Re: [rt-tests][PATCH] align thread wakeup times

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

* Nicholas Mc Guire | 2013-09-09 09:29:48 [+0200]:

>Hi !
Hi Nicholas,

>  This patch provides and additional -A/--align flag to cyclictest to align
> thread wakeup times of all threads as closly defined as possible.
>
>  When running multiple threads in cyclictest (-S or -t # option) the threads
> are launched in an unsynchronized manner. Basically the creation order and
> time for thread creation determines the start time. For provoking a maximum
> congestion situation (e.g. cache evictions) and to improve reproducibility
> or run conditions the start time should be defined distances appart. The 
> well defined distance is implemented as a offset parameter to -A/--align
> and will offset each threads start time by the parameter * the sequentially
> assigned thread number (par->tnum), together with the -d0 (distance in the
> intervals of the individual threads) this alignment option allows to get 
> the thread wakeup times as closely synchronized as possible.
>
>  The method to sync is simply that the thread with par->tnum == 0 is chosen
> to set a globally shared timestamp, and all other threads use this timestamp
> as their starting time rather than each calling clock_gettime() at startup.
> To ensure synchronization of the thread startup the setting of the global
> time is guarded by pthread_barriers.

I would rather fix current behaviour instead introducing yet another
option. By using -d0 I assume that all threads wakeup at the same time.
According to your patch this does not happen due to the thread creating
/ starting overhead.
Is there is a reason to keep this "faulty" behavior? If not I would vote
to make this what you suggest the default.

Sebastian