On Linux threads, from
pauillac.inria.fr
(Basically, Linux threads are a simple hack of unix
processes...)
K. Internals of LinuxThreads
K.1: What is the implementation model for LinuxThreads?
LinuxThreads follows the so-called "one-to-one" model: each thread is actually a separate process in the kernel. The kernel scheduler takes care of scheduling the threads, just like it schedules regular processes. The threads are created with the Linux clone() system call, which is a generalization of fork() allowing the new process to share the memory space, file descriptors, and signal handlers of the parent.
Advantages of the "one-to-one" model include:
minimal overhead on CPU-intensive multiprocessing (with about one thread per processor);
minimal overhead on I/O operations;
a simple and robust implementation (the kernel scheduler does most of the hard work for us).
The main disadvantage is more expensive context switches on mutex and condition operations, which must go through the kernel. This is mitigated by the fact that context switches in the Linux kernel are pretty efficient.
K.2: Have you considered other implementation models?
There are basically two other models. The "many-to-one" model relies on a user-level scheduler that context-switches between the threads entirely in user code; viewed from the kernel, there is only one process running. This model is completely out of the question for me, since it does not take advantage of multiprocessors, and require unholy magic to handle blocking I/O operations properly. There are several user-level thread libraries available for Linux, but I found all of them deficient in functionality, performance, and/or robustness.
The "many-to-many" model combines both kernel-level and user-level scheduling: several kernel-level threads run concurrently, each executing a user-level scheduler that selects between user threads. Most commercial Unix systems (Solaris, Digital Unix, IRIX) implement POSIX threads this way. This model combines the advantages of both the "many-to-one" and the "one-to-one" model, and is attractive because it avoids the worst-case behaviors of both models -- especially on kernels where context switches are expensive, such as Digital Unix. Unfortunately, it is pretty complex to implement, and requires kernel support which Linux does not provide. Linus Torvalds and other Linux kernel developers have always been pushing the "one-to-one" model in the name of overall simplicity, and are doing a pretty good job of making kernel-level context switches between threads efficient. LinuxThreads is just following the general direction they set. |
