clk: timestamps

[Yann Droneaud]

Hi,

Le mardi 26 juin 2018 à 19:44 +0530, Subhashini Rao Beerisetty a
écrit :
> 
> In the kernel code I see it supports CLOCK_REALTIME \ CLOCK_MONOTONIC
>  \ CLOCK_MONOTONIC_RAW timestamps. Can someone explain what’s major
> difference between those three modes and when to use which one?
> 

A generic answer could be found here:
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/time.h.html

> I’ve N number of Linux machines in the network with the same software
> running. Basically I need to collect the timestamps in kernel mode in
> all the machines and then compare, which(either CLOCK_REALTIME or
> CLOCK_MONOTIC_RAW or MONOTONIC) one would be the correct way?
> 

Probably none. Anyway, please find some hints below:

None of the monotonic clocks can be used because they're not supposed
to be synchronized accross the network

You're left with CLOCK_REALTIME, which is not synchronized accross the
network by default.

So before doing timestamp comparison, you would need to setup time
synchronisation between hosts on your network: NTP to synchronise hosts
 to the same seconds with couple of millisecond precision, PTP for sub
millisecond precision.

Time synchronisation is a tough problem but it's required if you want
to be able to compare timestamp accross hosts in a network.

Regards.

-- 
Yann Droneaud
OPTEYA