Clarify status of RNG being thread-safe #46280
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stdlib/Random/docs/src/index.md
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| In a multi-threaded program, you should generally use different RNG objects from different threads | ||
| or tasks in order to be thread-safe. However, the default RNG is thread-safe as of Julia 1.3 | ||
| (using a per-thread RNG up to version 1.6, and per-task thereafter). | ||
| In a multi-threaded program, you should generally use different RNG objects from different threads or tasks in order to be thread-safe and deterministic (if given a specific seed). However, the default global RNG is thread-safe as of Julia 1.3 (using a per-thread RNG up to version 1.6, and per-task thereafter). The global RNG refers to per-thread RNG instances; If you instantiate a non-global RNG, you should create a new RNG instance for each thread to avoid possible data-races. |
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The
The global RNG refers to per-thread RNG instances; If you instantiate a non-global RNG, you should create a new RNG instance for each thread to avoid possible data-races.
seems like it is just saying the same thing as the earlier stuff in the paragraph again?
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Here was my confusion so maybe there's a clearer way to get this across. Per the prior docs:
- The global RNG is thread safe
- Xoshiro is used as the global RNG
- Therefore, if I use a newly instantiated Xoshiro (call it
X), I can have code in multiple threads referenceXwithout issues.
#3 is not correct because the Global RNG is constructed in a way that makes it thread safe and that's the point that I don't think comes across previously and what
The global RNG refers to per-thread RNG instances; If you instantiate a non-global RNG, you should create a new RNG instance for each thread to avoid possible data-races.
Is trying to differentiate from what was stated before.
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I tried to clarify in the latest commit
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| The default global RNG is thread-safe as of Julia 1.3 (using a per-thread RNG up to version 1.6, and per-task thereafter). The global RNG refers to per-thread RNG instances; If you instantiate a non-global RNG, you should create a new RNG instance for each thread to avoid possible data-races. That is, even if using the `Xoshiro`, the manually created RNG must be done per thread (the global RNG is thread-safe not via `Xoshiro itself, but by the details of the global RNG handling). |
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I find this new wording too confusing.
The first point is that we do not talk about the "global RNG", at this point in the page we didn't define it. Nowadays we should only talk about the "default RNG" in the manual, which is the RNG used by default when no explicit RNG is passed.
Otherwise, it looks like all the information was already there. If you want to insist on your point, I would just add to the already existing paragraph something along the lines of:
do note that while
Xoshiroand the default RNG both implement the same RNG algorithm, the default RNG is thread-safe butXoshirois not.
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Here's what I wrote on Zulip about this:
Maybe something from that comment can be used to clarify the docs. |
I found the documentation about the thread-safe nature of the default RNG to be confusing. I understand now that the thread-safe nature is due to the construction of the
DEFAULT_RNGand not a characteristic of, e.g.Xoshiro.See this short discussion on Zulip for more where this change in documentation was proposed: https://julialang.zulipchat.com/#narrow/stream/274208-helpdesk-.28published.29/topic/Threaded.20.60randn.60.20and.20benchmarking.20error/near/291960004