Re: [dpdk-dev] [PATCH 1/3] mbuf: embedding timestamp into the packet

[Oleg Kuporosov <olegk@mellanox.com>]

Hello Konstantin,

> 
> My vote also would be to have timestamp in the second cache line.
> About moving seqn to the 2-nd cache line too - that's probably a fair point.

It may impact throughput till ~6% for applications required such embedded 
Timestamps.
 
> About the rest of the patch:
> Do you really need to put that code into the PMDs itself?
> Can't the same result be achieved by using RX callbacks?
> Again that approach would work with any PMD and there would be no need
> to modify PMD code itself.

Correct, the approach with using callbacs (rte_eth_timesync_read_[r|t]x_timestamp())
Has also some Cons for this use case:
- FSI needs the most accurate stamping as possible by reasons were described in
Cover letter
- callback will be called from user app and so you have to take into account 
Difference between time when packet was released by NIC and callback call
- such difference is not easy to estimate correctly due to dependency on CPU type,
Its frequency and current load conditions
- even estimated it would be hard without additional performance penalty to align
Packet with timestamp, taking account that call may actually placed from
Different thread or even process.

It looks the least impacting and correct way is to have timestamp in rte_mbuf and fill
It in Rx burst procedure.

> Another thing, that I am in doubt why to use system time?
> Wouldn't raw HW TSC value (rte_rdtsc()) do here?

System time is being used for periodic clock synchronization between wall
clock and NIC to estimate NIC clock deviation. It is in assumption the system itself is
synchronized by PTP from master clock. It is run on context of control thread.

Thanks,
Oleg.