terry mcintyre wrote:
Regarding the time used by RDTSC:
From
http://smallcode.weblogs.us/2007/12/07/performance-measurements-with-rdtsc/
Intel and Agner Fog recommend measuring the overhead
of RDTSC function and subtracting it from your result.
The overhead is relatively low (150-200 clock cycles)
and it occurs in all tested functions, so you can
neglect it when measuring long functions (e.g., 100
000 clock cycles).
NOTE: the page above recommends flushing the cache
before calling RDTSC, when using it for timing
purposes. There is probably no need to do so when
grabbing LSBs.
I wonder, however, if the LSBs would be as random as
all that, when called frequently in
quasi-deterministic code which takes a predictable
number of cycles between invocations.
I'm not interested in using it here to measure performance, but as a
possible way to have a very fast and very high quality random number
function. But this won't happen if RDTSC isn't fast and it doesn't
appear to be very fast.
I don't expect RDTSC to be very random either, I'm more interested in
the chaos it presents to the random number system which is produced even
with very little agitation added to the system. Even an occasional 1 bit
change would cure many of the problems of some fast random number
generators.
If you were to call this random number generator consecutively, many
time in a tight loop, I suspect that you will get a LOT of variation in
the number of cycles that have passed between calls, certainly enough to
make it completely unpredictable in the long run. Like weather
predictions you might predict the next day (or call) with some level of
reasonable accuracy but not a specific day in the next month.
Every deterministic pseudo random number generator in existence always
has some kind of structure. The main difference between what we consider
good and bad ones is how well that structure is obfuscated or hidden
from view. We try to be clever so that statistical tests cannot see the
structure. One of primary methods to "hide" this structure is to make it
so big (by long cycle lengths) that we can only see a tiny portion of
it. For instance if every zillion'th bit alternated between 0 and 1, it
would be hard to observe.
So if you can add a small amount of non-determinism you can probably
"bust up" the hidden structure.
At any rate, it seems like it's not workable as a fast alternative to
RNG. It might be combined with a slow very high quality generator to
produce numbers that are somewhat closer to true random numbers.
- Don
Terry McIntyre <[EMAIL PROTECTED]>
“Wherever is found what is called a paternal government, there is found state
education. It has been discovered that the best way to insure implicit
obedience is to commence tyranny in the nursery.”
Benjamin Disraeli, Speech in the House of Commons [June 15, 1874]
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