Re: arca-vm-26

Jamie Lokier (lkd@tantalophile.demon.co.uk)
Mon, 25 Jan 1999 14:38:26 +0000


I wrote:
> I don't think the source has to be _that_ good, so a linear feedback
> shift register should do. You can periodically mix in entropy from the
> random device into the LFSR value to keep it "fairly" random. (Say,
> whenever entropy enters the randomness pool),

I've changed my mind. For a VMA skiplist, I would try a polynomial
congruential generator similar to the kind used by BSD random(), as that
can also be extremely fast, and is pretty good.

Benjamin Saller Bender wrote:
> Can you imagine the VM or the network layer blocking to wait for
> the entorpy pool to refill though. The better the random() the better the
> spread on the tree, it becomes more important as N grows.

The LFSR suggestion doesn't involve any blocking. An LFSR, which is
very fast, provides random bits though they are not particularly good.
A skip list only needs bits, which suits an LFSR. New entropy is used
to stir the LFSR value to keep it fair -- to avoid identifiable long
term patterms. That is only done when new entropy arrives, not when a
random bit is required.

-- Jamie

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