+<sect2><title>Threading (a.k.a Multiprocessing)</>
+
+<para>&SBCL; (as of version 0.8.3, on Linux x86 only) supports a
+fairly low-level threading interface that maps onto the host operating
+system's concept of threads or lightweight processes.
+
+<sect3><title>Lisp-level view</title>
+
+<para>A rudimentary interface to creating and managing multiple threads
+can be found in the <literal>sb-thread</literal> package. This is
+intended for public consumption, so look at the exported symbols and
+their documentation strings.
+
+<para>Dynamic bindings to symbols are per-thread. Signal handlers
+are per-thread.
+
+<para><function>sb-ext:quit</function> exits the current thread, not
+necessarily the whole environment. The environment will be shut down
+when the last thread exits.
+
+<para>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.
+
+<para>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 `sessions' such that each
+view has its own collection of foreground/background/stopped threads.
+<function>sb-thread:make-listener-thread</function> can be used to
+start a new thread in its own `session'.
+
+<para>Mutexes and condition variables are available for
+managing access to shared data: see
+
+<itemizedlist>
+<listitem>
+<programlisting>(apropos "mutex" :sb-thread)</programlisting>
+<listitem>
+<programlisting>(apropos "condition" :sb-thread)</programlisting>
+<listitem> <para>and the <structname>waitqueue</structname> structure
+</para>
+</listitem>
+</itemizedlist>
+
+and poke around in their documentation strings.
+
+<sect3><title>Implementation (Linux x86)</title>
+
+<para>On Linux x86, this is implemented using
+<function>clone()</function> and does not involve pthreads. This is
+not because there is anything wrong with pthreads <emphasis>per
+se</emphasis>, 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</command> output.
+
+<para>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.
+
+<para>Threads waiting on queues (e.g. for locks or condition
+variables) are put to sleep using <function>sigtimedwait()</function>
+and woken with SIGCONT.
+
+<para>&SBCL; at present will alway have at least two tasks running as
+seen from Linux: when the first process has done startup
+initialization (mapping files in place, installing signal handlers
+etc) it creates a new thread to run the Lisp startup and initial
+listener. The original thread stays around to reap dead subthreads
+and deallocate their resources (e.g. stacks) when they exit.
+
+<para>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.
+
+<para>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.
+
+<para>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 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. Arbitration for the input stream
+is managed by locking on <varname>sb-thread::*session-lock*</varname>
+
+<para>A thread can be created in a new Lisp 'session' (new terminal or
+window) using <function>sb-thread:make-listener-thread</function>.
+These sessions map directly onto POSIX sessions, so that pressing
+Control-C in the wrong window will not interrupt them - this has been
+found to be embarrassing.
+
+<sect2><title>Support For Unix</>
projects / sbcl.git / blobdiff