In order to adapt to the Apache 2.0 threads architecture (for threaded MPMs), mod_perl 2.0 needs to use thread-safe Perl interpreters, also known as ithreads (interpreter threads). This mechanism is enabled at compile time and ensures that each Perl interpreter instance is reentrant—that is, multiple Perl interpreters can be used concurrently within the same process without locking, as each instance has its own copy of any mutable data (symbol tables, stacks, etc.). This of course requires that each Perl interpreter instance is accessed by only one thread at any given time.
The first mod_perl generation has only a single PerlInterpreter, which is constructed by the parent process, then inherited across the forks to child processes. mod_perl 2.0 has a configurable number of PerlInterpreters and two classes of interpreters, parent and clone. A parent is like in mod_perl 1.0, where the main interpreter created at startup time compiles any preloaded Perl code. A clone is created from the parent using the Perl API perl_clone( ) function. At request time, parent interpreters are used only for making more clones, as the clones are the interpreters that actually handle requests. Care is taken by Perl to copy only mutable data, which means that no runtime locking is required and read-only data such as the syntax tree is shared from the parent, which should reduce the overall mod_perl memory footprint.
Rather than creating a PerlInterperter for each thread, by default mod_perl creates a pool of interpreters. The pool mechanism helps cut down memory usage a great deal. As already mentioned, the syntax tree is shared between all cloned interpreters. If your server is serving more than just mod_perl requests, having a smaller number of PerlInterpreters than the number of threads will clearly cut down on memory usage. Finally, perhaps the biggest win is memory reuse: as calls are made into Perl subroutines, memory allocations are made for variables when they are used for the first time. Subsequent use of variables may allocate more memory; e.g., if a scalar variable needs to hold a longer string than it did before, or an array has new elements added. As an optimization, Perl hangs onto these allocations, even though their values go out of scope. mod_perl 2.0 has much better control over which PerlInterpreters are used for incoming requests. The interpreters are stored in two linked lists, one for available interpreters and another for busy ones. When needed to handle a request, one interpreter is taken from the head of the available list, and it's put back at the head of the same list when it's done. This means that if, for example, you have ten interpreters configured to be cloned at startup time, but no more than five are ever used concurrently, those five continue to reuse Perl's allocations, while the other five remain much smaller, but ready to go if the need arises.
The interpreters pool mechanism has been abstracted into an API known as tipool (thread item pool). This pool, currently used to manage a pool of PerlInterpreter objects, can be used to manage any data structure in which you wish to have a smaller number of items than the number of configured threads.
It's important to notice that the Perl ithreads implementation ensures that Perl code is thread-safe, at least with respect to the Apache threads in which it is running. However, it does not ensure that functions and extensions that call into third-party C/C++ libraries are thread-safe. In the case of non-thread-safe extensions, if it is not possible to fix those routines, care needs to be taken to serialize calls into such functions (either at the XS or Perl level). See Perl 5.8.0's perlthrtut manpage.
Note that while Perl data is thread-private unless explicitly shared and threads themselves are separate execution threads, the threads can affect process-scope state, affecting all the threads. For example, if one thread does chdir("/tmp"), the current working directory of all threads is now /tmp. While each thread can correct its current working directory by storing the original value, there are functions whose process-scope changes cannot be undone. For example, chroot( ) changes the root directory of all threads, and this change is not reversible. Refer to the perlthrtut manpage for more information.