CODEWITHSUNDEEP
objective-cmacoscocoareal-time
Brandon
Brandon

Reputation: 678

Using mach_absolute_time() to slow down a simulation. Is this the proper way?

I am writing an application for OS X (Obj-C/Cocoa) that runs a simulation and displays the results to the user. In one case, I want the simulation to run in "real-time" so that the user can watch it go by at the same speed it would happen in real life. The simulation is run with a specific timestep, dt. Right now, I am using mach_absolute_time() to slow down the simulation. When I profile this code, I see that by far, most of my CPU time is spent in mach_absolute_time() and my CPU is pegged at 100%. Am I doing this right? I figured that if I'm slowing down the simulation such that the program isn't simulating anything most of the time then CPU usage should be down but mach_absolute_time() obviously isn't a "free call" so I feel like there might be a better way?

double nextT = mach_absolute_time();
while (runningSimulation)
{
    if (mach_absolute_time() >= nextT)
    {
         nextT += dt_ns;
         // Compute the next "frame" of the simulation
         // ....
    }
}

Upvotes: 2

Views: 847

Answers (2)

parapura rajkumar
parapura rajkumar

Reputation: 24413

You are doing busy spinning. If there is lot of time before you need to simulate again considering sleeping instead.

But any sleep won't guarantee that it would sleep exactly for the duration specified. Depending on how accurate you want it to be, you can sleep for a little less and afterwards spin for the rest.

Upvotes: 3

bbum
bbum

Reputation: 162722

Do not spin at all.

That is the first rule of writing GUI apps where battery life and app responsiveness matter.

sleep() or nanosleep() can be made to work, but only if used on something other than the main thread.

A better solution is to use any of the time based constructs in GCD as that'll make more efficient use of system resources.

If you want the simulation to appear smooth to the user, you'll really want to lock the slowed version to the refresh rate of the screen. On iOS, there is CADisplayLink. I don't know of a direct equivalent on the Mac.

Upvotes: 5

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