RMM: RAPIDS Memory Manager

Achieving optimal performance in GPU-centric workflows frequently requires customizing how host and device memory are allocated. For example, using "pinned" host memory for asynchronous host <-> device memory transfers, or using a device memory pool sub-allocator to reduce the cost of dynamic device memory allocation.

The goal of the RAPIDS Memory Manager (RMM) is to provide:

• A common interface that allows customizing device and host memory allocation
• A collection of implementations of the interface
• A collection of data structures that use the interface for memory allocation

For information on the interface RMM provides and how to use RMM in your C++ code, see below.

NOTE: For the latest stable README.md ensure you are on the main branch.

Installation

Conda

RMM can be installed with Conda (miniconda, or the full Anaconda distribution) from the rapidsai channel:

# for CUDA 10.2
conda install -c nvidia -c rapidsai -c conda-forge -c defaults \
    rmm cudatoolkit=10.2
# for CUDA 10.1
conda install -c nvidia -c rapidsai -c conda-forge -c defaults \
    rmm cudatoolkit=10.1
# for CUDA 10.0
conda install -c nvidia -c rapidsai -c conda-forge -c defaults \
    rmm cudatoolkit=10.0

We also provide nightly Conda packages built from the HEAD of our latest development branch.

Note: RMM is supported only on Linux, and with Python versions 3.7 and later.

Note: The RMM package from Conda requires building with GCC 7 or later. Otherwise, your application may fail to build.

See the Get RAPIDS version picker for more OS and version info.

Building from Source

Get RMM Dependencies

Compiler requirements:

• gcc version 7.0 or higher required
• nvcc version 9.0 or higher recommended
• cmake version 3.18 or higher

CUDA/GPU requirements:

• CUDA 9.0+
• NVIDIA driver 396.44+
• Pascal architecture or better

You can obtain CUDA from https://developer.nvidia.com/cuda-downloads

Script to build RMM from source

To install RMM from source, ensure the dependencies are met and follow the steps below:

• Clone the repository and submodules

$ git clone --recurse-submodules https://github.com/rapidsai/rmm.git
$ cd rmm

Follow the instructions under "Create the conda development environment cudf_dev" in the cuDF README.

• Create the conda development environment cudf_dev

# create the conda environment (assuming in base `cudf` directory)
$ conda env create --name cudf_dev --file conda/environments/dev_py35.yml
# activate the environment
$ source activate cudf_dev

• Build and install librmm using cmake & make. CMake depends on the nvcc executable being on your path or defined in CUDACXX environment variable.

$ mkdir build                                       # make a build directory
$ cd build                                          # enter the build directory
$ cmake .. -DCMAKE_INSTALL_PREFIX=/install/path     # configure cmake ... use $CONDA_PREFIX if you're using Anaconda
$ make -j                                           # compile the library librmm.so ... '-j' will start a parallel job using the number of physical cores available on your system
$ make install                                      # install the library librmm.so to '/install/path'

• Building and installing librmm and rmm using build.sh. Build.sh creates build dir at root of git repository. build.sh depends on the nvcc executable being on your path or defined in CUDACXX environment variable.

$ ./build.sh -h                                     # Display help and exit
$ ./build.sh -n librmm                              # Build librmm without installing
$ ./build.sh -n rmm                                 # Build rmm without installing
$ ./build.sh -n librmm rmm                          # Build librmm and rmm without installing
$ ./build.sh librmm rmm                             # Build and install librmm and rmm

• To run tests (Optional):

$ cd build (if you are not already in build directory)
$ make test

• Build, install, and test the rmm python package, in the python folder:

$ python setup.py build_ext --inplace
$ python setup.py install
$ pytest -v

Done! You are ready to develop for the RMM OSS project.

Caching third-party dependencies

RMM uses CPM.cmake to handle third-party dependencies like spdlog, Thrust, GoogleTest, GoogleBenchmark. In general you won't have to worry about it. If CMake finds an appropriate version on your system, it uses it (you can help it along by setting CMAKE_PREFIX_PATH to point to the installed location). Otherwise those dependencies will be downloaded as part of the build.

