clvk

clvk is a prototype implementation of OpenCL 3.0 on top of Vulkan using clspv as the compiler.

Supported applications

Full list

Getting dependencies

clvk depends on the following external projects:

• clspv and its dependencies
• OpenCL-Headers
• SPIRV-Headers
• SPIRV-Tools
• SPIRV-LLVM-Translator

clvk also (obviously) depends on a Vulkan implementation. The build system supports a number of options there (see Building section).

To fetch all the dependencies needed to build and run clvk, please run:

git submodule update --init --recursive
./external/clspv/utils/fetch_sources.py --deps llvm

Building

clvk uses CMake for its build system.

Getting started

To build with the default configuration options, just use following:

mkdir -p build
cd build
cmake ../
make -j$(nproc)

Other options

The build system allows a number of things to be configured.

Vulkan implementation

You can select the Vulkan implementation that clvk will target with the CLVK_VULKAN_IMPLEMENTATION build system option. Two options are currently supported:

• -DCLVK_VULKAN_IMPLEMENTATION=system instructs the build system to use the Vulkan implementation provided by your system as detected by CMake. This is the default.

• -DCLVK_VULKAN_IMPLEMENTATION=loader enables building against a copy of the Vulkan Loader sources provided by the user using CLVK_VULKAN_LOADER_DIR. The configuration of the loader to target Vulkan ICDs is left to the user.

• -DCLVK_VULKAN_IMPLEMENTATION=custom instructs the build system to use the values provided by the user manually using -DVulkan_INCLUDE_DIRS and -DVulkan_LIBRARIES.

Tests

It is possible to disable the build of the tests by passing -DCLVK_BUILD_TESTS=OFF.

It is also possible to disable only the build of the tests linking with the static OpenCL library by passing -DCLVK_BUILD_STATIC_TESTS=OFF.

Assertions

Assertions can be controlled with the CLVK_ENABLE_ASSERTIONS build option. They are enabled by default in Debug builds and disabled in other build types.

OpenCL conformance tests

Passing -DCLVK_BUILD_CONFORMANCE_TESTS=ON will instruct CMake to build the OpenCL conformance tests. This is not expected to work out-of-the box at the moment.

Clspv compilation

You can select the compilation style that clvk will use with Clspv via the CLVK_CLSPV_ONLINE_COMPILER option. By default, Clspv is run in a separate process.

• -DCLVK_CLSPV_ONLINE_COMPILER=1 will cause clvk to compile kernels in the same process via the Clspv C++ API.

You can build clvk using an external Clspv source tree by setting -DCLSPV_SOURCE_DIR=/path/to/clspv/source/.

SPIRV components

All needed SPIRV components are added to clvk using git submodules. It is possible to disable the build of those component or to reuse already existing sources:

SPIRV-Headers

SPIRV_HEADERS_SOURCE_DIR can be overriden to use another SPIRV-Headers repository.

SPIRV-Tools

SPIRV_TOOLS_SOURCE_DIR can be overriden to use another SPIRV-Tools repository. You can also disable the build of SPIRV-Tools by setting -DCLVK_BUILD_SPIRV_TOOLS=OFF.

SPIRV-LLVM-Translator

LLVM_SPIRV_SOURCE can be overriden to use another SPIRV-LLVM-Translator repository. Note that it is not used if the compiler support is disabled (enabled by default).

Building for Android

clvk can be built for Android using the Android NDK toolchain.

1. Download and extract the NDK toolchain to a directory (/path/to/ndk)
2. Pass the following options to CMake:
   • -DCMAKE_TOOLCHAIN_FILE=/path/to/ndk/build/cmake/android.toolchain.cmake
   • -DANDROID_ABI=<ABI_FOR_THE_TARGET_DEVICE>, most likely arm64-v8a
   • -DCLVK_VULKAN_IMPLEMENTATION=loader
   • -DCLVK_VULKAN_LOADER_DIR=/path/to/Vulkan-Loader (see above)
3. That should be it!

Using

Via the OpenCL ICD Loader

clvk supports the cl_khr_icd OpenCL extension that makes it possible to use the OpenCL ICD Loader.

Directly

To use clvk to run an OpenCL application, you just need to make sure that the clvk shared library is picked up by the dynamic linker.

When clspv is not built into the shared library (which is currently the default), you also need to make sure that clvk has access to the clspv binary. If you wish to move the built library and clspv binary out of the build tree, you will need to make sure that you provide clvk with a path to the clspv binary via the CLVK_CLSPV_PATH environment variable (see Environment variables).

Unix-like systems (Linux, macOS)

The following ought to work on Unix-like systems:

$ LD_LIBRARY_PATH=/path/to/build /path/to/application

# Running the included simple test
$ LD_LIBRARY_PATH=./build ./build/simple_test

With perfetto traces

Perfetto is a production-grade open-source stack for performance instrumentation and trace analysis. It offers services and libraries and for recording system-level and app-level traces, native + java heap profiling, a library for analyzing traces using SQL and a web-based UI to visualize and explore multi-GB traces.

