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(mysql.info)Thread packages

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Differences between different thread packages
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MySQL is very dependent on the thread package used. So when choosing a
good platform for MySQL, the thread package is very important.

There are at least three types of thread packages:

   * User threads in a single process. Thread switching is managed with
     alarms and the threads library manages all non-thread-safe
     functions with locks. Read, write and select operations are
     usually managed with a thread-specific select that switches to
     another thread if the running threads have to wait for data.  If
     the user thread packages are integrated in the standard libs
     (FreeBSD and BSDI threads) the thread package requires less
     overhead than thread packages that have to map all unsafe calls
     (MIT-pthreads, FSU Pthreads and RTS threads).  In some
     environments (for example, SCO), all system calls are thread safe
     so the mapping can be done very easily (FSU Pthreads on SCO).
     Downside: All mapped calls take a little time and it's quite
     tricky to be able to handle all situations. There are usually also
     some system calls that are not handled by the thread package (like
     MIT-pthreads and sockets). Thread scheduling isn't always optimal.

   * User threads in separate processes. Thread switching is done by the
     kernel and all data are shared between threads.  The thread package
     manages the standard thread calls to allow sharing data between
     threads.  LinuxThreads is using this method.  Downside: Lots of
     processes. Thread creating is slow. If one thread dies the rest
     are usually left hanging and you must kill them all before
     restarting. Thread switching is somewhat expensive.

   * Kernel threads. Thread switching is handled by the thread library
     or the kernel and is very fast. Everything is done in one process,
     but on some systems, `ps' may show the different threads. If one
     thread aborts, the whole process aborts. Most system calls are
     thread safe and should require very little overhead.  Solaris,
     HP-UX, AIX and OSF1 have kernel threads.

In some systems kernel threads are managed by integrating user level
threads in the system libraries. In such cases, the thread switching
can only be done by the thread library and the kernel isn't really
"thread aware".