How to determine which processes are using how much entropy from /dev/urandom

The short answer is 0, because entropy is not consumed.

There is a common misconception that entropy is consumed — that each time you read a random bit, this removes some entropy from the random source. This is wrong. You do not “consume” entropy. Yes, the Linux documentation gets it wrong.

During the life cycle of a Linux system, there are two stages:

1. Initially, there is not enough entropy. /dev/random will block until it thinks it has amassed enough entropy; /dev/urandom happily provides low-entropy data.
2. After a while, enough entropy is present in the random generator pool. /dev/random assigns a bogus rate of “entropy leek” and blocks now and then; /dev/urandom happily provides crypto-quality random data.

FreeBSD gets it right: on FreeBSD, /dev/random (or /dev/urandom, which is the same thing) blocks if it doesn't have enough entropy, and once it does, it keeps spewing out random data. On Linux, neither /dev/random nor /dev/urandom is the useful thing.

In practice, use /dev/urandom, and make sure when you provision your system that the entropy pool is fed (from disk, network and mouse activity, from a hardware source, from an external machine, …).

While you could try to read how many bytes get read from /dev/urandom, this is completely pointless. Reading from /dev/urandom does not deplete the entropy pool. Each consumer uses up 0 bits of entropy per any unit of time you care to name.