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

[Sangjin Han <sangjin@eecs.berkeley.edu>]

Why LFENCE, rather than CPUID? I guess LFENCE does not prevent
out-of-order execution for non-load instructions across it.

This link has detailed information on RDTSC, RDTSCP, and CPUID:
http://www.intel.com/content/dam/www/public/us/en/documents/white-papers/ia-32-ia-64-benchmark-code-execution-paper.pdf

Sangjin

On Mon, Jan 27, 2014 at 3:58 AM, didier.pallard
<didier.pallard@6wind.com> wrote:
> 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.
>
>
>