@menu
* Things Which Might Be In The Next ANSI Standard::
-* Threading::
* Support For Unix::
* Customization Hooks for Users::
* Tools To Help Developers::
@end itemize
-
-@node Threading
-@comment node-name, next, previous, up
-@subsection Threading (a.k.a Multiprocessing)
-
-SBCL supports a fairly low-level threading interface that maps onto
-the host operating system's concept of threads or lightweight
-processes. This means that threads may take advantage of hardware
-multiprocessing on machines that have more than one CPU, but it does
-not allow Lisp control of the scheduler. This is found in the
-SB-THREAD package.
-
-This requires x86 and Linux kernel 2.6 or systems with NPTL backports.
-
-@subsubsection Special variables
-
-The interaction of special variables with multiple threads is mostly
-as one would expect, but users of other Lisps are warned that the
-behaviour of locally bound specials differs in places from what they
-may expect.
-
-@itemize
-@item
-global special values are visible across all threads;
-@item
-bindings (e.g. using LET) are local to the thread;
-@item
-initial values in a new thread are taken from the thread that created it.
-@end itemize
-
-@subsubsection Mutex support
-
-Mutexes are used for controlling access to a shared resource. One
-thread is allowed to hold the mutex, others which attempt to take it
-will be made to wait until it's free. Threads are woken in the order
-that they go to sleep.
-
-There isn't a timeout on mutex acquisition, but the usual WITH-TIMEOUT
-macro (which throws a TIMEOUT condition after n seconds) can be used
-if you want a bounded wait.
-
-@lisp
-(defpackage :demo (:use "CL" "SB-THREAD" "SB-EXT"))
-
-(in-package :demo)
-
-(defvar *a-mutex* (make-mutex :name "my lock"))
-
-(defun thread-fn ()
- (let ((id (current-thread-id)))
- (format t "Thread ~A running ~%" id)
- (with-mutex (*a-mutex*)
- (format t "Thread ~A got the lock~%" id)
- (sleep (random 5)))
- (format t "Thread ~A dropped lock, dying now~%" id)))
-
-(make-thread #'thread-fn)
-(make-thread #'thread-fn)
-
-@end lisp
-
-@subsubsection Waitqueue/condition variables
-
-These are based on the POSIX condition variable design, hence the
-annoyingly CL-conflicting name. For use when you want to check a
-condition and sleep until it's true. For example: you have a shared
-queue, a writer process checking ``queue is empty'' and one or more
-readers that need to know when ``queue is not empty''. It sounds
-simple, but is astonishingly easy to deadlock if another process runs
-when you weren't expecting it to.
-
-There are three components:
-
-@itemize
-@item the condition itself (not represented in code)
-@item the condition variable (a.k.a waitqueue) which proxies for it
-@item a lock to hold while testing the condition
-@end itemize
-
-Important stuff to be aware of:
-
-@itemize
-@item when calling condition-wait, you must hold the mutex. condition-wait will drop the mutex while it waits, and obtain it again before returning for whatever reason;
-
-@item likewise, you must be holding the mutex around calls to condition-notify;
-
-@item a process may return from condition-wait in several circumstances: it is not guaranteed that the underlying condition has become true. You must check that the resource is ready for whatever you want to do to it.
-
-@end itemize
-
-@lisp
-(defvar *buffer-queue* (make-waitqueue))
-(defvar *buffer-lock* (make-mutex :name "buffer lock"))
-
-(defvar *buffer* (list nil))
-
-(defun reader ()
- (with-mutex (*buffer-lock*)
- (loop
- (condition-wait *buffer-queue* *buffer-lock*)
- (loop
- (unless *buffer* (return))
- (let ((head (car *buffer*)))
- (setf *buffer* (cdr *buffer*))
- (format t "reader ~A woke, read ~A~%"
- (current-thread-id) head))))))
-
-(defun writer ()
- (loop
- (sleep (random 5))
- (with-mutex (*buffer-lock*)
- (let ((el (intern
- (string (code-char
- (+ (char-code #\A) (random 26)))))))
- (setf *buffer* (cons el *buffer*)))
- (condition-notify *buffer-queue*))))
-
-(make-thread #'writer)
-(make-thread #'reader)
-(make-thread #'reader)
-
-@end lisp
-
-@subsubsection Sessions/Debugging
-
-If the user has multiple views onto the same Lisp image (for example,
-using multiple terminals, or a windowing system, or network access)
-they are typically set up as multiple @dfn{sessions} such that each
-view has its own collection of foreground/background/stopped threads.
-A thread which wishes to create a new session can use
-@code{sb-thread:with-new-session} to remove itself from the current
-session (which it shares with its parent and siblings) and create a
-fresh one.
-# See also @code{sb-thread:make-listener-thread}.
-
-Within a single session, threads arbitrate between themselves for the
-user's attention. A thread may be in one of three notional states:
-foreground, background, or stopped. When a background process
-attempts to print a repl prompt or to enter the debugger, it will stop
-and print a message saying that it has stopped. The user at his
-leisure may switch to that thread to find out what it needs. If a
-background thread enters the debugger, selecting any restart will put
-it back into the background before it resumes. Arbitration for the
-input stream is managed by calls to @code{sb-thread:get-foreground}
-(which may block) and @code{sb-thread:release-foreground}.
-
-@code{sb-ext:quit} terminates all threads in the current session, but
-leaves other sessions running.
-
-
-@subsubsection Implementation (Linux x86)
-
-On Linux x86, threading is implemented using @code{clone()} and does
-not involve pthreads. This is not because there is anything wrong
-with pthreads @emph{per se}, but there is plenty wrong (from our
-perspective) with LinuxThreads. SBCL threads are mapped 1:1 onto
-Linux tasks which share a VM but nothing else - each has its own
-process id and can be seen in e.g. @command{ps} output.
-
-Per-thread local bindings for special variables is achieved using the
-%fs segment register to point to a per-thread storage area. This may
-cause interesting results if you link to foreign code that expects
-threading or creates new threads, and the thread library in question
-uses %fs in an incompatible way.
-
-Queues require the @code{sys_futex()} system call to be available:
-this is the reason for the NPTL requirement. We test at runtime that
-this system call exists.
-
-Garbage collection is done with the existing Conservative Generational
-GC. Allocation is done in small (typically 8k) regions: each thread
-has its own region so this involves no stopping. However, when a
-region fills, a lock must be obtained while another is allocated, and
-when a collection is required, all processes are stopped. This is
-achieved by sending them signals, which may make for interesting
-behaviour if they are interrupted in system calls. The streams
-interface is believed to handle the required system call restarting
-correctly, but this may be a consideration when making other blocking
-calls e.g. from foreign library code.
-
-Large amounts of the SBCL library have not been inspected for
-thread-safety. Some of the obviously unsafe areas have large locks
-around them, so compilation and fasl loading, for example, cannot be
-parallelized. Work is ongoing in this area.
-
-A new thread by default is created in the same POSIX process group and
-session as the thread it was created by. This has an impact on
-keyboard interrupt handling: pressing your terminal's intr key
-(typically @kbd{Control-C}) will interrupt all processes in the
-foreground process group, including Lisp threads that SBCL considers
-to be notionally `background'. This is undesirable, so background
-threads are set to ignore the SIGINT signal.
-
-@code{sb-thread:make-listener-thread} in addition to creating a new
-Lisp session makes a new POSIX session, so that pressing
-@kbd{Control-C} in one window will not interrupt another listener -
-this has been found to be embarrassing.
-
-
@node Support For Unix
@comment node-name, next, previous, up
@subsection Support For Unix
projects / sbcl.git / blobdiff