This small program is designed to estimate entropy available from the system timer.
Note: all observations apply only to a small sample of systems, namely two Intel Haswell-generation Linux 4.x machines.
By plotting a histogram of the low bits of each call it can be observed that numbers are well distributed — there is no obvious bias at any level.
By plotting a histogram of the differences between calls, it immediately becomes obvious that the time of each cycle of the loop is nearly constant, however not exactly constant.
By plotting a histogram of the low bits of the differences (and varying the number of bits), it can be seen that there is bias in the differences at anything above the lowest bit, but in total there appears to be roughly 2-3 bits of entropy per call.
Tweak the three constants, and toggle which graphs are plotted. Make sure gnuplot is installed, then:
cargo run