Are we reading the same document?

There is what is contained in the PPD; and there is what is contained in the source files.

For example: Here are a couple of comment blocks drawn from the latest Parrot sources: 

Create a local copy of the PMC if necessary. (No copy is made if it
is marked shared.) This includes workarounds for Parrot_clone() not
doing the Right Thing with subroutines (specifically, code segments
aren't preserved and it is difficult to do so as long as
Parrot_clone() depends on freezing).

[download]
Fixup a PMC to be sharable. Right now, reassigns the vtable to one
owned by some master interpreter, so the PMC can be safely reused
after thread death.

In the future the PMC returned might be different than the one
passed, e.g., if we need to reallocate the PMC in a different
interpreter.

[download]

How do you interpret those?

My interpretation is that

  1. Parrot generated code is not reentrant. It cannot be called from a multiple threads concurrently.
  2. Parrot data structures (PMCs) are allocated on a per thread (interpreter) basis, and have to be cloned (copied by freezing and thawing) in order to be shared.

That is, as best as I can divine, repeating all the mistakes of the Perl 5 implementation.

To understand the fundamental (and I do not use that word lightly here) importance of reentrancy to threading, read some wisdom from people who have done it. Pay particular attention to section 12.3.

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.
"Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."

In reply to Re^2: Slow evolution of Perl = Perl is a closed Word (NQP, parrot concurrency == Oh dear.) by BrowserUk
in thread Slow evolution of Perl = Perl is a closed Word by Anonymous Monk

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