GDRCopy

A low-latency GPU memory copy library based on NVIDIA GPUDirect RDMA technology.

Introduction

While GPUDirect RDMA is meant for direct access to GPU memory from third-party devices, it is possible to use these same APIs to create perfectly valid CPU mappings of the GPU memory.

The advantage of a CPU driven copy is the very small overhead involved. That might be useful when low latencies are required.

What is inside

GDRCopy offers the infrastructure to create user-space mappings of GPU memory, which can then be manipulated as if it was plain host memory (caveats apply here).

A simple by-product of it is a copy library with the following characteristics: - very low overhead, as it is driven by the CPU. As a reference, currently a cudaMemcpy can incur in a 6-7us overhead.

• An initial memory pinning phase is required, which is potentially expensive, 10us-1ms depending on the buffer size.

• Fast H-D, because of write-combining. H-D bandwidth is 6-8GB/s on Ivy Bridge Xeon but it is subject to NUMA effects.

• Slow D-H, because the GPU BAR, which backs the mappings, can't be prefetched and so burst reads transactions are not generated through PCIE

The library comes with a few tests like: - gdrcopy_sanity, which contains unit tests for the library and the driver. - gdrcopy_copybw, a minimal application which calculates the R/W bandwidth for a specific buffer size. - gdrcopy_copylat, a benchmark application which calculates the R/W copy latency for a range of buffer sizes. - gdrcopy_apiperf, an application for benchmarking the latency of each GDRCopy API call. - gdrcopy_pplat, a benchmark application which calculates the round-trip ping-pong latency between GPU and CPU.

Requirements

GPUDirect RDMA requires NVIDIA Data Center GPU or NVIDIA RTX GPU (formerly Tesla and Quadro) based on Kepler or newer generations, see GPUDirect RDMA. For more general information, please refer to the official GPUDirect RDMA design document.

The device driver requires GPU display driver >= 418.40 on ppc64le and >= 331.14 on other platforms. The library and tests require CUDA >= 6.0.

DKMS is a prerequisite for installing GDRCopy kernel module package. On RHEL or SLE, however, users have an option to build kmod and install it instead of the DKMS package. See Build and installation section for more details.

# On RHEL
# dkms can be installed from epel-release. See https://fedoraproject.org/wiki/EPEL.
$ sudo yum install dkms

# On Debian - No additional dependency

# On SLE / Leap
# On SLE dkms can be installed from PackageHub.
$ sudo zypper install dkms rpmbuild

CUDA and GPU display driver must be installed before building and/or installing GDRCopy. The installation instructions can be found in https://developer.nvidia.com/cuda-downloads.

GPU display driver header files are also required. They are installed as a part of the driver (or CUDA) installation with runfile. If you install the driver via package management, we suggest - On RHEL, sudo dnf module install nvidia-driver:latest-dkms. - On Debian, sudo apt install nvidia-dkms-<your-nvidia-driver-version>. - On SLE, sudo zypper install nvidia-gfx<your-nvidia-driver-version>-kmp.

The supported architectures are Linux x86_64, ppc64le, and arm64. The supported platforms are RHEL8, RHEL9, Ubuntu20_04, Ubuntu22_04, SLE-15 (any SP) and Leap 15.x.

Root privileges are necessary to load/install the kernel-mode device driver.

DMA-BUF mmap backend

GDRCopy can export GPU memory as a Linux dma-buf via the CUDA driver and map it into user space with a plain mmap() on the dma-buf file descriptor. This backend does not require the GDRCopy kernel module (gdrdrv) and is intended for environments where gdrdrv is not installed or loaded.

Requirements

• CUDA driver 13.3 or newer

Backend selection

On gdr_open(), GDRCopy tries gdrdrv first. If gdrdrv is not installed or fails to open, it falls back to the dma-buf mmap backend (provided the driver supports). To force the dma-buf backend even when gdrdrv is available, set environment variable GDRCOPY_USE_DMABUF_MMAP=1 before calling gdr_open(). To check at runtime which backend is active:

int using_dmabuf;
gdr_get_attribute(g, GDR_ATTR_USING_DMA_BUF_MMAP, &using_dmabuf);
// using_dmabuf != 0  -> dma-buf mmap backend is in use

Mapping type

CPU cacheability is decided by the CUDA driver at pin time and cannot be changed afterwards. The mapping type depends on the pin flag and if the platform is coherent.

Pin flag	Coherent platform	Non-coherent platform
Default	GDR_MAPPING_TYPE_CACHING	GDR_MAPPING_TYPE_WC
GDR_PIN_FLAG_FORCE_PCIE	GDR_MAPPING_TYPE_WC	GDR_MAPPING_TYPE_WC

The dmabuf backend does not support user-requested mapping types: the type the pin produces is the only type gdr_map_v2 will accept. Passing an explicit cache flag (GDR_MAP_FLAG_REQ_CACHE_MAPPING, GDR_MAP_FLAG_REQ_WC_MAPPING, …) that asks for anything other than the default returns EINVAL.