If you frequently start new builds from scratch, consider setting the environment variable CPM_SOURCE_CACHE to an external download directory to avoid repeated downloads of the third-party dependencies.

Using RMM in a downstream CMake project

The installed RMM library provides a set of config files that makes it easy to integrate RMM into your own CMake project. In your CMakeLists.txt, just add

find_package(rmm [VERSION])
# ...
target_link_libraries(<your-target> (PRIVATE|PUBLIC) rmm::rmm)

Since RMM is a header-only library, this does not actually link RMM, but it makes the headers available and pulls in transitive dependencies. If RMM is not installed in a default location, use CMAKE_PREFIX_PATH or rmm_ROOT to point to its location.

One of RMM's dependencies is the Thrust library, so the above automatically pulls in Thrust by means of a dependency on the rmm::Thrust target. By default it uses the standard configuration of Thrust. If you want to customize it, you can set the variables THRUST_HOST_SYSTEM and THRUST_DEVICE_SYSTEM; see Thrust's CMake documentation.

Using RMM in C++

The first goal of RMM is to provide a common interface for device and host memory allocation. This allows both users and implementers of custom allocation logic to program to a single interface.

To this end, RMM defines two abstract interface classes:

• rmm::mr::device_memory_resource for device memory allocation
• rmm::mr::host_memory_resource for host memory allocation

These classes are based on the std::pmr::memory_resource interface class introduced in C++17 for polymorphic memory allocation.

device_memory_resource

rmm::mr::device_memory_resource is the base class that defines the interface for allocating and freeing device memory.

It has two key functions:

1. void* device_memory_resource::allocate(std::size_t bytes, cuda_stream_view s)

   • Returns a pointer to an allocation of at least bytes bytes.

2. void device_memory_resource::deallocate(void* p, std::size_t bytes, cuda_stream_view s)

   • Reclaims a previous allocation of size bytes pointed to by p.
   • p must have been returned by a previous call to allocate(bytes), otherwise behavior is undefined

It is up to a derived class to provide implementations of these functions. See available resources for example device_memory_resource derived classes.

Unlike std::pmr::memory_resource, rmm::mr::device_memory_resource does not allow specifying an alignment argument. All allocations are required to be aligned to at least 256B. Furthermore, device_memory_resource adds an additional cuda_stream_view argument to allow specifying the stream on which to perform the (de)allocation.

cuda_stream_view and cuda_stream

rmm::cuda_stream_view is a simple non-owning wrapper around a CUDA cudaStream_t. This wrapper's purpose is to provide strong type safety for stream types. (cudaStream_t is an alias for a pointer, which can lead to ambiguity in APIs when it is assigned 0.) All RMM stream-ordered APIs take a rmm::cuda_stream_view argument.

rmm::cuda_stream is a simple owning wrapper around a CUDA cudaStream_t. This class provides RAII semantics (constructor creates the CUDA stream, destructor destroys it). An rmm::cuda_stream can never represent the CUDA default stream or per-thread default stream; it only ever represents a single non-default stream. rmm::cuda_stream cannot be copied, but can be moved.

cuda_stream_pool

rmm::cuda_stream_pool provides fast access to a pool of CUDA streams. This class can be used to create a set of cuda_stream objects whose lifetime is equal to the cuda_stream_pool. Using the stream pool can be faster than creating the streams on the fly. The size of the pool is configurable. Depending on this size, multiple calls to cuda_stream_pool::get_stream() may return instances of rmm::cuda_stream_view that represent identical CUDA streams.

Thread Safety

All current device memory resources are thread safe unless documented otherwise. More specifically, calls to memory resource allocate() and deallocate() methods are safe with respect to calls to either of these functions from other threads. They are not thread safe with respect to construction and destruction of the memory resource object.

Note that a class thread_safe_resource_adapter is provided which can be used to adapt a memory resource that is not thread safe to be thread safe (as described above). This adapter is not needed with any current RMM device memory resources.

