On Jul 19, 2013, at 1:05 AM, Kasper Nielsen <kaspe...@gmail.com> wrote:
Thanks so much for your feedback! Your points are well taken. > Telling people that your random source passes the die-hard test but at the > same time only using the current time as the seed is just pure evil. > Imagine spinning up a couple of thousands images with a JVM on Amazon. You > are going to see collisions among the seed values a lot sooner than you > think. Especially if the granularity of System.nanoTime() is not actual 1 > nanosecond. You need to use some kind of random source for the seeding or > at least mix in the process ID/host ID with the current time. > Obviously RDRAND would be a good choice if available. This is a problem > Random and TLR suffers from as well. I agree that this is a problem, and in fact have already looked at ways to get other environmental information to mix into the initial seed. The problem is making it Write Once, Run Anywhere---what environmental information other than time-of-day is reliably available on all platforms? It should be noted that the programmer has an easily available workaround: use the constructor that accepts a 64-bit seed. If the programmer gathers environmental data and then uses this constructor, if every JVM image manages to have a different seed, then all the RNGs will be different. Of course, either way, we are unable to guarantee much more than that they will have started in different places on the period of length 2^64. The REALLY good thing to do, if all those JVM images are collaborating, would be for one of the processes to create one original RNG, then split it and send it the new one (via serialization?? TBD) to another process, then further recursively split as needed. > Also I'm really worried about the seed value of just 64 bit. That is not a > lot if SR is sold as a quality random number generator. People are going to > experience collisions (albeit rarely) while using it. Again I agree with you, and I have been working this week on a 128-bit-seed version, still being tested on both the quality and speed dimensions. --Guy
