[FFmpeg-devel] [PATCHv3 1/3] lavu/rand: add 64 bit random number generator

Ronald S. Bultje rsbultje at gmail.com
Tue Mar 15 15:56:34 CET 2016


On Tue, Mar 15, 2016 at 10:29 AM, Derek Buitenhuis <
derek.buitenhuis at gmail.com> wrote:

> On 3/15/2016 4:46 AM, Ganesh Ajjanagadde wrote:
> > Concretely, it is nearly as fast as av_lfg_get (which only returns 32
> bits),
> > and has a much smaller cache (128 bits). Thus, the timings should also
> > be more stable.
> >
> > This is needed because av_lfg_get<<32 | av_lfg_get is far slower, and
> > likely less random as measured by BigCrush - most LFG's perform
> > quite poorly on the BigCrush suite:
> > http://www6.tw.freebsd.org/distfiles/testu01.pdf.
> > In particular, FFmpeg's seems to be Ran055 in the paper, see pg31.
> This doesn't explain how it benefits our uses of PRNGs. We are a set of
> multimedia
> libraries, not a general utilities library. Most of our use is for noise
> generation
> or unit tests, neither of which benefits much/at all from this. For
> encrypted streams,
> we pass in keys via options (e.g. MP4 CENC), mostly.

Might be related to aacenc? But yes, we need to know overall speed gain of
some useful end user feature before/after this.


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