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 supported 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 NOR-MAL NORMAL
MAL 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 indirectly 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 pol-icy policy
icy 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 pol-icy policy
icy 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 nor-mal normal
mal 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 manual.
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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