Behavior differences vs. the gdrdrv backend

• Persistent mappings. All dma-buf mappings are persistent; GDR_ATTR_USE_PERSISTENT_MAPPING always returns 1.
• One fd per pinned buffer. Each gdr_pin_buffer consumes one dma-buf file descriptor until gdr_unpin_buffer. Applications that pin many buffers should account for the process FD limit.
• No timing fields. gdr_get_info_v2 returns tm_cycles = 0 and cycles_per_ms = 0.
• No invalidation callback. gdr_get_callback_flag always returns 0.
• GDR API compatibility — Standard GDRCopy APIs remain unchanged

Build and installation

We provide three ways for building and installing GDRCopy.

rpm package

# For RHEL:
$ sudo yum groupinstall 'Development Tools'
$ sudo yum install dkms rpm-build make

# For SLE:
$ sudo zypper in dkms rpmbuild

$ cd packages
$ CUDA=<cuda-install-top-dir> ./build-rpm-packages.sh
$ sudo rpm -Uvh gdrcopy-kmod-<version>dkms.noarch.<platform>.rpm
$ sudo rpm -Uvh gdrcopy-<version>.<arch>.<platform>.rpm
$ sudo rpm -Uvh gdrcopy-devel-<version>.noarch.<platform>.rpm

DKMS package is the default kernel module package that build-rpm-packages.sh generates. To create kmod package, -m option must be passed to the script. Unlike the DKMS package, the kmod package contains a prebuilt GDRCopy kernel module which is specific to the NVIDIA driver version and the Linux kernel version used to build it.

deb package

$ sudo apt install build-essential devscripts debhelper fakeroot pkg-config dkms
$ cd packages
$ CUDA=<cuda-install-top-dir> ./build-deb-packages.sh
$ sudo dpkg -i gdrdrv-dkms_<version>_<arch>.<platform>.deb
$ sudo dpkg -i libgdrapi_<version>_<arch>.<platform>.deb
$ sudo dpkg -i gdrcopy-tests_<version>_<arch>.<platform>.deb
$ sudo dpkg -i gdrcopy_<version>_<arch>.<platform>.deb

from source

$ make prefix=<install-to-this-location> CUDA=<cuda-install-top-dir> all install
$ sudo ./insmod.sh

Notes

Compiling the gdrdrv driver requires the NVIDIA driver source code, which is typically installed at /usr/src/nvidia-<version>. Our make file automatically detects and picks that source code. In case there are multiple versions installed, it is possible to pass the correct path by defining the NVIDIA_SRC_DIR variable, e.g. export NVIDIA_SRC_DIR=/usr/src/nvidia-520.61.05/nvidia before building the gdrdrv module.

There are two major flavors of NVIDIA driver: 1) proprietary, and 2) opensource. We detect the flavor when compiling gdrdrv based on the source code of the NVIDIA driver. Different flavors come with different features and restrictions: - gdrdrv compiled with the opensource flavor will provide functionality and high performance on all platforms. However, you will not be able to load this gdrdrv driver when the proprietary NVIDIA driver is loaded. - gdrdrv compiled with the proprietary flavor can always be loaded regardless of the flavor of NVIDIA driver you have loaded. However, it may have suboptimal performance on coherent platforms such as Grace-Hopper. Functionally, it will not work correctly on Intel CPUs with Linux kernel built with confidential compute (CC) support, i.e. CONFIG_ARCH_HAS_CC_PLATFORM=y, WHEN CC is enabled at runtime.

Tests

Execute provided tests: ```shell $ gdrcopy_sanity Total: 28, Passed: 28, Failed: 0, Waived: 0

List of passed tests: basic_child_thread_pins_buffer_cumemalloc basic_child_thread_pins_buffer_vmmalloc basic_cumemalloc basic_small_buffers_mapping basic_unaligned_mapping basic_vmmalloc basic_with_tokens data_validation_cumemalloc data_validation_vmmalloc invalidation_access_after_free_cumemalloc invalidation_access_after_free_vmmalloc invalidation_access_after_gdr_close_cumemalloc invalidation_access_after_gdr_close_vmmalloc invalidation_fork_access_after_free_cumemalloc invalidation_fork_access_after_free_vmmalloc invalidation_fork_after_gdr_map_cumemalloc invalidation_fork_after_gdr_map_vmmalloc invalidation_fork_child_gdr_map_parent_cumemalloc invalidation_fork_child_gdr_map_parent_vmmalloc invalidation_fork_child_gdr_pin_parent_with_tokens invalidation_fork_map_and_free_cumemalloc invalidation_fork_map_and_free_vmmalloc invalidation_two_mappings_cumemalloc invalidation_two_mappings_vmmalloc invalidation_unix_sock_shared_fd_gdr_map_cumemalloc invalidation_unix_sock_shared_fd_gdr_map_vmmalloc invalidation_unix_sock_shared_fd_gdr_pin_buffer_cumemalloc invalidation_unix_sock_shared_fd_gdr_pin_buffer_vmmalloc

