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a06df8d7c1
The motivation for upgrading our PRNG is the recent buildbot failures in the ZTS' tests/functional/fault/decompress_fault test. The probability of a failure in that test is 0.8^256, which is ~1.6e-25 out of 1, yet we have observed multiple test failures in it. This suggests a problem with our random number generation. The xorshift128+ generator that we were using has been replaced by newer generators that have "better statistical properties". After doing some reading, it turns out that these generators have "low linear complexity of the lowest bits", which could explain the ZTS test failures. We do two things to try to fix this: 1. We upgrade from xorshift128+ to xoshiro256++ 1.0. 2. We tweak random_get_pseudo_bytes() to copy the higher order bytes first. It is hoped that this will fix the test failures in tests/functional/fault/decompress_fault, although I have not done simulations. I am skeptical that any simulations I do on a PRNG with a period of 2^256 - 1 would be meaningful. Since we have raised the minimum kernel version to 3.10 since this was first implemented, we have the option of using the Linux kernel's get_random_int(). However, I am not currently prepared to do performance tests to ensure that this would not be a regression (for the time being), so we opt for upgrading our PRNG to a newer one from Sebastiano Vigna. Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de> Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu> Closes #13983 |
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