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A Mach *thread* belongs to exactly one [[task]], and is the means of execution.
The task supplies the resources.

Mach threads are implemented inside the [[kernel]], like in many operating
systems, unlike some user-level thread packages (aka "green" threads).

A thread (theoretically) runs concurrently with all the other threads of a
system.  If the system provides several processors, they can be used for
simultaneously running either several threads of the same task, or several
threads of different tasks.  <!-- This is called SMP; the processors use
*shared memory*.  --> [[!tag open_issue_documentation]] <!-- This needs a new
page, also covering Mach's `processor_set`s, and non-SMP, but still
multiprocessor systems.  --> (But this is currently not support in [[GNU
Mach|gnumach]].)

It is easy for the kernel to switch execution from one thread to another one
inside the same task: essentially, it only involves exchanging a few processor
registers' state.

Threads have scheduling parameters and maintain various statistics about
themselves.

On GNU/Hurd, APIs for Mach threads and thereabouts are provided by the
[[hurd/libthreads]] (cthreads), and [[libpthread]] (POSIX Threads) packages.

A task backing a thread is the basis for a [[UNIX process|unix/process]].