Date: Sun, 16 Aug 2015 23:23:02 +0300 From: Solar Designer <solar@...nwall.com> To: john-dev@...ts.openwall.com Subject: Re: FMT_OMP_BAD On Sun, Aug 16, 2015 at 11:04:17PM +0300, Solar Designer wrote: > The worst performance impact of enabling OpenMP is seen for: > > Ratio: 0.55939 real, 0.55939 virtual dynamic_1400:Raw > Ratio: 0.64531 real, 0.64531 virtual dynamic_1401:Only one salt > > This is followed by my bitslice DES formats, for which the impact is > 20%. That's pretty bad. They start collecting much larger groups of > candidate passwords when OpenMP is enabled, and this may result in > higher cache miss rate. This doesn't explain why the performance impact > is roughly the same regardless of iteration count, though. There must > be something else. I'll need to check if this same performance impact > is seen in core tree. The issue is also seen in core. It mostly goes away with newer gcc ("scl enable devtoolset-3 bash" on super), with the remaining performance impact of enabling OpenMP at between 5% and 6% then. It also mostly goes away (even for code generated by the old gcc) when running 2 threads/core. So I guess it's primarily worse code generation or/and cost of the more complicated addressing modes in the thread-safe code. Alexander
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