Stream-ordered Memory Allocation

rmm::mr::device_memory_resource is a base class that provides stream-ordered memory allocation. This allows optimizations such as re-using memory deallocated on the same stream without the overhead of synchronization.

A call to device_memory_resource::allocate(bytes, stream_a) returns a pointer that is valid to use on stream_a. Using the memory on a different stream (say stream_b) is Undefined Behavior unless the two streams are first synchronized, for example by using cudaStreamSynchronize(stream_a) or by recording a CUDA event on stream_a and then calling cudaStreamWaitEvent(stream_b, event).

The stream specified to device_memory_resource::deallocate should be a stream on which it is valid to use the deallocated memory immediately for another allocation. Typically this is the stream on which the allocation was last used before the call to deallocate. The passed stream may be used internally by a device_memory_resource for managing available memory with minimal synchronization, and it may also be synchronized at a later time, for example using a call to cudaStreamSynchronize().

For this reason, it is Undefined Behavior to destroy a CUDA stream that is passed to device_memory_resource::deallocate. If the stream on which the allocation was last used has been destroyed before calling deallocate or it is known that it will be destroyed, it is likely better to synchronize the stream (before destroying it) and then pass a different stream to deallocate (e.g. the default stream).

Note that device memory data structures such as rmm::device_buffer and rmm::device_uvector follow these stream-ordered memory allocation semantics and rules.

Available Resources

RMM provides several device_memory_resource derived classes to satisfy various user requirements. For more detailed information about these resources, see their respective documentation.

cuda_memory_resource

Allocates and frees device memory using cudaMalloc and cudaFree.

managed_memory_resource

Allocates and frees device memory using cudaMallocManaged and cudaFree.

Note that managed_memory_resource cannot be used with NVIDIA Virtual GPU Software (vGPU, for use with virtual machines or hypervisors) because NVIDIA CUDA Unified Memory is not supported by NVIDIA vGPU.

pool_memory_resource

A coalescing, best-fit pool sub-allocator.

fixed_size_memory_resource

A memory resource that can only allocate a single fixed size. Average allocation and deallocation cost is constant.

binning_memory_resource

Configurable to use multiple upstream memory resources for allocations that fall within different bin sizes. Often configured with multiple bins backed by fixed_size_memory_resources and a single pool_memory_resource for allocations larger than the largest bin size.

Default Resources and Per-device Resources

RMM users commonly need to configure a device_memory_resource object to use for all allocations where another resource has not explicitly been provided. A common example is configuring a pool_memory_resource to use for all allocations to get fast dynamic allocation.

To enable this use case, RMM provides the concept of a "default" device_memory_resource. This resource is used when another is not explicitly provided.

Accessing and modifying the default resource is done through two functions:

• device_memory_resource* get_current_device_resource()

  • Returns a pointer to the default resource for the current CUDA device.
  • The initial default memory resource is an instance of cuda_memory_resource.
  • This function is thread safe with respect to concurrent calls to it and set_current_device_resource().
  • For more explicit control, you can use get_per_device_resource(), which takes a device ID.

• device_memory_resource* set_current_device_resource(device_memory_resource* new_mr)

  • Updates the default memory resource pointer for the current CUDA device to new_resource
  • Returns the previous default resource pointer
  • If new_resource is nullptr, then resets the default resource to cuda_memory_resource
  • This function is thread safe with respect to concurrent calls to it and get_current_device_resource()
  • For more explicit control, you can use set_per_device_resource(), which takes a device ID.

Example

rmm::mr::cuda_memory_resource cuda_mr;
// Construct a resource that uses a coalescing best-fit pool allocator
rmm::mr::pool_memory_resource<rmm::mr::cuda_memory_resource> pool_mr{&cuda_mr};
rmm::mr::set_current_device_resource(&pool_mr); // Updates the current device resource pointer to `pool_mr`
rmm::mr::device_memory_resource* mr = rmm::mr::get_current_device_resource(); // Points to `pool_mr`

Multiple Devices

A device_memory_resource should only be used when the active CUDA device is the same device that was active when the device_memory_resource was created. Otherwise behavior is undefined.

