Time irregularity

[Vort]

One method of providing uniform amount of resources to program is using sync=slowdown option.
However in reality, even with ips=1000000, resources to program are delivered with changing (oscillating) speed.
It makes hard to use programs (games), which requires correctly timed inputs from user.

For example, try to track single car movement in Frogger (frogger.zip):

bochs_irregular_time.mp4

It moves with approximately equal speed, then jumps forward, then moves evenly again and so on.
Even ingame music suffers from this effect.

Same game in DOSBox produce smooth movement.

Version: d9d07c0.

[stlintel]

I wonder if it is related to jumpy TSC in Windows XP ...

[Vort]

Frogger knows nothing about Power Management Timer.
These are separate problems.

[Vort]

Something is wrong with implementation of sync=slowdown.
If I add hack to call msleep a little less often, time jumps almost disappear:

diff --git a/bochs/iodev/slowdown_timer.cc b/bochs/iodev/slowdown_timer.cc
index 2d715f50c..cf4c456dd 100644
--- a/bochs/iodev/slowdown_timer.cc
+++ b/bochs/iodev/slowdown_timer.cc
@@ -144,7 +144,8 @@ void bx_slowdown_timer_c::handle_timer()
 #if BX_HAVE_USLEEP
     usleep(s.Q);
 #elif BX_HAVE_MSLEEP
-    msleep(usectomsec((Bit32u)s.Q));
+    if (rand() % 100 > 20)
+        msleep(usectomsec((Bit32u)s.Q));
 #elif BX_HAVE_SLEEP
     sleep(usectosec(s.Q));
 #else 

This is not proper solution of course. More investigation is needed.

[Vort]

I have no idea why, but my next hack allows to get smooth animations in games for me:

diff --git a/bochs/iodev/slowdown_timer.cc b/bochs/iodev/slowdown_timer.cc
index 2d715f50c..3535804b9 100644
--- a/bochs/iodev/slowdown_timer.cc
+++ b/bochs/iodev/slowdown_timer.cc
@@ -26,6 +26,8 @@
 #include "pc_system.h"
 #include "slowdown_timer.h"
 
+#include <chrono>
+
 #include <errno.h>
 #if !defined(_MSC_VER)
 #include <unistd.h>
@@ -59,6 +61,12 @@ bx_slowdown_timer_c::bx_slowdown_timer_c()
   s.timer_handle=BX_NULL_TIMER_HANDLE;
 }
 
+uint64_t get_monotonic_microseconds()
+{
+  return std::chrono::duration_cast<std::chrono::microseconds>(
+    std::chrono::steady_clock::now().time_since_epoch()).count();
+}
+
 void bx_slowdown_timer_c::init(void)
 {
   // Return early if slowdown timer not selected
@@ -73,7 +81,7 @@ void bx_slowdown_timer_c::init(void)
   if(s.MAXmultiplier<1)
     s.MAXmultiplier=1;
 
-  s.start_time = sectousec(time(NULL));
+  s.start_time = get_monotonic_microseconds();
   s.start_emulated_time = bx_pc_system.time_usec();
   s.lasttime=0;
   if (s.timer_handle == BX_NULL_TIMER_HANDLE) {
@@ -103,7 +111,7 @@ void bx_slowdown_timer_c::handle_timer()
 {
   Bit64u total_emu_time = (bx_pc_system.time_usec()) - s.start_emulated_time;
   Bit64u wanttime = s.lasttime+s.Q;
-  Bit64u totaltime = sectousec(time(NULL)) - s.start_time;
+  Bit64u totaltime = get_monotonic_microseconds() - s.start_time;
   Bit64u thistime=(wanttime>totaltime)?wanttime:totaltime;
 
 #if BX_SLOWDOWN_PRINTF_FEEDBACK 

I just arbitrarily changed less precise time() with more precise <chrono> equivalent.
Probably I broke something in the process (especially compatibility), but I hope it will give ideas about how to make proper fix.

I tested this hack not only with 1M IPS Frogger, but also with 5M IPS Superfrog. It is playable now.