* Re: Using rdtsc to timestamp RTT of packets
2023-03-06 1:01 Using rdtsc to timestamp RTT of packets fwefew 4t4tg
@ 2023-03-06 7:56 ` Gabor LENCSE
2023-03-06 9:46 ` Van Haaren, Harry
2023-03-06 16:56 ` Stephen Hemminger
2 siblings, 0 replies; 5+ messages in thread
From: Gabor LENCSE @ 2023-03-06 7:56 UTC (permalink / raw)
To: users
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Please see my comments inline.
On 3/6/2023 2:01 AM, fwefew 4t4tg wrote:
> I convinced myself that a viable way to measure timestamps between a
> request packet and its response packet can be the difference between
> two Intel rdtsc calls
I think it is a good solution: computationally inexpensive and accurate.
>
> The restrictions to valid use include:
>
> * RTT (time difference) must be calculated on the same CORE
>
Yes, it is safe.
But I think it is also OK, if the two cores used for taking sending TSC
and receiving TSC belong to the same physical CPU. At least it works me
well in my measurement program called siitperf.
> * fencing instructions (lfence) could be required
>
But it can also decrease the performance of your program.
> The time difference is OK provided,
>
> * it delivers at least microsecond resolution (rdtsc does)
> * the difference is always positive (end-start) or zero
> * the details of whether the clock runs or does not run at the
> processor speed is not material so long as there's sufficient
> resolution
> * DPDK gives me the frequency rte_rdtsc_cycles(); this way I can
> convert from a rdtsc difference to elapsed time
> * The OS doesn't reset the counter or pause it for interrupts or on
> halts
>
> I think rdtsc does all this. But then I read [1]:
>
> * The TSC is not always invariant
> * And of course context switches (if a thread is not pinned to a
> core) will invalidate any time difference
>
I start the packet sender and receiver threads by the
rte_eal_remote_launch() calls on the appropriate cores and it works fine
for me.
>
> * The TSC is not incremented when the processor enters a deep sleep.
> I don't care about this because I'll turn off the power saving
> modes anyway
>
> So I am not so sure.
I set the CPU clock frequency to a fixed value. (If I cannot do it from
BIOS, I use the tlp Linux package.)
>
> Now, of course, Mellanox can report time stamps. Is it actually
> possible to get HW NIC timestamps reported for every packet sent and
> received without overburdening the NIC? Based on what I can see for my
> case (Connect 4 LX) resolution is nanoseconds. So I am tempted to not
> fool around with rdtsc and just use NIC timestamps.
>
> What is praxis in DPDK programming when one needs RTTs?
I have implemented both the Latency and PDV (Packet Delay Variation)
measurements of RFC 8219 in siitperf using RDTSC. I am satisfied with
the result.
If you are interested, you can find the source code here:
https://github.com/lencsegabor/siitperf
And its latest version is documented in my paper:
http://www.hit.bme.hu/~lencse/publications/ECC-2022-SFNATxy-Tester-published.pdf
Best regards,
Gábor
>
> [1]
> https://stackoverflow.com/questions/42189976/calculate-system-time-using-rdtsc
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^ permalink raw reply [flat|nested] 5+ messages in thread
* RE: Using rdtsc to timestamp RTT of packets
2023-03-06 1:01 Using rdtsc to timestamp RTT of packets fwefew 4t4tg
2023-03-06 7:56 ` Gabor LENCSE
@ 2023-03-06 9:46 ` Van Haaren, Harry
2023-03-06 16:56 ` Stephen Hemminger
2 siblings, 0 replies; 5+ messages in thread
From: Van Haaren, Harry @ 2023-03-06 9:46 UTC (permalink / raw)
To: fwefew 4t4tg, users; +Cc: Gábor LENCSE
> From: fwefew 4t4tg <7532yahoo@gmail.com>
> Sent: Monday, March 6, 2023 1:01 AM
> To: users@dpdk.org
> Subject: Using rdtsc to timestamp RTT of packets
> I convinced myself that a viable way to measure timestamps between a request packet and its response packet can be the difference between two Intel rdtsc calls
> The restrictions to valid use include:
• RTT (time difference) must be calculated on the same CORE
For all CPU generations that you likely care-about (check lscpu | egrep "constant_tsc|nonstop_tsc" output) all CPUs on the same socket have a single TSC, and never stop.
> • DPDK gives me the frequency rte_rdtsc_cycles(); this way I can convert from a rdtsc difference to elapsed time
Correct, this gives a reference to convert "N TSC ticks" to real-world time "X of a second".
<snip>
> • The TSC is not always invariant
TSC is invariant/constant and non-stop based on the lscpu check above - so you can confirm your platform.
> • And of course context switches (if a thread is not pinned to a core) will invalidate any time difference
Context switches will not invalidate your timestamp - but if you're reacting to an interrupt which *then* timestamps "end" of measurement, then yes it is invalid/noisy. I'm trying to say, the context-switch jitter will make the measurement much more noisy - but "invalid" depends on requirements. Typically as DPDK has pinned & polling cores, this isn't a problem.
> Now, of course, Mellanox can report time stamps. Is it actually possible to get HW NIC timestamps reported for every packet sent and received without overburdening the NIC? > Based on what I can see for my case (Connect 4 LX) resolution is nanoseconds. So I am tempted to not fool around with rdtsc and just use NIC timestamps.
What is praxis in DPDK programming when one needs RTTs?
Some NICs have 1588 capabilities; these can timestamp *specific* packets on the wire to NS resolution. They typically can not timestamp *every* packet, or even 1 every ~million packets... but if measuring every Nth packet (with large N) then that might be worth trying, if the resolution and "closest-to-the-wire" timestamp are really required.
I would recommend going with a SW + TSC approach, for flexibility & ease of use; deploying in a cloud/virtio NIC environment will mean that ptp1588 or HW features are not available, so TSC is the "only" way to go then.
PTP 1588: https://doc.dpdk.org/api/rte__ethdev_8h.html#ad7aaa6d846a6e71edb4f6c737cdf60c7 and examples/ptpclient, and a HW PTP timestamping paper by "moongen" authors for reference (section 6) https://arxiv.org/pdf/1410.3322.pdf
On SW traffic-generation, latency, histograms etc, I've presented at DPDK Userspace last September on the topic: https://www.youtube.com/watch?v=Djcjq59H1uo
My conclusion (in the Q&A at end) is that for my use-cases, using TSC was a simpler/faster/easier option, and that resolution was much higher (and more than I required) for RTT in packet processing applications.
Hope that helps! -Harry
PS: apologies for formatting, it seems some HTML like mail was sent originally? Please send plain-text emails to mailing-lists.
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