I’ve noticed that on my Linux workstation (an older CPU with a current graphics card and dual 4K monitors) taking screenshots by pressing “PrintScreen” in the GNOME desktop seems really slow: specifically, there is a long delay of a second or more between tapping the key and the start of the audiovisual “camera flash” feedback.
Profiling with the “perf” tool and checking the actual source for gnome-shell confirmed three things:
- gnome-shell doesn’t start the camera flash effect until after the screenshot is saved to disk as a PNG file, meaning the delay is mostly up to the PNG image compression.
- PNG compression is slow, and single-threaded! It takes about 1.25s to save a 7680×2160 desktop screenshot PNG on this machine; 0.75s on my faster laptop. A top of the line modern workstation can probably hit 0.5s, but that’s still a longish delay for feedback of a keyboard event.
- The two biggest contributors to CPU time are filtering the output pixels before compression (part of libpng), and the actual “deflate” compression (part of zlib).
I’ve divided the problem up into two areas to work on, one of which looks easy and one of which looks tractable but is a little more work to do right:
- Patch gnome-shell to split the screenshot provider into separate “capture” and “write” steps, then start the camera flash effect after capture to run in parallel with the compression. On most machines the screenshot will already be compressed before the visual effect finishes, making it seem instantaneous! On corner cases like mine, it’ll still feel fast, but the PNG may not be available until after the flash finishes.
- Create a library to write PNGs using multithreading [for the actual filter/compression stages] (either as a helper for libpng, or standalone), and figure out how to get it used in various places. (My screenshot case would be gdk-pixbuf.)
The patch for gnome-shell is mostly written but I haven’t yet figured out how to build and test the latest gnome-shell on Fedora 28. :) Will poke at that in a bit.
For parallelizing the actual compression, I found a number of tools to do multithreaded gzip/deflate wrapping around regular zlib, and one old half-finished experiment at specifically doing so for PNG saving. Learn more about them in our upcoming blog post part 2! (ooh! exciting!)