Re: rte_rdtsc() - what is the performance impact of using rte_rdtsc() time

[Stephen Hemminger <stephen@networkplumber.org>]

On Tue, 29 Aug 2023 20:25:54 +0530
Hari Haran <info2hariharan@gmail.com> wrote:

> Hi All,
> 
> Subject: rte_rdtsc() - what is the performance impact of using rte_rdtsc()
> time under lcore thread while(1)
> 
> Requirement:
> 
>    1. Store the packet received timestamp - based on it packets will be
>    removed from buffer if it exceeds the threshold timer of buffer
>    2. Two threads are available, One is lcore(dedicated core) and another
>    is pthread(not a dedicated core. In pthread, have to get the timestamp of
>    last received packet timestamp
> 
> 
> Query:
> 
>    1. For every packet reception in lcore thread under while(1), will get
>    the packet received timestamp using  rte_rdtsc() function. Whether usage of
>    rte_rdtsc() function adds more delay in packet processing?
>    2. Is there any way to convert rte_rdtsc() timestamp value to current
>    system time in pthread()? In pthread, the last packet received time needed
>    in the form of system time.
> 
> 
> Thanks in advance.
> 
> Regards,
> Hariharan

The problem is that rte_rdtsc() stops speculative execution so doing
lots of TSC instructions can hurt performance.

To correlate TSC timestamp to system time, you need to compute the offsets
once at startup. Alternatively, don't use rte_rdtsc() and instead use
clock_gettime() with the monotonic timer and the C library does the calculation
for you.