If a device_memory_resource is used with a stream associated with a different CUDA device than the device for which the memory resource was created, behavior is undefined.

Creating a device_memory_resource for each device requires care to set the current device before creating each resource, and to maintain the lifetime of the resources as long as they are set as per-device resources. Here is an example loop that creates unique_ptrs to pool_memory_resource objects for each device and sets them as the per-device resource for that device.

std::vector<unique_ptr<pool_memory_resource>> per_device_pools;
for(int i = 0; i < N; ++i) {
    cudaSetDevice(i); // set device i before creating MR
    // Use a vector of unique_ptr to maintain the lifetime of the MRs
    per_device_pools.push_back(std::make_unique<pool_memory_resource>());
    // Set the per-device resource for device i
    set_per_device_resource(cuda_device_id{i}, &per_device_pools.back());
}

Allocators

C++ interfaces commonly allow customizable memory allocation through an Allocator object. RMM provides several Allocator and Allocator-like classes.

polymorphic_allocator

A stream-ordered allocator similar to std::pmr::polymorphic_allocator. Unlike the standard C++ Allocator interface, the allocate and deallocate functions take a cuda_stream_view indicating the stream on which the (de)allocation occurs.

stream_allocator_adaptor

stream_allocator_adaptor can be used to adapt a stream-ordered allocator to present a standard Allocator interface to consumers that may not be designed to work with a stream-ordered interface.

Example:

rmm::cuda_stream stream;
rmm::mr::polymorphic_allocator<int> stream_alloc;

// Constructs an adaptor that forwards all (de)allocations to `stream_alloc` on `stream`.
auto adapted = rmm::mr::make_stream_allocator_adaptor(stream_alloc, stream);

// Allocates 100 bytes using `stream_alloc` on `stream`
auto p = adapted.allocate(100); 
...
// Deallocates using `stream_alloc` on `stream`
adapted.deallocate(p,100); 

thrust_allocator

thrust_allocator is a device memory allocator that uses the strongly typed thrust::device_ptr, making it usable with containers like thrust::device_vector.

See below for more information on using RMM with Thrust.

Device Data Structures

device_buffer

An untyped, uninitialized RAII class for stream ordered device memory allocation.

Example

cuda_stream_view s{...};
// Allocates at least 100 bytes on stream `s` using the *default* resource
rmm::device_buffer b{100,s}; 
void* p = b.data();                   // Raw, untyped pointer to underlying device memory

kernel<<<..., s.value()>>>(b.data()); // `b` is only safe to use on `s`

rmm::mr::device_memory_resource * mr = new my_custom_resource{...};
// Allocates at least 100 bytes on stream `s` using the resource `mr`
rmm::device_buffer b2{100, s, mr};

device_uvector<T>

A typed, uninitialized RAII class for allocation of a contiguous set of elements in device memory. Similar to a thrust::device_vector, but as an optimization, does not default initialize the contained elements. This optimization restricts the types T to trivially copyable types.

Example

cuda_stream_view s{...};
// Allocates uninitialized storage for 100 `int32_t` elements on stream `s` using the
// default resource
rmm::device_uvector<int32_t> v(100, s);
// Initializes the elements to 0
thrust::uninitialized_fill(thrust::cuda::par.on(s.value()), v.begin(), v.end(), int32_t{0}); 

rmm::mr::device_memory_resource * mr = new my_custom_resource{...};
// Allocates uninitialized storage for 100 `int32_t` elements on stream `s` using the resource `mr`
rmm::device_uvector<int32_t> v2{100, s, mr}; 

device_scalar

A typed, RAII class for allocation of a single element in device memory. This is similar to a device_uvector with a single element, but provides convenience functions like modifying the value in device memory from the host, or retrieving the value from device to host.

