What is the difference between different thread packages?

MySQL is very dependent on the used thread package. 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 not thread- safe functions with locks. Read, write and select 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-threads, FSU-threads and RTS threads). In some environments (for example SCO), all system calls are thread- safe so the mapping can be done very easily (FSU-threads 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-threads and sockets). Thread scheduling isn't always optimal.
• User threads in separate processes. Thread switching is done by the kernel and all data is 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 one has to 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 'ps' may on some systems show the different threads. If one thread aborts the whole process aborts. Most system calls are threadsafe and should require very little overhead. Solaris, HP/UX, AIX and OSF1 has 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'.

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