Before this change, an idle Audacity on a recent Linux laptop uses 6%
of CPU. This is because DrawOverlays() is called every 50ms through a
timer in TrackPanel, which instanciates a wxClientDC. This is quite
expensive and dominates the profiles: creating wxClientDCs is 84% of
the CPU audacity spends while idle, according to callgrind. With this
change, we avoid creating wxClientDCs when it's not necessary.
After this change, and idle Audacity consumes 1.6% of CPU, and most of
the time is spend in gtk/wxwidgets processing events.
Here are the perf stats of an idle Audacity before the change:
524.425485 task-clock (msec) # 0.060 CPUs utilized
825 context-switches # 0.002 M/sec
36 cpu-migrations # 0.069 K/sec
0 page-faults # 0.000 K/sec
1,198,433,346 cycles # 2.285 GHz
1,243,329,771 instructions # 1.04 insn per cycle
308,073,049 branches # 587.449 M/sec
5,801,494 branch-misses # 1.88% of all branches
8.808129958 seconds time elapsed
and after:
149.110455 task-clock (msec) # 0.016 CPUs utilized
908 context-switches # 0.006 M/sec
32 cpu-migrations # 0.215 K/sec
0 page-faults # 0.000 K/sec
313,372,582 cycles # 2.102 GHz
167,401,770 instructions # 0.53 insn per cycle
41,857,947 branches # 280.718 M/sec
1,189,566 branch-misses # 2.84% of all branches
9.076940003 seconds time elapsed
