162 lines
6.3 KiB
Markdown
162 lines
6.3 KiB
Markdown
<!---
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Copyright (C) 2020-2021 Intel Corporation
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SPDX-License-Identifier: MIT
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-->
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# Frequently asked questions (OpenCL)
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For general questions,
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see the [main FAQ](https://github.com/intel/compute-runtime/blob/master/FAQ.md).
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## OpenCL version
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### Which version of OpenCL is supported?
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See [README.md](https://github.com/intel/compute-runtime/blob/master/README.md).
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## Known Issues and Limitations
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OpenCL compliance of a driver built from open-source components should not be
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assumed by default. Intel will clearly designate / tag specific builds to
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indicate production quality including formal compliance. Other builds should be
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considered experimental.
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### What is the functional delta to the "Beignet" driver?
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Intel's former open-source [Beignet driver](https://01.org/beignet) provided
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sharing capabilities with MESA OpenGL driver. Our intention is to provide
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these capabilities in NEO in the future.
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NEO supports platforms starting with Gen8 graphics (formerly Broadwell).
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For earlier platforms, please use Beignet driver.
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## Feature: cl_intel_va_api_media_sharing extension
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### Where can I learn more about this extension?
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See the enabling [guide](cl_intel_va_api_media_sharing.md).
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## Feature: cl_cache
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### What is cl_cache?
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This is a mechanism to cache binary representations of OpenCL kernels provided in text form by
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the application. By storing the binary representations, compiling is required only the first time,
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which improves performance.
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### How can cl_cache be enabled?
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In the working directory, manually create *cl_cache* directory.
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The driver will use this directory to store the binary representations of the compiled kernels.
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Note: This will work on all supported OSes.
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### Configuring cl_cache location
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Cached kernels can be stored in a different directory than the default one.
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This is useful when the application is installed into a directory
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for which the user doesn't have permissions.
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#### Linux configuration
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Set the environment variable named `cl_cache_dir` to new location of cl_cache directory.
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#### Example:
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If the application's directory is `/home/user/Document`, by default cl_cache will be stored in
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`/home/user/Document/cl_cache`. If the new path should be `/home/user/Desktop/cl_cache_place`,
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set environment variable `cl_cache_dir` to `/home/user/Desktop/cl_cache_place`.
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```bash
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export cl_cache_dir=/home/user/Desktop/cl_cache_place
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```
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Subsequent application runs with passed source code and `cl_cache_dir` environment variable set will
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reuse previously cached kernel binaries instead of compiling kernels from source.
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#### Windows configuration
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To set the new location of cl_cache directory - in the registry `HKEY_LOCAL_MACHINE\SOFTWARE\Intel\IGFX\OCL`:
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1. add key `cl_cache_dir`
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1. add string value named <path_to_app> to `cl_cache_dir` key
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1. set data of added value to desired location of cl_cache
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#### Example:
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If application is located in `C:\Program Files\application\app.exe`,
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by default cl_cache will be stored in `C:\Program Files\application\cl_cache`.
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If the new path should be `C:\Users\USER\Documents\application\cl_cache`,
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to subkey `HKEY_LOCAL_MACHINE\SOFTWARE\Intel\IGFX\OCL\cl_cache_dir`
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add string value named `C:\Program Files\application\app.exe`
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with data `C:\Users\USER\Documents\application\cl_cache`.
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e.g.
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string value : `HKEY_LOCAL_MACHINE\SOFTWARE\Intel\IGFX\OCL\cl_cache_dir\C:\Program Files\application\app.exe`
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data : `C:\Users\USER\Documents\application\cl_cache`
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Neo will look for string value (REG_SZ) `C:\Program Files\application\app.exe`
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in key `HKEY_LOCAL_MACHINE\SOFTWARE\Intel\IGFX\OCL\cl_cache_dir`.
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Data of this string value will be used as new cl_cache dump directory for this specific application.
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### What are the known limitations of cl_cache?
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1. Not thread safe.
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(Workaround: Make sure your clBuildProgram calls are executed in thread safe fashion.)
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1. Binary representation may not be compatible between various versions of NEO and IGC drivers.
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(Workaround: Manually empty *cl_cache* directory prior to update)
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1. Cache is not automatically cleaned. (Workaround: Manually empty *cl_cache* directory)
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1. Cache may exhaust disk space and cause further failures.
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(Workaround: Monitor and manually empty *cl_cache* directory)
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1. Cache is not process safe.
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## Feature: Out of order queues
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### Implementation details of out of order queues implementation
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Current implementation of out of order queues allows multiple kernels to be run concurently.
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This allows for better device utilization in scenarios where single kernel doesn't fill whole device.
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More details can be found here:
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* [Sample applications](https://github.com/intel/compute-samples/tree/master/compute_samples/applications/commands_aggregation)
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* [IWOCL(*) presentation](https://www.iwocl.org/wp-content/uploads/iwocl-2019-michal-mrozek-intel-breaking-the-last-line-of-performance-border.pdf)
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### Known issues and limitations
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1. Turning on profiling on out of order command queue serializes kernel execution.
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1. Blocking command queue with user events blocks all further submissions until event is unblocked.
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1. Commands blocked by user events, when unblocked are serialized as well.
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## Feature: Double-precision emulation (FP64)
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By default NEO driver enables double precision operations only on platforms with supporting hardware.
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This is signified by exposing the "cl_khr_fp64" extension in the extension string.
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For other platforms, this support can be emulated by the compiler (IGC).
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### How do I enable emulation?
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FP64 emulation can only be enabled on Linux. There are two settings that have to be set.
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#### Runtime setting:
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There are two ways you can enable this feature in NEO:
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* Set an environment variable **OverrideDefaultFP64Settings** to **1**:
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`OverrideDefaultFP64Settings=1`
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* In **igdrcl.config** configuration file in the same directory as application binary
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(you may have to create this file) add a line as such:
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`OverrideDefaultFP64Settings = 1`
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#### Compiler setting:
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IGC reads flags only from environment, so set **IGC_EnableDPEmulation** to **1** as such:
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`IGC_EnableDPEmulation=1`
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After both settings have been set you can run the application normally.
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### Known issues and limitations
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Intel does not claim full specification conformance when using emulated mode.
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We reserve the right to not fix issues that appear only in emulation mode.
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Performance degradation is to be expected and has not been measured by Intel. |