Overview
| PyPi module | https://pypi.org/project/accurate-timed-loop.html |
| git repository | https://bitbucket.org/arrizza-public/accurate-timed-loop |
| git command | git clone git@bitbucket.org:arrizza-public/accurate-timed-loop.git |
| Verification Report | https://arrizza.com/web-ver/python-accurate-timed-loop-report.html |
| Version Info |
|
- installation: see https://arrizza.com/setup-common
Summary
This is a python module that provides a way to have an accurate timed loop.
For example if you need to do an activity every 250ms +/-10ms, this loop will do that.
Sample code
see sample.py for a full example
import accurate_timed_loop
loop_delay = 0.250 # seconds
total_wait = 25.0 # seconds
for elapsed, start_time in accurate_timed_loop.accurate_wait(total_wait, loop_delay):
# ... do task every 250 mS
pass
Scripts
- See Quick Start for information on using scripts.
- See xplat-utils submodule for information on the submodule.
Accuracy and Limitations
The sample.py does testing and shows that on Windows MSYS2 the std deviation error is roughly 4mS in a 250mS loop. This means that 95% of loops will be +/-8 mS of the requested loop_delay.
expected elapsed diff1(ms) actual(s) diff2(ms)
1 0.000000 0.000000 0.000 0.000000 0.000
2 0.250000 0.257294 7.294 0.257294 7.294
<snip>
100 24.750000 24.764093 14.093 24.764093 14.093
101 25.000000 25.015579 15.579 25.015579 15.579
Stats:
loop count : 101 loops
Error Range : 0.000 to 24.406 mS
Error Stddev : 5.009 mS
Error Average : 8.863 mS
Recommended adj: 0.012200
sample rc=0
doit overall rc=0
This value is specific to Windows and to the PC that it is running on.
To make it more accurate for your PC and OS use the fixed_adjustment parameter. Set it so the minimum and maximum are roughly symmetrical around 0. The Stdev and Average error values at that point should be minimal.
import accurate_timed_loop
loop_delay = 0.250 # seconds
total_wait = 25.0 # seconds
adj = 0.009228 # macOS
for elapsed, start_time in accurate_timed_loop.accurate_wait(total_wait, loop_delay, fixed_adjustment=adj):
# ... do task every 250 mS
pass
Notes:
- Re-run this several times, and tweak the fixed adjustment.
- The sample.py reports a "Recommended adj" that usually results in better accuracy.
- macOS and Ubuntu tend to be less variant than Windows
This report shows that std deviation is much better.
expected elapsed diff1(ms) actual(s) diff2(ms)
1 0.000000 0.000000 0.000 0.000000 0.000
2 0.250000 0.251537 1.537 0.251537 1.537
<snip>
101 25.000000 24.989502 -10.498 24.989502 -10.498
102 25.250000 25.241386 -8.614 25.241386 -8.614
Stats:
loop count : 102 loops
Error Range : -9.228 to 5.864 mS
Error Stddev : 1.238 mS
Error Average : 4.953 mS
Recommended adj: 0.009228
sample rc=0
doit overall rc=0
Limitations:
- there is NO guarantee that the average error will always be that low or that consistent
- the following runs were on a macOS
# === first run:
Stats:
loop count : 102 loops
Error Range : -9.486 to 4.613 mS
Error Stddev : 1.962 mS
Error Average : 5.775 mS
Recommended adj: 0.009486
# === second run:
Stats:
loop count : 102 loops
Error Range : -9.587 to 3.287 mS
Error Stddev : 2.163 mS
Error Average : 6.745 mS
Recommended adj: 0.009587
# === third run:
Stats:
loop count : 102 loops
Error Range : -9.472 to 6.782 mS
Error Stddev : 1.546 mS
Error Average : 5.597 mS
Recommended adj: 0.009472
# === fourth run:
Stats:
loop count : 101 loops
Error Range : -9.518 to 10.365 mS
Error Stddev : 1.865 mS
Error Average : 5.410 mS
Recommended adj: 0.009518
# === fifth run:
Stats:
loop count : 101 loops
Error Range : -9.369 to 13.726 mS
Error Stddev : 2.196 mS
Error Average : 5.614 mS
Recommended adj: 0.009369
- if you use the adj parameter the incoming "elapsed" parameter will not be after your expected delay.
For example these two came in:
- at 24.749 seconds instead of the expected 24.750 seconds
- at 24.999 seconds instead of the expected 25.000 seconds
expected elapsed diff1(ms) actual(s) diff2(ms)
100 24.750000 24.749573 -0.427 24.749573 -0.427
101 25.000000 24.999601 -0.399 24.999601 -0.399