Why is Perl usually installed without thread support?

Question

Perlbrew installs per default Perl without thread support. Is this just a legacy habit or could a Perl installation with thread support generate problems?

Answer 1

Compiling Perl with thread supports adds a lot of overhead due to all the locking, even if you don't use threads in your program. I measured about 15% overhead for a simple benchmark just by using Perl compiled with threads support.

Answer 2

Because it isn't robust and performant enough to be the default.

Besides that, you have to consider CPAN. There are too many modules written in C without concern for threads.

I say this with all due love and respect as an ex-Perl6 / Parrot developer. Perl5 wasn't designed from the ground up with threading in mind (at least when I was involved in the community).

At this point, I don't think enough people care to change it. The future was going to be Perl6, and Parrot had threads very early. To destabilize Perl 5 at this point in its lifecycle is probably questionable.

It isn't like Perl is unique in this, Linux was the same way for a long time (as-in there was a big kernel lock that had to be dealt with). Most projects start like that, but some take it further than others before addressing it.

Answer 3

As @steffen-ultisch said, it is a performance issue.

But, if one so wish, it is possible to easily install Perl both with and without threads, so you can use the version more proper to a given script.

The perlbrew installation, say for Perl 5.22.1, is:

perlbrew install-multiple 5.22.1 --both=thread