Mac OS X Manual Page For getiopolicy

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getiopolicy_np(3) BSD Library Functions Manual getiopolicy_np(3)

NAME
getiopolicy_np, setiopolicy_np -- manipulate the I/O policy of a process
or thread

LIBRARY
Standard C Library (libc, -lc)

SYNOPSIS
#include <sys/resource.h>

int
getiopolicy_np(int iotype, int scope);

int
setiopolicy_np(int iotype, int scope, int policy);

DESCRIPTION
The getiopolicy_np() and setiopolicy_np() functions are provided to get
or set the I/O policy of the current process or the current thread. The
policy of the I/O of the given type iotype can be get or set for the
given scope.

The I/O type is specified in the argument iotype. The currently sup-ported supported
ported I/O type is IOPOL_TYPE_DISK, which means the I/O policy for I/Os
to local disks can be get or set. I/Os to local disks are I/Os sent to
the media without going through a network, including I/Os to internal and
external hard drives, optical media in internal and external drives,
flash drives, floppy disks, ram disks, and mounted disk images which
reside on these media, but not including remote volumes mounted through
networks (AFP, SMB, NFS, etc) or disk images residing on remote volumes.

The scope that the I/O policy takes effect is specified in the argument
scope as follows:

IOPOL_SCOPE_PROCESS The I/O policy of all I/Os issued by the current
process is get or set.

IOPOL_SCOPE_THREAD The I/O policy of all I/Os issued by the current
thread is get or set.

In getiopolicy_np(), the I/O policy of the given I/O type and scope is
returned. In setiopolicy_np(), the argument policy is an integer which
contains the new I/O policy to be set for the given I/O type and scope.
The I/O policy can have the following values:

IOPOL_DEFAULT This is the default I/O policy for the first process and
every new created thread.

IOPOL_NORMAL I/Os with NORMAL policy are called NORMAL I/Os. They
are handled by the system using best-effort.

IOPOL_THROTTLE I/Os with THROTTLE policy are called THROTTLE I/Os. If
a THROTTLE I/O request occurs within a small time window
(usually a fraction of a second) of another NORMAL I/O
request, the thread that issues the THROTTLE I/O is
forced to sleep for a certain interval. This slows down
the thread that issues the THROTTLE I/O so that NORMAL
I/Os can utilize most of the disk I/O bandwidth.

IOPOL_PASSIVE The PASSIVE I/Os are a special type of NORMAL I/O that
are processed the same as NORMAL I/Os but are ignored by
the THROTTLE I/Os so that the threads issuing THROTTLE
I/Os are not slowed down by PASSIVE I/Os. The PASSIVE
I/O policy is useful for server type applications. The
I/Os generated by these applications are called passive
I/Os because these I/Os are caused directly or indi-rectly indirectly
rectly by the I/O requests they receive from client
applications. For example, when an image file is
mounted by DiskImages, DiskImages generate passive I/Os.
DiskImages should mark these I/Os using the PASSIVE I/O
policy so that when client applications that issue
THROTTLE I/Os access the volume managed by DiskImages,
these client applications will not be slowed down by the
I/Os generated by DiskImages.

The I/O policy of a new created process is inherited from its parent
process. The I/O policy of an I/O request depends on the I/O policy of
both the current thread and the current process. If the I/O policy of
the current thread is IOPOL_DEFAULT, the I/O policy of the current
process is used; if the I/O policy of the current thread is not
IOPOL_DEFAULT, the I/O policy of the current thread overrides the I/O
policy of the current process; if the I/O policy of the current process
is IOPOL_DEFAULT, the policy of I/Os issued by this process is NORMAL.
For example, given the following thread and process I/O policy in the
first two columns, the I/O policy of all I/Os issued by the thread is
given in the third column:

Process I/O Policy Thread I/O Policy I/O Policy
DEFAULT DEFAULT NORMAL
DEFAULT PASSIVE PASSIVE
THROTTLE DEFAULT THROTTLE
THROTTLE PASSIVE PASSIVE
PASSIVE NORMAL NORMAL

The thread or process with THROTTLE I/O policy enabled may be slowed down
when it issues reads, but will not be slowed down when it issues writes.
If it issues far more writes than reads (e.g., an application downloading
large amounts of data through the network), these writes compete with the
normal I/Os of other processes and may have an adverse effect on the I/O
throughput or latency of those processes.

RETURN VALUES
The getiopolicy_np() call returns the I/O policy of the given I/O type
and scope. If error happens, -1 is returned. The setiopolicy_np() call
returns 0 if there is no error, or -1 if there is an error. When error
happens, the error code is stored in the external variable errno.

ERRORS
Getiopolicy_np() and setiopolicy_np() will fail if:

[EINVAL] Io_type or scope is not one of the values defined in
this manual.

In addition to the errors indicated above, setiopolicy_np() will fail if:

[EINVAL] Policy is not one of the values defined in this man-ual. manual.
ual.

SEE ALSO
nice(3), getpriority(2), setpriority(2), renice(8)

HISTORY
The getiopolicy_np() and setiopolicy_np() function call first appeared in
Mac OS X 10.5 (Leopard) .

BSD July 18, 2006 BSD

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