-- https://github.com/google/perfetto/tree/v29.0#perfetto---system-profiling-app-tracing-and-trace-analysis

Perfetto can be enabled by passing the following options to CMake:

• -DCLVK_PERFETTO_ENABLE=ON
• -DCLVK_PERFETTO_SDK_DIR=/path/to/perfetto/sdk

The perfetto SDK can be found in the Perfetto Github repository

If you already have a perfetto library in your system, you still need to provide the path to the SDK directory so the build system can find perfetto.h. But you should also provide the following option to CMake:

• -DCLVK_PERFETTO_LIBRARY=<your_perfetto_library_name>

By default, clvk will use Perfetto's InProcess backend, which means that you just have to run your application to generate traces. Environment variables can be used to control the maximum size of traces and what file they are saved to.

If you'd rather use Perfetto's System backend, pass the following option to CMake:

• -DCLVK_PERFETTO_BACKEND=System

Once traces have been generated, you can view them using the perfetto trace viewer.

Windows

Copy OpenCL.dll into a system location or alongside the application executable you want to run with clvk.

Tuning clvk

clvk can be tuned to improve the performance of specific workloads or on specific platforms. While we try to have the default parameters set at their best values for each platform, they can be changed for specific applications. One of the best way to know whether something can be improved is to use traces to understand what should be changed.

Group size

clvk is grouping commands and waiting for a call to clFlush or any blocking calls (clFinish, clWaitForEvents, etc.) to submit those groups for execution.

clvk's default group flushing behaviour can be controlled using the following two variables to flush groups as soon as a given number of commands have been grouped:

• CLVK_MAX_CMD_GROUP_SIZE
• CLVK_MAX_FIRST_CMD_GROUP_SIZE

Batch size

clvk relies on vulkan to offload workoad to the GPU. As such, it is better to batch OpenCL commands (translated into vulkan commands) into a vulkan command buffer. But doing that may increase the latency to start running commands.

The size of those batches can be controlled using the following two variables:

• CLVK_MAX_CMD_BATCH_SIZE
• CLVK_MAX_FIRST_CMD_BATCH_SIZE

Environment variables

The behaviour of a few things in clvk can be controlled by environment variables. Here's a quick guide:

• CLVK_LOG controls the level of logging

  • 0: only print fatal messages (default)
  • 1: print errors as well
  • 2: print warnings as well
  • 3: print information messages as well
  • 4: print all debug messages

• CLVK_LOG_COLOUR controls colour logging

  • 0: disabled
  • 1: enabled (default when the output is a terminal)

• CLVK_LOG_DEST controls where the logging output goes

  • stderr: logging goes to the standard error (default)
  • stdout: logging goes to the standard output
  • file:<fname>: logging goes to <fname>. The file will be created if it does not exist and will be truncated.

• CLVK_CLSPV_PATH to provide a path to the clspv binary to use

• CLVK_LLVMSPIRV_BIN to provide a path to the llvm-spirv binary to use

• CLVK_VALIDATION_LAYERS allows to enable Vulkan validation layers

  • 0: disabled (default)
  • 1: enabled

• CLVK_CLSPV_OPTIONS to provide additional options to pass to clspv

• CLVK_QUEUE_PROFILING_USE_TIMESTAMP_QUERIES to use timestamp queries to measure the CL_PROFILING_COMMAND_{START,END} profiling infos on devices that do not support VK_EXT_calibrated_timestamps.

  • 0: disabled (default)
  • 1: enabled

  WARNING: the values will not use the same time base as that used for CL_PROFILING_COMMAND_{QUEUED,SUBMIT} but this allows to get closer-to-the-execution timestamps.

• CLVK_SPIRV_VALIDATION controls SPIR-V validation behaviour.

  • 0: skip validation
  • 1: warn when validation fails
  • 2: fail compilation and report an error when validation fails (default)

• CLVK_SKIP_SPIRV_CAPABILITY_CHECK to avoid checking whether the Vulkan device supports all of the SPIR-V capabilities declared by each SPIR-V module.

• CLVK_MAX_BATCH_SIZE to control the maximum number of commands that can be recorded in a single command buffer.

• CLVK_KEEP_TEMPORARIES to keep temporary files created during program build, compilation and link operations.

  • 0: disabled (default)
  • 1: enabled

• CLVK_CACHE_DIR specifies a directory used for caching compiled program data between applications runs. The user is responsible for ensuring that this directory is not used concurrently by more than one application.

• CLVK_COMPLIER_TEMP_DIR specifies a directory used to create a temporary folder to store compiled program data used in a single run. This folder shall have write permission (default: current directory).

• CLVK_MAX_CMD_GROUP_SIZE specifies the maximum number of commands in a group. When a group reaches this number, it is automatically flushed.

• CLVK_MAX_FIRST_CMD_GROUP_SIZE specifies the maximum number of commands in a group when there is no group to be processed or being processed in the queue.

• CLVK_MAX_CMD_BATCH_SIZE specifies the maximum number of commands per batch. When this number is reached, the batch will be added to the current group of commands, and a new batch will be created.

• CLVK_MAX_FIRST_CMD_BATCH_SIZE specifies the maximum number of commands per batch when there is no batch to be processed or being processed in the queue.

• CLVK_PERFETTO_TRACE_MAX_SIZE specifies the maximum size (in kB) of traces generated by Perfetto. It only applies when using Perfetto with the InProcess backend.

• CLVK_PERFETTO_TRACE_DEST specifies the filename to use for the traces generated by Perfetto (default: clvk.perfetto-trace).

Limitations

• Only one device per CL context
• No support for out-of-order queues
• No support for device partitioning
• No support for native kernels
• All the limitations implied by the use of clspv
• ... and problably others