$ gdrcopy_copybw GPU id:0; name: Tesla V100-SXM2-32GB; Bus id: 0000:06:00 GPU id:1; name: Tesla V100-SXM2-32GB; Bus id: 0000:07:00 GPU id:2; name: Tesla V100-SXM2-32GB; Bus id: 0000:0a:00 GPU id:3; name: Tesla V100-SXM2-32GB; Bus id: 0000:0b:00 GPU id:4; name: Tesla V100-SXM2-32GB; Bus id: 0000:85:00 GPU id:5; name: Tesla V100-SXM2-32GB; Bus id: 0000:86:00 GPU id:6; name: Tesla V100-SXM2-32GB; Bus id: 0000:89:00 GPU id:7; name: Tesla V100-SXM2-32GB; Bus id: 0000:8a:00 selecting device 0 testing size: 131072 rounded size: 131072 gpu alloc fn: cuMemAlloc device ptr: 7f1153a00000 map_d_ptr: 0x7f1172257000 info.va: 7f1153a00000 info.mapped_size: 131072 info.page_size: 65536 info.mapped: 1 info.wc_mapping: 1 page offset: 0 user-space pointer:0x7f1172257000 writing test, size=131072 offset=0 num_iters=10000 write BW: 9638.54MB/s reading test, size=131072 offset=0 num_iters=100 read BW: 530.135MB/s unmapping buffer unpinning buffer closing gdrdrv

$ gdrcopy_copylat GPU id:0; name: Tesla V100-SXM2-32GB; Bus id: 0000:06:00 GPU id:1; name: Tesla V100-SXM2-32GB; Bus id: 0000:07:00 GPU id:2; name: Tesla V100-SXM2-32GB; Bus id: 0000:0a:00 GPU id:3; name: Tesla V100-SXM2-32GB; Bus id: 0000:0b:00 GPU id:4; name: Tesla V100-SXM2-32GB; Bus id: 0000:85:00 GPU id:5; name: Tesla V100-SXM2-32GB; Bus id: 0000:86:00 GPU id:6; name: Tesla V100-SXM2-32GB; Bus id: 0000:89:00 GPU id:7; name: Tesla V100-SXM2-32GB; Bus id: 0000:8a:00 selecting device 0 device ptr: 0x7fa2c6000000 allocated size: 16777216 gpu alloc fn: cuMemAlloc

map_d_ptr: 0x7fa2f9af9000 info.va: 7fa2c6000000 info.mapped_size: 16777216 info.page_size: 65536 info.mapped: 1 info.wc_mapping: 1 page offset: 0 user-space pointer: 0x7fa2f9af9000

gdr_copy_to_mapping num iters for each size: 10000 WARNING: Measuring the API invocation overhead as observed by the CPU. Data might not be ordered all the way to the GPU internal visibility. Test Size(B) Avg.Time(us) gdr_copy_to_mapping 1 0.0889 gdr_copy_to_mapping 2 0.0884 gdr_copy_to_mapping 4 0.0884 gdr_copy_to_mapping 8 0.0884 gdr_copy_to_mapping 16 0.0905 gdr_copy_to_mapping 32 0.0902 gdr_copy_to_mapping 64 0.0902 gdr_copy_to_mapping 128 0.0952 gdr_copy_to_mapping 256 0.0983 gdr_copy_to_mapping 512 0.1176 gdr_copy_to_mapping 1024 0.1825 gdr_copy_to_mapping 2048 0.2549 gdr_copy_to_mapping 4096 0.4366 gdr_copy_to_mapping 8192 0.8141 gdr_copy_to_mapping 16384 1.6155 gdr_copy_to_mapping 32768 3.2284 gdr_copy_to_mapping 65536 6.4906 gdr_copy_to_mapping 131072 12.9761 gdr_copy_to_mapping 262144 25.9459 gdr_copy_to_mapping 524288 51.9100 gdr_copy_to_mapping 1048576 103.8028 gdr_copy_to_mapping 2097152 207.5990 gdr_copy_to_mapping 4194304 415.2856 gdr_copy_to_mapping 8388608 830.6355 gdr_copy_to_mapping 16777216 1661.3285

gdr_copy_from_mapping num iters for each size: 100 Test Size(B) Avg.Time(us) gdr_copy_from_mapping 1 0.9069 gdr_copy_from_mapping 2 1.7170 gdr_copy_from_mapping 4 1.7169 gdr_copy_from_mapping 8 1.7164 gdr_copy_from_mapping 16 0.8601 gdr_copy_from_mapping 32 1.7024 gdr_copy_from_mapping 64 3.1016 gdr_copy_from_mapping 128 3.4944 gdr_copy_from_mapping 256 3.6400 gdr_copy_from_mapping 512 2.4394 gdr_copy_from_mapping 1024 2.8022 gdr_copy_from_mapping 2048 4.6615 gdr_copy_from_m