回复： Is there any method to obtain real-world date and time in Genode?

[王硕]

Hi all,
Thanks to Stefan, your solution may be good for others, but for my part, it's not decent. Rdtsc varies on different computers of different CPUs, and my project will run on thousands of computers. It's impossible to adjust it to each computer.
Moreover, thanks to w_schmidt. My requirement is to record and display system date and time in an audit log app our team made before. When I was using Rtc, it made my genode project very slow. The mouse even cannot move, and the experience of operating was terrible. When I deleted lines related to Rtc, the system returned to normal‍.  


 Shuo Wang, 
University of Chinese Academy of Sciences.


 


Hi Stefan,

>When you say RTC is too slow, what drift did you perceived?

I may add this to the topic. I tried something similiar to measure 
performance.

First i was a bit irritated that i only get values which are different by 
one million units, until i looked at the code.
https://github.com/genodelabs/genode/blob/master/repos/os/src/drivers/rtc/x86/main.cc 
, line 218
    return mktime(day, mon, year, hour, min, sec) * 1000000ULL;
-> this only returns something which looks like a timestamp, but has no 
milliseconds at all.

The second thing is the timing. Using the RDTSC call from
https://github.com/genodelabs/genode/blob/master/repos/os/include/x86_64/trace/timestamp.h
I tried the following
    time1 = timestamp();
    msleep(1000)
    time2= timestamp();
which showed that a second of wait is quite equal to the number of time 
cycles.

Then i tried to emasure the rtc call.
time1 = timestamp();
sometestvariable = rtc.get_current_time();
time2= timestamp();

Funnilly this seems to take 0.995 seconds.

So RDTSC seems to be better for perf. measurements.... if and only if you 
have a fixed processor speed (and a x86 compatible which is not from cyrix, 
and you always take the same cpu, and...).
On modern processors if speed steps are used the rate of ticks is not 
constant (e.g. due to power saving measures triggered by BIOS).
Also if you have multiple processors you will always need to look up the 
corresponding processor. Some newer intel processors return the TSC always 
according to the fastest possible processor speed (which is fine).

There are more problems doing measurements with TSC. Because of this, 
microsoft for example gives the advice to never use the TSC and use methods 
from the windows api. Also in POSIX RDTSC should not be used, but 
clock_gettime(). So also on linux you should not use TSC but clock_gettime() 
/ CLOCK_MONOTONIC.

Maybe something to think about in future releases of Genode.

Best regards,
Wolfgang

-----Ursprüngliche Nachricht----- 
From: Stefan Kalkowski
Sent: Thursday, October 23, 2014 11:31 AM
To: genode-main at lists.sourceforge.net
Subject: Re: Is there any method to obtain real-world date and time in 
Genode?

Hello,

On 10/21/2014 07:33 PM, 王硕 wrote:
> Dear Norman and all Genodians,

the Genode OS framework is neither Norman's exclusive project, nor
limited to the team working at Genode Labs, but at least meant as an
open source community project. Although, Norman and the team at Genode
Labs have the greatest influence on the project, and are mostly
competent contacts, with this mailing list you address *all* Genode
developers and enthusiasts. So _please_: avoid addressing people
personally in the first place.

>
> I want to obtain‍ real-world date and time in Genode (base-foc), so
> would you like to tell me how to achieve it. Rtc is too slow, and Rdtsc‍
> can't obtain real time.

When you say RTC is too slow, what drift did you perceived?

Joking apart, your problem is a basic problem in computer science not
limited, or special with respect to Genode. I assume with "RTC is too
slow" you meant that reading the RTC every time you need a timestamp is
to expensive. If you look at existent systems like for instance Linux,
the problem is solved in the following way: Calendar time is read
exactly once while booting, and stored in a variable. From that on
another less expensive system timer is used to add periodically
wall-clock time to the initial time-stamp.

To break it down to your problem:

* Initially read the RTC time, as well as the RDTSC
* Whenever you want to read the current time, read the RDTSC, and add
the difference between the actual and the last RDTSC value to your
stored calendar time, and you are done

Regards
Stefan

>
> ------------------
>
> Shuo Wang,
> University of Chinese Academy of Sciences.
>
>
>
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-- 
Stefan Kalkowski
Genode Labs

http://www.genode-labs.com/ · http://genode.org/

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