[dpdk-dev] [PATCH] timer: add lfence before TSC read

[didier.pallard]

Yes, i will add a new function that includes the lfence;

for the performance penalty, we did not see noticable performance impact 
on our full software, so we did not see any reason to use 2 functions, 
but it's certainly because we make a very limited number of calls to 
rdtsc and it's true that it is highly application dependant, so 2 
functions are probably better. But if using the unaccurate function, you 
may have some hard time the first time you want to debug or do some 
precise measures, since the measure is not always done when expected. 
And generally, especially when debugging, you're not focusing at first 
on the function you're using to debug...
i don't know how to do to be sure that people will be aware of the 
problem and do not lose time on the same problem, i will try to add some 
kind of warning in rte_rdtsc function itself.
But perhaps should it be better to use the precise version as default 
one and let the optimized version with another name to be use on purpose 
when accuracy is not important; By default, i think we generaly suppose 
a time reading function to be accurate...

thanks
didier

On 01/27/2014 10:57 AM, Thomas Monjalon wrote:
> 24/01/2014 12:42, François-Frédéric Ozog:
>> IMHO, adding the lfence for all cases is introducing an un-necessary
>> performance penalty.
>>
>> What about adding rte_rdtsc_sync() or rte_rdtsc_serial() with the comment
>> about the rdtsc instruction behavior so that developers can choose which
>> form they want?
> Yes it could be a good idea in some cases. Didier, could you try to add such
> function ?
>
> But in some debugging cases we need to have high precision for almost all
> timestamps. Here I don't know what is the smartest solution.
>
> Thank you for commenting. Hope we'll find a good fix.