Example

cuda_stream_view s{...};
// Allocates uninitialized storage for a single `int32_t` in device memory
rmm::device_scalar<int32_t> a{s}; 
a.set_value(42, s); // Updates the value in device memory to `42` on stream `s`

kernel<<<...,s.value()>>>(a.data()); // Pass raw pointer to underlying element in device memory

int32_t v = a.value(s); // Retrieves the value from device to host on stream `s`

host_memory_resource

rmm::mr::host_memory_resource is the base class that defines the interface for allocating and freeing host memory.

Similar to device_memory_resource, it has two key functions for (de)allocation:

1. void* host_memory_resource::allocate(std::size_t bytes, std::size_t alignment)

   • Returns a pointer to an allocation of at least bytes bytes aligned to the specified alignment

2. void host_memory_resource::deallocate(void* p, std::size_t bytes, std::size_t alignment)

   • Reclaims a previous allocation of size bytes pointed to by p.

Unlike device_memory_resource, the host_memory_resource interface and behavior is identical to std::pmr::memory_resource.

Available Resources

new_delete_resource

Uses the global operator new and operator delete to allocate host memory.

pinned_memory_resource

Allocates "pinned" host memory using cuda(Malloc/Free)Host.

Host Data Structures

RMM does not currently provide any data structures that interface with host_memory_resource. In the future, RMM will provide a similar host-side structure like device_buffer and an allocator that can be used with STL containers.

Using RMM with Thrust

RAPIDS and other CUDA libraries make heavy use of Thrust. Thrust uses CUDA device memory in two situations:

1. As the backing store for thrust::device_vector, and
2. As temporary storage inside some algorithms, such as thrust::sort.

RMM provides rmm::mr::thrust_allocator as a conforming Thrust allocator that uses device_memory_resources.

Thrust Algorithms

To instruct a Thrust algorithm to use rmm::mr::thrust_allocator to allocate temporary storage, you can use the custom Thrust CUDA device execution policy: rmm::exec_policy(stream).

thrust::sort(rmm::exec_policy(stream, ...);

The first stream argument is the stream to use for rmm::mr::thrust_allocator. The second stream argument is what should be used to execute the Thrust algorithm. These two arguments must be identical.

Logging

RMM includes two forms of logging. Memory event logging and debug logging.

Memory Event Logging and logging_resource_adaptor

Memory event logging writes details of every allocation or deallocation to a CSV (comma-separated value) file. In C++, Memory Event Logging is enabled by using the logging_resource_adaptor as a wrapper around any other device_memory_resource object.

Each row in the log represents either an allocation or a deallocation. The columns of the file are "Thread, Time, Action, Pointer, Size, Stream".

The CSV output files of the logging_resource_adaptor can be used as input to REPLAY_BENCHMARK, which is available when building RMM from source, in the gbenchmarks folder in the build directory. This log replayer can be useful for profiling and debugging allocator issues.

The following C++ example creates a logging version of a cuda_memory_resource that outputs the log to the file "logs/test1.csv".

std::string filename{"logs/test1.csv"};
rmm::mr::cuda_memory_resource upstream;
rmm::mr::logging_resource_adaptor<rmm::mr::cuda_memory_resource> log_mr{&upstream, filename};

If a file name is not specified, the environment variable RMM_LOG_FILE is queried for the file name. If RMM_LOG_FILE is not set, then an exception is thrown by the logging_resource_adaptor constructor.

In Python, memory event logging is enabled when the logging parameter of rmm.reinitialize() is set to True. The log file name can be set using the log_file_name parameter. See help(rmm.reinitialize) for full details.

Debug Logging

RMM includes a debug logger which can be enabled to log trace and debug information to a file. This information can show when errors occur, when additional memory is allocated from upstream resources, etc. The default log file is rmm_log.txt in the current working directory, but the environment variable RMM_DEBUG_LOG_FILE can be set to specify the path and file name.

There is a CMake configuration variable RMM_LOGGING_LEVEL, which can be set to enable compilation of more detailed logging. The default is INFO. Available levels are TRACE, DEBUG, INFO, WARN, ERROR, CRITICAL and OFF.

The log relies on the spdlog library.

