docs/multiple-iothreads.txt: add documentation on IOThread programming

Copyright (c) 2014 Red Hat Inc.

This work is licensed under the terms of the GNU GPL, version 2 or later.  See

the COPYING file in the top-level directory.

This document explains the IOThread feature and how to write code that runs

outside the QEMU global mutex.

The main loop and IOThreads

---------------------------

QEMU is an event-driven program that can do several things at once using an

event loop.  The VNC server and the QMP monitor are both processed from the

same event loop, which monitors their file descriptors until they become

readable and then invokes a callback.

The default event loop is called the main loop (see main-loop.c).  It is

possible to create additional event loop threads using -object

iothread,id=my-iothread.

Side note: The main loop and IOThread are both event loops but their code is

not shared completely.  Sometimes it is useful to remember that although they

are conceptually similar they are currently not interchangeable.

Why IOThreads are useful

------------------------

IOThreads allow the user to control the placement of work.  The main loop is a

scalability bottleneck on hosts with many CPUs.  Work can be spread across

several IOThreads instead of just one main loop.  When set up correctly this

can improve I/O latency and reduce jitter seen by the guest.

The main loop is also deeply associated with the QEMU global mutex, which is a

scalability bottleneck in itself.  vCPU threads and the main loop use the QEMU

global mutex to serialize execution of QEMU code.  This mutex is necessary

because a lot of QEMU's code historically was not thread-safe.

The fact that all I/O processing is done in a single main loop and that the

QEMU global mutex is contended by all vCPU threads and the main loop explain

why it is desirable to place work into IOThreads.

The experimental virtio-blk data-plane implementation has been benchmarked and

shows these effects:

ftp://public.dhe.ibm.com/linux/pdfs/KVM_Virtualized_IO_Performance_Paper.pdf

How to program for IOThreads

----------------------------

The main difference between legacy code and new code that can run in an

IOThread is dealing explicitly with the event loop object, AioContext

(see include/block/aio.h).  Code that only works in the main loop

implicitly uses the main loop's AioContext.  Code that supports running

in IOThreads must be aware of its AioContext.

AioContext supports the following services:

 * File descriptor monitoring (read/write/error on POSIX hosts)

 * Event notifiers (inter-thread signalling)

 * Timers

 * Bottom Halves (BH) deferred callbacks

There are several old APIs that use the main loop AioContext:

 * LEGACY qemu_aio_set_fd_handler() - monitor a file descriptor

 * LEGACY qemu_aio_set_event_notifier() - monitor an event notifier

 * LEGACY timer_new_ms() - create a timer

 * LEGACY qemu_bh_new() - create a BH

 * LEGACY qemu_aio_wait() - run an event loop iteration

Since they implicitly work on the main loop they cannot be used in code that

runs in an IOThread.  They might cause a crash or deadlock if called from an

IOThread since the QEMU global mutex is not held.

Instead, use the AioContext functions directly (see include/block/aio.h):

 * aio_set_fd_handler() - monitor a file descriptor

 * aio_set_event_notifier() - monitor an event notifier

 * aio_timer_new() - create a timer

 * aio_bh_new() - create a BH

 * aio_poll() - run an event loop iteration

The AioContext can be obtained from the IOThread using

iothread_get_aio_context() or for the main loop using qemu_get_aio_context().

Code that takes an AioContext argument works both in IOThreads or the main

loop, depending on which AioContext instance the caller passes in.

How to synchronize with an IOThread

-----------------------------------

AioContext is not thread-safe so some rules must be followed when using file

descriptors, event notifiers, timers, or BHs across threads:

1. AioContext functions can be called safely from file descriptor, event

notifier, timer, or BH callbacks invoked by the AioContext.  No locking is

necessary.

2. Other threads wishing to access the AioContext must use

aio_context_acquire()/aio_context_release() for mutual exclusion.  Once the

context is acquired no other thread can access it or run event loop iterations

in this AioContext.

aio_context_acquire()/aio_context_release() calls may be nested.  This

means you can call them if you're not sure whether #1 applies.

There is currently no lock ordering rule if a thread needs to acquire multiple

AioContexts simultaneously.  Therefore, it is only safe for code holding the

QEMU global mutex to acquire other AioContexts.

Side note: the best way to schedule a function call across threads is to create

a BH in the target AioContext beforehand and then call qemu_bh_schedule().  No

acquire/release or locking is needed for the qemu_bh_schedule() call.  But be

sure to acquire the AioContext for aio_bh_new() if necessary.

The relationship between AioContext and the block layer

-------------------------------------------------------

The AioContext originates from the QEMU block layer because it provides a

scoped way of running event loop iterations until all work is done.  This

feature is used to complete all in-flight block I/O requests (see

bdrv_drain_all()).  Nowadays AioContext is a generic event loop that can be

used by any QEMU subsystem.

The block layer has support for AioContext integrated.  Each BlockDriverState

is associated with an AioContext using bdrv_set_aio_context() and

bdrv_get_aio_context().  This allows block layer code to process I/O inside the

right AioContext.  Other subsystems may wish to follow a similar approach.

Block layer code must therefore expect to run in an IOThread and avoid using

old APIs that implicitly use the main loop.  See the "How to program for

IOThreads" above for information on how to do that.

If main loop code such as a QMP function wishes to access a BlockDriverState it

must first call aio_context_acquire(bdrv_get_aio_context(bs)) to ensure the

IOThread does not run in parallel.

Long-running jobs (usually in the form of coroutines) are best scheduled in the

BlockDriverState's AioContext to avoid the need to acquire/release around each

bdrv_*() call.  Be aware that there is currently no mechanism to get notified

when bdrv_set_aio_context() moves this BlockDriverState to a different

AioContext (see bdrv_detach_aio_context()/bdrv_attach_aio_context()), so you

may need to add this if you want to support long-running jobs.