP, potential TCP options like timestamps... Now flent might report actual tcp-rates and I am out to lunch, but I have a hard time reconciling  how flent/netperf, can actually learn about all the additional overhead... As far as I can tell all netperf sees is the payload it sends and the payload it receives...
Tl;dr: Overhead unknown to flent effortlessly explains why the TCP good put as reported by flent falls short of the shaped rate as that rate contains all overheads.


Best Regards
        Sebastian

On June 3, 2015 7:45:47 AM GMT+02:00, Aaron Wood <woody77 at gmail.com> wrote:
>I wrote this up on my blog, where I can intersperse text and graphs a
>bit
>better:
>
>http://burntchrome.blogspot.com/2015/06/htb-rate-limiting-not-quite-lining-up.html
>
>Basically, I ran a series of tcp_download tests, using increasing
>ingress
>rates with sqm_scripts, and then used flent's box-plots to put the
>results
>into a combined image for comparing.
>
>On the 3800, it never meets the rate, but it's only off by maybe 5%. 
>But
>on my new WRT1900AC, it's wildly off, even over the same performance
>range
>(I tested it from 80-220Mbps rates in 20Mbps jumps, and saw from
>40-150Mbps.
>
>I have no idea where to start looking for the cause.  But for now, I'm
>just
>setting my ingress rate MUCH higher than I should, because it's working
>out
>to the right value as a result.
>
>-Aaron
>
>
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