illumos: manual page: lgrp_affinity

LGRP_AFFINITY_GET(3LGRP) Locality Group Library Functions

NAME

lgrp_affinity_get, lgrp_affinity_set - get or set lgroup affinity

SYNOPSIS

cc [ flag ... ] file... -llgrp [ library ... ]
#include <sys/lgrp_user.h>

lgrp_affinity_t lgrp_affinity_get(idtype_t idtype, id_t id,
lgrp_id_t lgrp);

int lgrp_affinity_set(idtype_t idtype, id_t id, lgrp_id_t lgrp,
lgrp_affinity_t affinity);

DESCRIPTION

The lgrp_affinity_get() function returns the affinity that the LWP or set
of LWPs specified by the idtype and id arguments have for the given
lgroup.

The lgrp_affinity_set() function sets the affinity that the LWP or set of
LWPs specified by idtype and id have for the given lgroup. The lgroup
affinity can be set to LGRP_AFF_STRONG, LGRP_AFF_WEAK, or LGRP_AFF_NONE.

If the idtype is P_PID, the affinity is retrieved for one of the LWPs in
the process or set for all the LWPs of the process with process ID (PID)
id. The affinity is retrieved or set for the LWP of the current process
with LWP ID id if idtype is P_LWPID. If id is P_MYID, then the current
LWP or process is specified.

The operating system uses the lgroup affinities as advice on where to run
a thread and allocate its memory and factors this advice in with other
constraints. Processor binding and processor sets can restrict which
lgroups a thread can run on, but do not change the lgroup affinities.

Each thread can have an affinity for an lgroup in the system such that
the thread will tend to be scheduled to run on that lgroup and allocate
memory from there whenever possible. If the thread has affinity for more
than one lgroup, the operating system will try to run the thread and
allocate its memory on the lgroup for which it has the strongest
affinity, then the next strongest, and so on up through some small,
system-dependent number of these lgroup affinities. When multiple
lgroups have the same affinity, the order of preference among them is
unspecified and up to the operating system to choose. The lgroup with
the strongest affinity that the thread can run on is known as its "home
lgroup" (see lgrp_home(3LGRP)) and is usually the operating system's
first choice of where to run the thread and allocate its memory.

There are different levels of affinity that can be specified by a thread
for a particular lgroup. The levels of affinity are the following from
strongest to weakest:

LGRP_AFF_STRONG /* strong affinity */
LGRP_AFF_WEAK /* weak affinity */
LGRP_AFF_NONE /* no affinity */

The LGRP_AFF_STRONG affinity serves as a hint to the operating system
that the calling thread has a strong affinity for the given lgroup. If
this is the thread's home lgroup, the operating system will avoid
rehoming it to another lgroup if possible. However, dynamic
reconfiguration, processor offlining, processor binding, and processor
set binding and manipulation are examples of events that can cause the
operating system to change the thread's home lgroup for which it has a
strong affinity.

The LGRP_AFF_WEAK affinity is a hint to the operating system that the
calling thread has a weak affinity for the given lgroup. If a thread has
a weak affinity for its home lgroup, the operating system interprets this
to mean that thread does not mind whether it is rehomed, unlike
LGRP_AFF_STRONG. Load balancing, dynamic reconfiguration, processor
binding, or processor set binding and manipulation are examples of events
that can cause the operating system to change a thread's home lgroup for
which it has a weak affinity.

The LGRP_AFF_NONE affinity signifies no affinity and can be used to
remove a thread's affinity for a particular lgroup. Initially, each
thread has no affinity to any lgroup. If a thread has no lgroup
affinities set, the operating system chooses a home lgroup for the thread
with no affinity set.

RETURN VALUES

Upon successful completion, lgrp_affinity_get() returns the affinity for
the given lgroup.

Upon successful completion, lgrp_affinity_set() return 0.

Otherwise, both functions return -1 and set errno to indicate the error.

ERRORS

The lgrp_affinity_get() and lgrp_affinity_set() functions will fail if:

EINVAL
The specified lgroup, affinity, or ID type is not valid.

EPERM
The effective user of the calling process does not have
appropriate privileges, and its real or effective user ID does
not match the real or effective user ID of one of the LWPs.

ESRCH
The specified lgroup or LWP(s) was not found.