Intel® oneAPI DPC++/C++ Compiler Developer Guide and Reference

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OpenMP* Memory Spaces and Allocators

For storage and retrieval variables, OpenMP* provides memory known as memory spaces. Different memory spaces have different traits. Depending on how a variable is to be used and accessed determines which memory space is appropriate for allocation of the variable.
Each memory space has a unique allocator that is used to allocate and deallocate memory in that space. The allocators allocate variables in contiguous space that does not overlap any other allocation in the memory space. Multiple memory spaces with different traits may map to a single memory resource.
The behavior of the allocator is affected by the allocator traits that you specify. The allocator traits, their possible values, and their default values are shown in the following table:
Allocator Trait
Values That Can Be Specified
Default Value
access
  • all
  • cgroup
  • pteam
  • thread
All
alignment
A positive integer value that is a power of 2 specifying number of bytes
1 byte
fallback
  • abort_fb
  • allocator_fb
  • default_mem_fb
  • null_fb
default_mem_fb
fb_data
An allocator handle
None
partition
  • blocked
  • environment
  • interleaved
  • nearest
environment
pinned
  • true
  • false
false
pool_size
a positive integer value
Implementation defined
sync_hint
  • contended
  • uncontended
  • private
  • serialized
contended
The
access
 trait specifies the accessibility of the allocated memory. The following are values you can specify for
access
:
  • all
    This value indicates that the allocated memory must be accessible by all threads in the device where the memory allocation occurs.
    This is the default setting.
  • cgroup
    This value indicates that the allocated memory must be accessible by all threads of the same contention group as the thread that requested the allocation. Accessing the allocated memory thread that is not part of the same contention group results in undefined behavior.
  • pteam
    This value indicates that the allocated memory is accessible by all threads that bind to the same parallel region as the thread that requests the allocations. Access to the memory by a thread that does not bind to the same parallel region as the thread that allocated the memory results in undefined behavior.
  • thread
    This value indicates that the memory allocated is accessible only by the thread that allocated it. Attempts to allocate the memory by another thread result in undefined behavior.
The
alignment
 trait specifies how allocated variables will be aligned. Variables will be byte-aligned to at least the value specified for this trait. The default setting is 1 byte. Alignment can also be affected by directives and OpenMP runtime allocator routines that specify alignment requirements.
The
fallback
 trait indicates how an allocator behaves if it is unable to satisfy an allocation request. The following are values you can specify for
fallback
:
  • abort_fb
    This value indicates that the program terminates if the allocation request fails.
  • allocator_fb
    If this value is specified and the allocation request fails, the allocation will be tried by the allocator specified by the
    fb_data
     trait.
  • default_mem_fb
    This value indicates that a failed allocation request will be retried in the
    omp_default_mem_space
     memory space. All traits for the
    omp_default_mem_space
     allocator should be set to the default trait values, except the
    fallback
     trait should be set to
    null_fb
    . This is the default setting.
  • null_fb
    This value indicates the allocator returns a zero value when an allocation request fails.
The
fb_data
 trait lets you specify a fall back allocator to be used if the requested allocator fails to satisfy the allocation request. The
fallback
 trait of the failing allocator must be set to
allocator_fb
 in order for the allocator specified by the
fb_data
 trait to be used.
The
partition
 trait describes the partitioning of allocated memory over the storage resources represented by the memory space of the allocator. The following are values you can specify for
partition
:
  • blocked
    This value indicates the allocated memory is partitioned into blocks of memory of approximately equal size with one block per storage resource.
  • environment
    This value indicates the allocated memory placement is determined by the runtime execution environment. This is the default setting.
  • interleaved
    This value indicates the allocated memory is distributed in a round-robin fashion across the storage resources.
  • nearest
    This value indicates that the allocated memory will be placed in the storage resource nearest to the thread that requested the allocation.
If the
pinned
 trait has the value
true
, the allocator ensures each allocation made by the allocator will remain in the storage resource at the same location where it was allocated until it is deallocated. The default setting is
false
.
The value of
pool_size
 is the total number of bytes of storage available to an allocator when there have been no allocations. The following affect
pool_size
:
  • If the
    access
     trait has the value all, the value of
    pool_size
     is the limit for all allocations for all threads having access to the allocator.
  • If the
    access
     trait of the allocator has the value
    cgroup
    , the value of
    pool_size
     is the limit for allocations made from the threads within the same contention group.
  • For allocators with the
    access
     access trait value of
    pteam
    , the value of
    pool_size
     is the limit for allocations made within the same parallel team.
  • If the
    access
     trait has the value
    thread
    , the value of
    pool_size
     is the limit for allocations made from each thread using the allocator.
  • An allocation request for more space than the value of
    pool_size
     results in the allocator not fulfilling the allocation request.
The
sync_hint
 trait describes the way that multiple threads can access an allocator. The following are values you can specify for
sync_hint
:
  • contended
     or
    uncontended
    Value
    contended
     indicates that many threads are anticipated to make simultaneous allocation requests while the value
    uncontended
     indicates that few threads are anticipated to make simultaneous allocation. The default setting is
    contended
    .
  • private
    This value indicates that all allocation requests will come from the same thread. Specifying
    private
     when this is not the case and two or more threads make allocation requests by the same allocator results in undefined behavior.
  • serialized
    This value indicates that only one thread will request an allocation at a given time. The behavior is undefined if two threads request an allocation simultaneously by an allocator whose
    sync_hint
     value is serialized.
There are five predefined memory spaces in OpenMP:
  • The system default memory is referred to as
    omp_default_mem_space
    .
  • Large capacity memory is referred to as
    omp_large_cap_mem_space
    .
  • High bandwidth memory is referred to as
    omp_high_bw_mem_space
    .
  • Low latency memory is referred to as
    omp_low_lat_mem_space
    .
  • Memory designed for optimal storage of constant values is referred to as
    omp_const_mem_space
    .
    It can be initialized with compile-time constant expressions or by using a firstprivate clause.
    Writing to variables in
    omp_const_mem_space
     results in undefined behavior.
There are three additional predefined memory spaces that are extensions to the OpenMP standard:
  • omp_target_host_mem_space
     is host memory that is accessible by the device.
  • omp_target_shared_mem_space
     is memory that can migrate between the host and the device.
  • omp_target_device_mem_space
     is memory that is accessible to the device.
The following table shows the predefined memory allocators, the memory space they are associated with, and the non-default memory trait values they possess.
Allocator Name
Associated Memory Space
Non-Default Trait Values
omp_default_mem_alloc
omp_default_mem_space
fallback=null_fb
omp_large_cap_mem_alloc
omp_large_cap_mem_space
none
omp_low_lat_mem_alloc
omp_low_lat_mem_space
none
omp_high_bw_mem_alloc
omp_high_bw_mem_space
none
omp_const_mem_alloc
omp_const_mem_space
none
omp_cgroup_mem_alloc
implementation/system defined
access=cgroup
omp_pteam_mem_alloc
implementation/system defined
access=pteam
omp_thread_mem_alloc
implementation/system defined
access=thread
omp_target_host_mem_alloc
omp_target_host_mem_space
none
omp_target_shared_mem_alloc
omp_target_shared_mem_space
none
omp_target_device_mem_alloc
omp_target_device_mem_space
none

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