5.0005 threads (introduced with perl 5.005) by default shared all data and data structures. This turned out to be almost impossible to make thread-safe, since almost any perl-level "read" operation can actually end up modifying an SV (scalar value). For example:

my $x = 1;
print $x; # whoops $x has been modified: converted from int to string
$y = \$x; # whoops $x has been modified: its ref count has increased.
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To get this to work right would involve locking before just about any operation. So that threads model was abandoned.

There was a separate effort to allow fork emulation under Windows (which doesn't support fork()). This worked by collecting all perl's state into a single interpreter struct and allocating all SVs from per-interpreter pools. When fork() was called, the interpreter and all its SVs etc etc would be copied, and a new thread created which ran using that new data. So each "process" (actually just a thread) had its complete own copy of everything and could run independently without affecting any other threads; no (or very little) locking required. This first appeared with 5.6.

Then someone had the idea of exposing this interface at the perl level (rather than just via fork() under windows). Thus was born the threads.pm module, which did a similar thing to the fork (cloned the current state), but started the new thread with fresh code rather than running from the same point as the caller (a la fork()). Someone also added threads::shared, which via a mechanism similar to tying, allowed data structures to be shared across threads. These came out with 5.8.

Dave.

dave the m wrote:

5.0005 threads (introduced with perl 5.005) by default shared all data and data structures. This turned out to be almost impossible to make thread-safe, since almost any perl-level "read" operation can actually end up modifying an SV (scalar value).

Is this required by the language, or, evolving from your examples:

my $x = 1;
print $x  #don't care if modified!
our $package_X;
print $package_x; #Now I care!
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I wouldn't see a simple 'my' var as needing sharing, unless you take a reference to it..., package vars might be ideal for something like a Fortran COMMON section, if I remember what I'm talking about... i.e. GLOBAL vars/package (that would be shared).

But if I print $x, does it have to modify "$x", or -- rather why not leave it alone and have print modify a copy -- it's not like it is being stored somewhere that a shared implementation might expect to be able to access it's 'mutated form' ;-).

As for your 3rd line, referring to the ref count, that's definitely something the interpreter would need to track, but wouldn't be hard to implement on the x86 as, as long as the counter is arch-word (32/64bit) aligned, an inc/dec operation is atomic.

The thing that is annoying about the current model is, from my understanding, the limitation on having to pre-declare something as shared or not -- which would, it seems, preclude using it with object oriented programming where specific objects could have global state (and need locking in the presence of multiple writers) -- but not multiple readers.

But the good news, as I understand you saying, is that the current code uses native OS threads -- it's just that they don't share much [if any] data...that's slightly better than I thought it might be given that under linux today, a fork-exec you can choose multiple levels of sharing and code segments of compiled programs can automatically share the same code memory (presuming they weren't built statically).

Thanks for the info....

Is this required by the language

There's nothing in the language that precludes a 5.0005-style threading implementation. The difficulty was in retrospectively trying to make the existing implementation thread-safe, where it had never been designed for that possibility. This is one of the (many) reasons why it was concluded that a complete from-the-ground-up rewrite of the perl interpreter was required, i.e. perl6.

The main drawbacks of the ithreads model are: that cloning the existing interpreter when creating a new thread is slow; that it uses lots of memory, since the new interpreter doesn't make any use of the OS facilities that a fork() would, of sharing memory by default with copy-on-write pages; and that having shared variables is slow, clunky and is memory-heavy.

Dave.

Perl does have real threads. The rest is irrelevant history not worth digging over.