The question have been asked in here http://www.gossamer-threads.com/lists/linux/kernel/1210167 but I don't see an answer.
AFAIK /proc/sys/kernel/random/entropy_avail should return the size of available entropy but should not consume it. At least I don't see any reason for that.
However, I have been noticing the same thing as OP for at least a year and now I executed in quick succession
% cat /proc/sys/kernel/random/entropy_avail 3918 % cat /proc/sys/kernel/random/entropy_avail 3447 % cat /proc/sys/kernel/random/entropy_avail 2878 % cat /proc/sys/kernel/random/entropy_avail 2377 % cat /proc/sys/kernel/random/entropy_avail 1789 % cat /proc/sys/kernel/random/entropy_avail 1184 % cat /proc/sys/kernel/random/entropy_avail 577 % cat /proc/sys/kernel/random/entropy_avail 161 % cat /proc/sys/kernel/random/entropy_avail 133 % cat /proc/sys/kernel/random/entropy_avail 171 a while later I did the same with the same result, so I'm pretty sure the depletion of entropy is caused by the cat command.
Can anyone explain why this happens?