I compile ART myself on Ubuntu Linux 20.04. I’ve seen this issue for some time, it also happens with Rawtherapee but to a lesser degree. I just now saw the issue with ART 1.5.4-47-g3ccefd613.
In Multiple Editor Tabs Mode if I open a large number of Nikon NEF files, 10 for example, so I have 10 tabs, if I then change “input focus” from the ART window to a terminal window I see the CPU usage of ART go up, then if I click back on the ART window to again give ART “input focus” I again see the ART CPU usage go up. How much? For 10 tabs changing focus to the terminal window causes ART to use 4 seconds of CPU on my AMD 2700x 8 core 16 thread CPU. I have 32 GB of RAM and RAM usage is way below that limit plus when I check swap none is being used. When I then change focus back to ART I see another 3 seconds of CPU usage for a total of 7 seconds for each “input focus cycle”. If I close some of the tabs so I go from 10 tabs to say 3 then repeat the cycle of changing focus then the CPU usage does go down but its still about 2 or 3 seconds. If I close more tabs to go down to 1 tab and repeat the cycle the CPU usage again decreases but its still about 1 second. From the terminal window if I run “ps -lLfp PID” where PID is the ART process then I can see all the threads the ART process has and I also see the CPU usage is all in the main thread during this focus cycling, not the child threads. If I compile ART with “-DWITH_BENCHMARK=ON” I see messages when the 10 images open in the 10 tabs but nothing when I change focus back and forth, so apparently ART is not “re processing” the images. I downloaded a “gstack” command which uses gdb to print the stack trace of a given PID and all its children. When I run that gstack at the times of ART CPU usage when changing focus I see the main thread doing this (ART is now compiled as TYPE=RelWithDebugInfo):
Thread 1 (Thread 0x7fe66fb90e80 (LWP 49643)):
#0 0x00007fe67545f390 in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#1 0x00007fe67545f6a6 in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#2 0x00007fe67546168e in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#3 0x00007fe67545b5aa in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#4 0x00007fe6755b70bd in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#5 0x00007fe675469201 in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#6 0x00007fe675456a55 in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#7 0x00007fe675455874 in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#8 0x00007fe675456555 in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#9 0x00007fe6754565ab in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#10 0x00007fe6754565ab in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#11 0x00007fe6754565ab in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#12 0x00007fe6754565ab in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#13 0x00007fe6754565ab in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#14 0x00007fe6754565ab in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#15 0x00007fe6754565ab in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#16 0x00007fe6754565ab in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#17 0x00007fe6754565ab in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#18 0x00007fe6754565ab in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#19 0x00007fe6754565ab in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#20 0x00007fe6754565ab in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#21 0x00007fe6754565ab in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#22 0x00007fe6754565ab in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#23 0x00007fe6754565ab in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#24 0x00007fe6754565ab in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#25 0x00007fe67543c766 in () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#26 0x00007fe675170a56 in () at /lib/x86_64-linux-gnu/libgobject-2.0.so.0
#27 0x00007fe67518fb28 in g_signal_emit_valist () at /lib/x86_64-linux-gnu/libgobject-2.0.so.0
#28 0x00007fe6751900d3 in g_signal_emit () at /lib/x86_64-linux-gnu/libgobject-2.0.so.0
#29 0x00007fe675202f16 in () at /lib/x86_64-linux-gnu/libgdk-3.so.0
#30 0x00007fe6751ecf4d in () at /lib/x86_64-linux-gnu/libgdk-3.so.0
#31 0x00007fe675085a28 in () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#32 0x00007fe675084e8e in g_main_context_dispatch () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#33 0x00007fe675085240 in () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#34 0x00007fe675085533 in g_main_loop_run () at /lib/x86_64-linux-gnu/libglib-2.0.so.0
#35 0x00007fe67550f37d in gtk_main () at /lib/x86_64-linux-gnu/libgtk-3.so.0
#36 0x00007fe674944258 in Gtk::Main::run(Gtk::Window&) () at /lib/x86_64-linux-gnu/libgtkmm-3.0.so.1
#37 0x000055fa2d02f395 in main ()
So again it seems the CPU usage during focus change is not in ART code but in the GTK 3 libraries. I know next to nothing about GTK but I am wondering why simply changing input focus causes GTK to use a lot of ART CPU usage and that amount changes depending on the number of tabs open in ART. As I said at the start I see a similar pattern with Rawtherapee but the amount of CPU usage is roughly half when 10 tabs are open in Rawtherapee using the same 10 Nikon NEF images. I also tried deleting all the existing .arp and .pp3 files for those 10 images and the same focus cycling CPU usage happens.
I should also mention that the CPU usage is all in “user” time and not “sys” time, its not a ton of operating system calls.
So the bottom line is I am wondering where this CPU usage is coming from and can the ART code be “tweaked” to reduce that?
Then again I may just be crazy. 