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Make use of SGTimeStamp::sleepUntil for accurate sleeping.

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For use with frame rate throtteling make use of
SGTimeStamp::sleepUntil. This function should cover the
system dependent parts of accurate sleeps and should
move the system dependent code out of the time manager.
This commit is contained in:
Mathias Froehlich 2011-10-28 17:02:19 +02:00
parent 0bd7078c42
commit 81cab66c2c
1 changed files with 4 additions and 67 deletions
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71
src/Time/TimeManager.cxx
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@ -24,13 +24,6 @@
#include "TimeManager.hxx"
#ifdef _WIN32
# define WIN32_LEAN_AND_MEAN
# include <windows.h> // for Sleep()
#else
# include <unistd.h> // for usleep()
#endif
#include <simgear/timing/sg_time.hxx>
#include <simgear/structure/event_mgr.hxx>
#include <simgear/misc/sg_path.hxx>
@ -284,69 +277,13 @@ void TimeManager::computeFrameRate()
void TimeManager::throttleUpdateRate()
{
double throttle_hz = fgGetDouble("/sim/frame-rate-throttle-hz", 0.0);
SGTimeStamp currentStamp;
// common case, no throttle requested
if (throttle_hz <= 0.0) {
double throttle_hz = fgGetDouble("/sim/frame-rate-throttle-hz", 0.0);
if (throttle_hz <= 0)
return; // no-op
}
double frame_us = 1000000.0 / throttle_hz;
#define FG_SLEEP_BASED_TIMING 1
#if defined(FG_SLEEP_BASED_TIMING)
// sleep based timing loop.
//
// Calling sleep, even usleep() on linux is less accurate than
// we like, but it does free up the cpu for other tasks during
// the sleep so it is desirable. Because of the way sleep()
// is implemented in consumer operating systems like windows
// and linux, you almost always sleep a little longer than the
// requested amount.
//
// To combat the problem of sleeping too long, we calculate the
// desired wait time and shorten it by 2000us (2ms) to avoid
// [hopefully] over-sleep'ing. The 2ms value was arrived at
// via experimentation. We follow this up at the end with a
// simple busy-wait loop to get the final pause timing exactly
// right.
//
// Assuming we don't oversleep by more than 2000us, this
// should be a reasonable compromise between sleep based
// waiting, and busy waiting.
// sleep() will always overshoot by a bit so undersleep by
// 2000us in the hopes of never oversleeping.
frame_us -= 2000.0;
if ( frame_us < 0.0 ) {
frame_us = 0.0;
}
currentStamp.stamp();
double elapsed_us = (currentStamp - _lastStamp).toUSecs();
if ( elapsed_us < frame_us ) {
double requested_us = frame_us - elapsed_us;
#ifdef _WIN32
Sleep ((int)(requested_us / 1000.0)) ;
#else
usleep(requested_us) ;
#endif
}
#endif
// busy wait timing loop.
//
// This yields the most accurate timing. If the previous
// ulMilliSecondSleep() call is omitted this will peg the cpu
// (which is just fine if FG is the only app you care about.)
currentStamp.stamp();
SGTimeStamp next_time_stamp = _lastStamp;
next_time_stamp += SGTimeStamp::fromSec(1e-6*frame_us);
while ( currentStamp < next_time_stamp ) {
currentStamp.stamp();
}
// sleep for exactly 1/hz seconds relative to the past valid timestamp
SGTimeStamp::sleepUntil(_lastStamp + SGTimeStamp::fromSec(1/throttle_hz));
}
// periodic time updater wrapper