NAME

docs/pdds/clip/pdd25_threads.pod - Parrot Threads

ABSTRACT

This document defines the requirements and implementation strategy for Parrot's threading model.

VERSION

$Revision: $

DEFINITIONS

Concurrency

DESCRIPTION

Description of the subject.

IMPLEMENTATION

[Excerpt from Perl 6 and Parrot Essentials to seed discussion.]

Threads are a means of splitting a process into multiple pieces that execute simultaneously. It's a relatively easy way to get some parallelism without too much work. Threads don't solve all the parallelism problems your program may have. Sometimes multiple processes on a single system, multiple processes on a cluster, or processes on multiple separate systems are better. But threads do present a good solution for many common cases.

All the resources in a threaded process are shared between threads. This is simultaneously the great strength and great weakness of threads. Easy sharing is fast sharing, making it far faster to exchange data between threads or access shared global data than to share data between processes on a single system or on multiple systems. Easy sharing is dangerous, though, since without some sort of coordination between threads it's easy to corrupt that shared data. And, because all the threads are contained within a single process, if any one of them fails for some reason the entire process, with all its threads, dies.

With a low-level language such as C, these issues are manageable. The core data types, integers, floats, and pointers are all small enough to be handled atomically. Composite data can be protected with mutexes, special structures that a thread can get exclusive access to. The composite data elements that need protecting can each have a mutex associated with them, and when a thread needs to touch the data it just acquires the mutex first. By default there's very little data that must be shared between threads, so it's relatively easy, barring program errors, to write thread-safe code if a little thought is given to the program structure.

Things aren't this easy for Parrot, unfortunately. A PMC, Parrot's native data type, is a complex structure, so we can't count on the hardware to provide us atomic access. That means Parrot has to provide atomicity itself, which is expensive. Getting and releasing a mutex isn't really that expensive in itself. It has been heavily optimized by platform vendors because they want threaded code to run quickly. It's not free, though, and when you consider that running flat-out Parrot does one PMC operation per 100 CPU cycles, even adding an additional 10 cycles per operation can slow Parrot down by 10%.

For any threading scheme, it's important that your program isn't hindered by the platform and libraries it uses. This is a common problem with writing threaded code in C, for example. Many libraries you might use aren't thread-safe, and if you aren't careful with them your program will crash. While we can't make low-level libraries any safer, we can make sure that Parrot itself won't be a danger. There is very little data shared between Parrot interpreters and threads, and access to all the shared data is done with coordinating mutexes. This is invisible to your program, and just makes sure that Parrot itself is thread-safe.

When you think about it, there are really three different threading models. In the first one, multiple threads have no interaction among themselves. This essentially does with threads the same thing that's done with processes. This works very well in Parrot, with the isolation between interpreters helping to reduce the overhead of this scheme. There's no possibility of data sharing at the user level, so there's no need to lock anything.

In the second threading model, multiple threads run and pass messages back and forth between each other. Parrot supports this as well, via the event mechanism. The event queues are thread-safe, so one thread can safely inject an event into another thread's event queue. This is similar to a multiple-process model of programming, except that communication between threads is much faster, and it's easier to pass around structured data.

In the third threading model, multiple threads run and share data between themselves. While Parrot can't guarantee that data at the user level remains consistent, it can make sure that access to shared data is at least safe. We do this with two mechanisms.

First, Parrot presents an advisory lock system to user code. Any piece of user code running in a thread can lock a variable. Any attempt to lock a variable that another thread has locked will block until the lock is released. Locking a variable only blocks other lock attempts. It does not block plain access. This may seem odd, but it's the same scheme used by threading systems that obey the POSIX thread standard, and has been well tested in practice.

Secondly, Parrot forces all shared PMCs to be marked as such, and all access to shared PMCs must first acquire that PMC's private lock. This is done by installing an alternate vtable for shared PMCs, one that acquires locks on all its parameters. These locks are held only for the duration of the vtable function, but ensure that the PMCs affected by the operation aren't altered by another thread while the vtable function is in progress.

ATTACHMENTS

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FOOTNOTES

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REFERENCES

None.