[ruby-core:88151] [Ruby trunk Feature#14717] [PATCH] thread: allow disabling preempt

Issue #14717 has been updated by Eregon (Benoit Daloze).


normalperson (Eric Wong) wrote:
>  Giving control to other threads still happens with Thread.pass
>  or anything which currently releases GVL (including IO#read,
>  File.open, etc...).

At that point I wouldn't call them threads anymore,
I think threads usually imply preemption (not just on blocking actions but also timer-based).

Also, we'd be mixing two very different kinds of Threads, preemptive threads and auto-fibers.

>  For platforms without GVL, it can be a no-op.  I understand why
>  this can be a bad feature from that perspective (I hate GVL, too).
>  I mainly wanted this feature to give equivalance for
>  proposed auto-Fiber [Feature #13618] behavior.

No, because if it's a no-op it has different semantics.
Other Ruby implementations without GVL will want to support something like auto-fibers too, and have compatible API.
Designing this way seems to make it a MRI/GVL-only feature which is wrong.

However, I think making preemption an argument of the constructor, or a special Thread class/factory method
would help implementing it on Ruby implementations without a GVL.

Still, it's unclear to me how multiple "non-preemptive threads" would work (when there is no GVL).
I think auto-fibers need to be at a different level than Thread, so we can reason about such questions.
Being at Fiber level clarifies this: all fibers of a Thread never execute in parallel, but fibers of different Threads can.

I think it wouldn't make any sense in a Ruby implementation without GVL to have "a running non-preemptive thread/auto-fiber" prevent all other (preemptive) Threads to execute concurrently.

----------------------------------------
Feature #14717: [PATCH] thread: allow disabling preempt
https://bugs.ruby-lang.org/issues/14717#change-73176

* Author: normalperson (Eric Wong)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
In some cases, it may be beneficial to disable preemptibility to
have more predictable behavior than increasing Thread#priority.
Threads with preempt disabled will run until they either:

    a) encounter a thread blocking region
    b) call Thread.pass

This allows native threads to act cooperatively without
being interrupted by other threads.

Thread#preemptible? => true or false
Thread#preemptible = (true|false)


I plan to implement timer-based switching to "auto-fiber/threadlet/thriber/whatever-name"
[Feature #13618] to minimize migration costs from Thread.

However, I think based on the discussion in [Feature #13618]; having
predictability of non-preemptible threads is beneficial.  So implement it
for native Thread for now.

I will unify the behavior of [Feature #13618] with existing Thread.
I think green/native threads can be unified under Thread class similar to
how Fixnum/Bignum are both "Integer".


---Files--------------------------------
0001-thread-allow-disabling-preempt.patch (4.16 KB)


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