Note that to see logging below the INFO level, the C++ application must also call rmm::logger().set_level(), e.g. to enable all levels of logging down to TRACE, call rmm::logger().set_level(spdlog::level::trace) (and compile with -DRMM_LOGGING_LEVEL=TRACE).

Note that debug logging is different from the CSV memory allocation logging provided by rmm::mr::logging_resource_adapter. The latter is for logging a history of allocation / deallocation actions which can be useful for replay with RMM's replay benchmark.

Using RMM in Python Code

There are two ways to use RMM in Python code:

1. Using the rmm.DeviceBuffer API to explicitly create and manage device memory allocations
2. Transparently via external libraries such as CuPy and Numba

RMM provides a MemoryResource abstraction to control how device memory is allocated in both the above uses.

DeviceBuffers

A DeviceBuffer represents an untyped, uninitialized device memory allocation. DeviceBuffers can be created by providing the size of the allocation in bytes:

>>> import rmm
>>> buf = rmm.DeviceBuffer(size=100)

The size of the allocation and the memory address associated with it can be accessed via the .size and .ptr attributes respectively:

>>> buf.size
100
>>> buf.ptr
140202544726016

DeviceBuffers can also be created by copying data from host memory:

>>> import rmm
>>> import numpy as np
>>> a = np.array([1, 2, 3], dtype='float64')
>>> buf = rmm.to_device(a.tobytes())
>>> buf.size
24

Conversely, the data underlying a DeviceBuffer can be copied to the host:

>>> np.frombuffer(buf.tobytes())
array([1., 2., 3.])

MemoryResource objects

MemoryResource objects are used to configure how device memory allocations are made by RMM.

By default if a MemoryResource is not set explicitly, RMM uses the CudaMemoryResource, which uses cudaMalloc for allocating device memory.

rmm.reinitialize() provides an easy way to initialize RMM with specific memory resource options across multiple devices. See help(rmm.reinitialize) for full details.

For lower-level control, the rmm.mr.set_current_device_resource() function can be used to set a different MemoryResource for the current CUDA device. For example, enabling the ManagedMemoryResource tells RMM to use cudaMallocManaged instead of cudaMalloc for allocating memory:

>>> import rmm
>>> rmm.mr.set_current_device_resource(rmm.mr.ManagedMemoryResource())

⚠️ The default resource must be set for any device before allocating any device memory on that device. Setting or changing the resource after device allocations have been made can lead to unexpected behaviour or crashes. See Multiple Devices

As another example, PoolMemoryResource allows you to allocate a large "pool" of device memory up-front. Subsequent allocations will draw from this pool of already allocated memory. The example below shows how to construct a PoolMemoryResource with an initial size of 1 GiB and a maximum size of 4 GiB. The pool uses CudaMemoryResource as its underlying ("upstream") memory resource:

>>> import rmm
>>> pool = rmm.mr.PoolMemoryResource(
...     upstream=rmm.mr.CudaMemoryResource(),
...     initial_pool_size=2**30,
...     maximum_pool_size=2**32
... )
>>> rmm.mr.set_current_device_resource(pool)

Other MemoryResources include:

• FixedSizeMemoryResource for allocating fixed blocks of memory
• BinningMemoryResource for allocating blocks within specified "bin" sizes from different memory resources

MemoryResources are highly configurable and can be composed together in different ways. See help(rmm.mr) for more information.

Using RMM with CuPy

You can configure CuPy to use RMM for memory allocations by setting the CuPy CUDA allocator to rmm_cupy_allocator:

>>> import rmm
>>> import cupy
>>> cupy.cuda.set_allocator(rmm.rmm_cupy_allocator)

Using RMM with Numba

You can configure Numba to use RMM for memory allocations using the Numba EMM Plugin.

This can be done in two ways:

1. Setting the environment variable NUMBA_CUDA_MEMORY_MANAGER:

$ NUMBA_CUDA_MEMORY_MANAGER=rmm python (args)

2. Using the set_memory_manager() function provided by Numba:

>>> from numba import cuda
>>> import rmm
>>> cuda.set_memory_manager(rmm.RMMNumbaManager)