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Unix manual page for setiopolicy_np. (host=minya system=Darwin)
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 policies 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 only currently
supported I/O type is IOPOL_TYPE_DISK, which can mean either the I/O pol-
icy for I/Os to local disks or to remote volumes. 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. I/Os to remote volumes are I/Os that
require network activity to complete the operation. This is currently
only supported for remote volumes mounted by SMB or AFP.
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.
Policy can have the following values:
IOPOL_IMPORTANT I/Os with the IMPORTANT policy are unrestricted. This
policy should only be used for I/Os that are critical
to system responsiveness. This is the default I/O pol-
icy for new threads.
IOPOL_STANDARD The STANDARD policy is for work requested by the user,
but that is not the user's current focus. I/Os with
this policy may be delayed slightly to allow IMPORTANT
I/Os to complete quickly.
IOPOL_UTILITY The UTILITY policy is for short-running background
work. I/Os with this policy are throttled to prevent a
significant impact on the latency of IMPORTANT and
STANDARD I/Os.
IOPOL_THROTTLE The THROTTLE policy is for long-running I/O intensive
background work, such as backups, search indexing, or
file synchronization. I/Os with this policy will be
throttled to avoid impacting performance of higher pri-
ority I/Os.
IOPOL_PASSIVE The PASSIVE I/Os are a special type of I/O that are
ignored by the other policies so that the threads issu-
ing lower priority 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 pas-
sive I/Os. DiskImages should mark these I/Os using the
PASSIVE I/O policy so that when client applications
that access the volume managed by DiskImages, these
client applications will not be slowed down by the I/Os
generated by DiskImages.
I/Os with the STANDARD, UTILITY, and THROTTLE policies are called throt-
tleable I/Os and are of decreasing priority. If a throttleable request
occurs within a small time window of a request of higher priority, the
thread that issued the throttleable I/O is forced to a sleep for a short
period. (Both this window and the sleep period are dependent on the pol-
icy of the throttleable I/O.) This slows down the thread that issues the
throttleable I/O so that higher-priority I/Os can complete with low-
latency and receive a greater share of the disk bandwidth. Furthermore,
an IMPORTANT I/O request may bypass a previously issued throttleable I/O
request in kernel or driver queues and be sent to the device first. In
some circumstances, very large throttleable I/O requests will be broken
into smaller requests which are then issued serially.
The I/O policy of a newly created process is inherited from its parent
process. The I/O policy of an I/O request is the lowest priority policy
of the current thread and the current process.
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.
NOTES
The thread or process with a throttleable I/O policy enabled will be gen-
erally prevented from having an adverse effect on the throughput or
latency of higher priority I/Os of other processes. However, there are a
few considerations that users of the throttleable I/O policies should
keep in mind:
Consider using the F_NOCACHE fcntl(2) command to prevent caching when
using a throttleable I/O policy. This will reduce contention for avail-
able caches with IMPORTANT I/O.
Large read requests will automatically be broken up into smaller requests
to avoid stalling IMPORTANT I/O requests. However, due to the consis-
tency guarantees provided to contiguous writes, this can not be done
automatically for large writes. If a thread or process with a throt-
tleable I/O policy enabled will be issuing large writes, consider the use
of the F_SINGLE_WRITER fcntl(2) command. This will indicate to the sys-
tem that there is only one thread writing to the file and allow automatic
division of large writes.
Write-heavy throttleable I/O workloads may fill a drive's track (write)
cache. Subsequent higher priority writes must then wait for enough of
the track cache to be flushed before they can continue. If the writes
issued as throttleable I/O are small and not contiguous, many seeks may
be incurred before space is available for a subsequent higher priority
write. Issuers of throttleable I/O should attempt to issue their writes
sequentially or to locations in a single small area of the drive (i.e.
different positions in the same file) to ensure good spacial locality.
The F_FULLFSYNC fcntl(2) command can cause very long system-wide IO
stalls; use this command only if absolutely necessary.
SEE ALSO
nice(3), getpriority(2), setpriority(2), fcntl(2), open(2), renice(8)
HISTORY
The getiopolicy_np() and setiopolicy_np() function call first appeared in
Mac OS X 10.5 (Leopard) .
BSD April 30, 2013 BSD