Documentation/core-api/workqueue.rst

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=========
Workqueue
=========

:Date: September, 2010
:Author: Tejun Heo <tj@kernel.org>
:Author: Florian Mickler <florian@mickler.org>


Introduction
============

There are many cases where an asynchronous process execution context
is needed and the workqueue (wq) API is the most commonly used
mechanism for such cases.

When such an asynchronous execution context is needed, a work item
describing which function to execute is put on a queue.  An
independent thread serves as the asynchronous execution context.  The
queue is called workqueue and the thread is called worker.

While there are work items on the workqueue the worker executes the
functions associated with the work items one after the other.  When
there is no work item left on the workqueue the worker becomes idle.
When a new work item gets queued, the worker begins executing again.


Why Concurrency Managed Workqueue?
==================================

In the original wq implementation, a multi threaded (MT) wq had one
worker thread per CPU and a single threaded (ST) wq had one worker
thread system-wide.  A single MT wq needed to keep around the same
number of workers as the number of CPUs.  The kernel grew a lot of MT
wq users over the years and with the number of CPU cores continuously
rising, some systems saturated the default 32k PID space just booting
up.

Although MT wq wasted a lot of resource, the level of concurrency
provided was unsatisfactory.  The limitation was common to both ST and
MT wq albeit less severe on MT.  Each wq maintained its own separate
worker pool.  An MT wq could provide only one execution context per CPU
while an ST wq one for the whole system.  Work items had to compete for
those very limited execution contexts leading to various problems
including proneness to deadlocks around the single execution context.

The tension between the provided level of concurrency and resource
usage also forced its users to make unnecessary tradeoffs like libata
choosing to use ST wq for polling PIOs and accepting an unnecessary
limitation that no two polling PIOs can progress at the same time.  As
MT wq don't provide much better concurrency, users which require
higher level of concurrency, like async or fscache, had to implement
their own thread pool.

Concurrency Managed Workqueue (cmwq) is a reimplementation of wq with
focus on the following goals.

* Maintain compatibility with the original workqueue API.

* Use per-CPU unified worker pools shared by all wq to provide
  flexible level of concurrency on demand without wasting a lot of
  resource.

* Automatically regulate worker pool and level of concurrency so that
  the API users don't need to worry about such details.


The Design
==========

Annotation

Implementation Notes