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freebsd-dev/share/man/man4/aio.4
John Baldwin f3215338ef Refactor the AIO subsystem to permit file-type-specific handling and 
improve cancellation robustness.

Introduce a new file operation, fo_aio_queue, which is responsible for
queueing and completing an asynchronous I/O request for a given file.
The AIO subystem now exports library of routines to manipulate AIO
requests as well as the ability to run a handler function in the
"default" pool of AIO daemons to service a request.

A default implementation for file types which do not include an
fo_aio_queue method queues requests to the "default" pool invoking the
fo_read or fo_write methods as before.

The AIO subsystem permits file types to install a private "cancel"
routine when a request is queued to permit safe dequeueing and cleanup
of cancelled requests.

Sockets now use their own pool of AIO daemons and service per-socket
requests in FIFO order.  Socket requests will not block indefinitely
permitting timely cancellation of all requests.

Due to the now-tight coupling of the AIO subsystem with file types,
the AIO subsystem is now a standard part of all kernels.  The VFS_AIO
kernel option and aio.ko module are gone.

Many file types may block indefinitely in their fo_read or fo_write
callbacks resulting in a hung AIO daemon.  This can result in hung
user processes (when processes attempt to cancel all outstanding
requests during exit) or a hung system.  To protect against this, AIO
requests are only permitted for known "safe" files by default.  AIO
requests for all file types can be enabled by setting the new
vfs.aio.enable_usafe sysctl to a non-zero value.  The AIO tests have
been updated to skip operations on unsafe file types if the sysctl is
zero.

Currently, AIO requests on sockets and raw disks are considered safe
and are enabled by default.  aio_mlock() is also enabled by default.

Reviewed by:	cem, jilles
Discussed with:	kib (earlier version)
Sponsored by:	Chelsio Communications
Differential Revision:	https://reviews.freebsd.org/D5289
2016-03-01 18:12:14 +00:00

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.\"-
.\" Copyright (c) 2002 Dag-Erling Coïdan Smørgrav
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.\" $FreeBSD$
.\"
.Dd March 1, 2016
.Dt AIO 4
.Os
.Sh NAME
.Nm aio
.Nd asynchronous I/O
.Sh DESCRIPTION
The
.Nm
facility provides system calls for asynchronous I/O.
However, asynchronous I/O operations are only enabled for certain file
types by default.
Asynchronous I/O operations for other file types may block an AIO daemon
indefinitely resulting in process and/or system hangs.
Asynchronous I/O operations can be enabled for all file types by setting
the
.Va vfs.aio.enable_unsafe
sysctl node to a non-zero value.
.Pp
Asynchronous I/O operations on sockets and raw disk devices do not block
indefinitely and are enabled by default.
.Pp
The
.Nm
facility uses kernel processes
(also known as AIO daemons)
to service most asynchronous I/O requests.
These processes are grouped into pools containing a variable number of
processes.
Each pool will add or remove processes to the pool based on load.
Pools can be configured by sysctl nodes that define the minimum
and maximum number of processes as well as the amount of time an idle
process will wait before exiting.
.Pp
One pool of AIO daemons is used to service asynchronous I/O requests for
sockets.
These processes are named
.Dq soaiod<N> .
The following sysctl nodes are used with this pool:
.Bl -tag -width indent
.It Va kern.ipc.aio.num_procs
The current number of processes in the pool.
.It Va kern.ipc.aio.target_procs
The minimum number of processes that should be present in the pool.
.It Va kern.ipc.aio.max_procs
The maximum number of processes permitted in the pool.
.It Va kern.ipc.aio.lifetime
The amount of time a process is permitted to idle in clock ticks.
If a process is idle for this amount of time and there are more processes
in the pool than the target minimum,
the process will exit.
.El
.Pp
A second pool of AIO daemons is used to service all other asynchronous I/O
requests except for I/O requests to raw disks.
These processes are named
.Dq aiod<N> .
The following sysctl nodes are used with this pool:
.Bl -tag -width indent
.It Va vfs.aio.num_aio_procs
The current number of processes in the pool.
.It Va vfs.aio.target_aio_procs
The minimum number of processes that should be present in the pool.
.It Va vfs.aio.max_aio_procs
The maximum number of processes permitted in the pool.
.It Va vfs.aio.aiod_lifetime
The amount of time a process is permitted to idle in clock ticks.
If a process is idle for this amount of time and there are more processes
in the pool than the target minimum,
the process will exit.
.El
.Pp
Asynchronous I/O requests for raw disks are queued directly to the disk
device layer after temporarily wiring the user pages associated with the
request.
These requests are not serviced by any of the AIO daemon pools.
.Pp
Several limits on the number of asynchronous I/O requests are imposed both
system-wide and per-process.
These limits are configured via the following sysctls:
.Bl -tag -width indent
.It Va vfs.aio.max_buf_aio
The maximum number of queued asynchronous I/O requests for raw disks permitted
for a single process.
Asynchronous I/O requests that have completed but whose status has not been
retrieved via
.Xr aio_return 2
or
.Xr aio_waitcomplete 2
are not counted against this limit.
.It Va vfs.aio.num_buf_aio
The number of queued asynchronous I/O requests for raw disks system-wide.
.It Va vfs.aio.max_aio_queue_per_proc
The maximum number of asynchronous I/O requests for a single process
serviced concurrently by the default AIO daemon pool.
.It Va vfs.aio.max_aio_per_proc
The maximum number of outstanding asynchronous I/O requests permitted for a
single process.
This includes requests that have not been serviced,
requests currently being serviced,
and requests that have completed but whose status has not been retrieved via
.Xr aio_return 2
or
.Xr aio_waitcomplete 2 .
.It Va vfs.aio.num_queue_count
The number of outstanding asynchronous I/O requests system-wide.
.It Va vfs.aio.max_aio_queue
The maximum number of outstanding asynchronous I/O requests permitted
system-wide.
.El
.Sh SEE ALSO
.Xr aio_cancel 2 ,
.Xr aio_error 2 ,
.Xr aio_read 2 ,
.Xr aio_return 2 ,
.Xr aio_suspend 2 ,
.Xr aio_waitcomplete 2 ,
.Xr aio_write 2 ,
.Xr lio_listio 2 ,
.Xr sysctl 8
.Sh HISTORY
The
.Nm
facility appeared as a kernel option in
.Fx 3.0 .
The
.Nm
kernel module appeared in
.Fx 5.0 .
The
.Nm
facility was integrated into all kernels in
.Fx 11.0 .
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