[LibreQoS] Integration system, aka fun with graph theory

[Dave Taht]

Calling rdtsc directly used to be even faster than gettimeofday

https://github.com/dtaht/libv6/blob/master/erm/includes/get_cycles.h

On Mon, Oct 31, 2022 at 2:20 PM Herbert Wolverson via LibreQoS
<libreqos at lists.bufferbloat.net> wrote:
>
> I'd agree with color coding (when it exists - no rush, IMO) being configurable.
>
> From the "how much delay are we adding" discussion earlier, I thought I'd do a little bit of profiling of the BPF programs themselves. This is with the latest round of performance updates (https://github.com/thebracket/cpumap-pping/issues/2), so it's not measuring anything in production. I simply added a call to get the clock at the start, and again at the end - and log the difference. Measuring both XDP and TC BPF programs. (Execution goes (packet arrives)->(XDP cpumap sends it to the right CPU)->(egress)->(TC sends it to the right classifier, on the correct CPU and measures RTT latency). This is adding about two clock checks and a debug log entry to execution time, so measuring it is slowing it down.
>
> The results are interesting, and mostly tell me to try a different measurement system. I'm seeing a pretty wide variance. Hammering it with an iperf session and a queue capped at 5 gbit/s: most of the TC timings were 40 nanoseconds - not a packet that requires extra tracking, already in cache, so proceed. When the TCP RTT tracker fired and recorded a performance event, it peaked at 5,900 nanoseconds. So the tc xdp program seems to be adding a worst-case of 0.0059 ms to packet times. The XDP side of things is typically in the 300-400 nanosecond range, I saw a handful of worst-case numbers in the 3400 nanosecond range. So the XDP side is adding 0.00349 ms. So - assuming worst case (and keeping the overhead added by the not-so-great monitoring), we're adding 0.0093 ms to packet transit time with the BPF programs.
>
> With a much more sedate queue (ceiling 500 mbit/s), I saw much more consistent numbers. The vast majority of XDP timings were in the 75-150 nanosecond range, and TC was a consistent 50-55 nanoseconds when it didn't have an update to perform - peaking very occasionally at 1500 nanoseconds. Only adding 0.00155 ms to packet times is pretty good.
>
> It definitely performs best on long streams, probably because the previous lookups are all in cache. This is also making me question the answer I found to "how long does it take to read the clock?" I'd seen ballpark estimates of 53 nanoseconds. Given that this reads the clock twice, that can't be right. (I'm *really* not sure how to measure that one)
>
> Again - not a great test (I'll have to learn the perf system to do this properly - which in turn opens up the potential for flame graphs and some proper tracing). Interesting ballpark, though.
>
> On Mon, Oct 31, 2022 at 10:56 AM dan <dandenson at gmail.com> wrote:
>>
>>
>>
>> On Sun, Oct 30, 2022 at 8:21 PM Dave Taht via LibreQoS <libreqos at lists.bufferbloat.net> wrote:
>>>
>>> How about the idea of "metaverse-ready" metrics, with one table that is preseem-like and another that's
>>>
>>> blue =  < 8ms
>>> green = < 20ms
>>> yellow = < 50ms
>>> orange  = < 70ms
>>> red = > 70ms
>>
>>
>> These need configurable.  There are a lot of wisps that would have everything orange/red.  We're considering anything under 100ms good on the rural plans.   Also keep in mind that if you're tracking latence via pping etc, then you need some buffer in there for the internet at large.  <70ms to Amazon is one thing, they're very well connected, but <70ms to most of the internet isn't probably very realistic and would make most charts look like poop.
>
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Dave Täht CEO, TekLibre, LLC