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Old 19th October 2020, 21:09   #1  |  Link
Selur
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Should x264 be non-deterministic when '--non-deterministic' isn't used?

Over at videohelp, there is someone you get's non deterministic output even when he isn't using '--non-deterministic'.

Some folks say that:
a. x264 is always non-deterministic as soon as multithreading is used.
b. x264 is always non-deterministic as soon as VBV is used.
c. x264 is always non-deterministic as soon as VBV&multithreading are used together.
I though that x264 should be deterministic as long as '--non-deterministic' isn't used and if it isn't there is a bug?
Also when a., b. or c. is correct, what is the use of '--non-deterministic' ?

Can someone knowing the code answer this?


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Old 20th October 2020, 04:22   #2  |  Link
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I always believed a was true. Since the relative performance between threads is non-deterministic the code would need to wait for unencoded blocks it expected to be complete but I don't believe it does.

Of course, I am not someone knowing the code.
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Old 20th October 2020, 04:31   #3  |  Link
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According to the x264 mailing list from 2015, vbv+multithreading is intentionally non-deterministic.

https://mailman.videolan.org/piperma...il/011035.html
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Old 20th October 2020, 05:09   #4  |  Link
Asmodian
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There isn't much in the way of why there.

Is it only non-deterministic for case c then?
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Old 20th October 2020, 18:27   #5  |  Link
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Quote:
Originally Posted by Asmodian View Post
There isn't much in the way of why there.

Is it only non-deterministic for case c then?
Deterministic has a perf hit and doesn't offer any compression efficiency advantage. Non-deterministic really should be the default, with deterministic only used when pixel-exact technical comparisons are being done.

I've done HUGE amounts of x264 tuning over the last 10+ years, and I don't think I've ever had a need to use deterministic mode.
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Old 20th October 2020, 19:05   #6  |  Link
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Especially if running witht ou '-non-deterministic' doesn't really make x265 deterministic.
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Old 20th October 2020, 20:25   #7  |  Link
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Deterministic has a perf hit and doesn't offer any compression efficiency advantage. Non-deterministic really should be the default, with deterministic only used when pixel-exact technical comparisons are being done.
So it is only c. and because a. is not true --non-deterministic should always be used to avoid the issue I mentioned?

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I've done HUGE amounts of x264 tuning over the last 10+ years, and I don't think I've ever had a need to use deterministic mode.
I do have --non-deterministic in all my command lines but I am still curious.
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