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
policy 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, includ-
ing 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 policy 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 synchro-
nization. I/Os with this policy will be throttled to avoid impacting performance of higher priority 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 issuing lower prior-
ity 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 gen-
erate passive 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 DiskIm-
ages.
I/Os with the STANDARD, UTILITY, and THROTTLE policies are called throttleable 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 policy 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 band-
width. 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 pol-
icy 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 manual.
NOTES
The thread or process with a throttleable I/O policy enabled will be generally 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
available 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 con-
sistency 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
system 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