Environment Variables

This document describes environment variables that are having effect on DPC++ compiler and runtime.

Controlling DPC++ RT

Warning: the environment variables described in this document are used for development and debugging of DPC++ compiler and runtime. Their semantics are subject to change. Do not rely on these variables in production code.

Environment variable Values Description
SYCL_PI_TRACE Described below Enable specified level of tracing for PI.
SYCL_BE (deprecated) PI_OPENCL, PI_LEVEL_ZERO, PI_CUDA Force SYCL RT to consider only devices of the specified backend during the device selection. We are planning to deprecate SYCL_BE environment variable in the future. The specific grace period is not decided yet. Please use the new env var SYCL_DEVICE_FILTER instead.
SYCL_DEVICE_TYPE (deprecated) CPU, GPU, ACC, HOST Force SYCL to use the specified device type. If unset, default selection rules are applied. If set to any unlisted value, this control has no effect. If the requested device type is not found, a cl::sycl::runtime_error exception is thrown. If a non-default device selector is used, a device must satisfy both the selector and this control to be chosen. This control only has effect on devices created with a selector. We are planning to deprecate SYCL_DEVICE_TYPE environment variable in the future. The specific grace period is not decided yet. Please use the new env var SYCL_DEVICE_FILTER instead.
SYCL_DEVICE_FILTER backend:device_type:device_num See Section SYCL_DEVICE_FILTER below.
SYCL_PROGRAM_COMPILE_OPTIONS String of valid OpenCL compile options Override compile options for all programs.
SYCL_PROGRAM_LINK_OPTIONS String of valid OpenCL link options Override link options for all programs.
SYCL_USE_KERNEL_SPV Path to the SPIR-V binary Load device image from the specified file. If runtime is unable to read the file, cl::sycl::runtime_error exception is thrown.
SYCL_DUMP_IMAGES Any(*) Dump device image binaries to file. Control has no effect if SYCL_USE_KERNEL_SPV is set.
SYCL_PRINT_EXECUTION_GRAPH Described below Print execution graph to DOT text file.
SYCL_DISABLE_EXECUTION_GRAPH_CLEANUP Any(*) Disable cleanup of finished command nodes at host-device synchronization points.
SYCL_THROW_ON_BLOCK Any(*) Throw an exception on attempt to wait for a blocked command.
SYCL_DEVICELIB_INHIBIT_NATIVE String of device library extensions (separated by a whitespace) Do not rely on device native support for devicelib extensions listed in this option.
SYCL_DEVICE_ALLOWLIST A list of devices and their driver version following the pattern: BackendName:XXX,DeviceType:YYY,DeviceVendorId:0xXYZW,DriverVersion:{{X.Y.Z.W}}. Also may contain PlatformVersion, DeviceName and PlatformName. There is no fixed order of properties in the pattern. Filter out devices that do not match the pattern specified. BackendName accepts host, opencl, level_zero or cuda. DeviceType accepts host, cpu, gpu or acc. DeviceVendorId accepts uint32_t in hex form (0xXYZW). DriverVersion, PlatformVersion, DeviceName and PlatformName accept regular expression. Special characters, such as parenthesis, must be escaped. DPC++ runtime will select only those devices which satisfy provided values above and regex. More than one device can be specified using the piping symbol "|".
SYCL_QUEUE_THREAD_POOL_SIZE Positive integer Number of threads in thread pool of queue.
SYCL_DEVICELIB_NO_FALLBACK Any(*) Disable loading and linking of device library images
SYCL_PI_LEVEL_ZERO_MAX_COMMAND_LIST_CACHE Positive integer Maximum number of oneAPI Level Zero Command lists that can be allocated with no reuse before throwing an "out of resources" error. Default is 20000, threshold may be increased based on resource availabilty and workload demand.
SYCL_PI_LEVEL_ZERO_DISABLE_USM_ALLOCATOR Any(*) Disable USM allocator in Level Zero plugin (each memory request will go directly to Level Zero runtime)
SYCL_PI_LEVEL_ZERO_USM_ALLOCATOR MaxPoolableSize,Capacity,MaxPoolSize Values specified as positive integers. Defaults are 1, 4, 256. MaxPoolableSize is the maximum allocation size in MB that may be pooled. Capacity is the number of allocations in each size range that are freed by the program but retained in the pool for reallocation. Size ranges follow this pattern: 32, 48, 64, 96, 128, 192, and so on, i.e., powers of 2, with one range in between. MaxPoolSize is the maximum size of the pool in MB.
SYCL_PI_LEVEL_ZERO_BATCH_SIZE Integer Sets a preferred number of commands to batch into a command list before executing the command list. A value of 0 causes the batch size to be adjusted dynamically. A value greater than 0 specifies fixed size batching, with the batch size set to the specified value. The default is 0.
SYCL_PI_LEVEL_ZERO_FILTER_EVENT_WAIT_LIST Integer When set to 0, disables filtering of signaled events from wait lists when using the Level Zero backend. The default is 1.
SYCL_PI_LEVEL_ZERO_USE_COPY_ENGINE Integer Allows the use of copy engine, if available in the device, in Level Zero plugin to transfer SYCL buffer or image data between the host and/or device(s) and to fill SYCL buffer or image data in device or shared memory. The default is 1.
SYCL_PI_LEVEL_ZERO_USE_COPY_ENGINE_FOR_D2D_COPY (experimental) Integer Allows the use of copy engine, if available in the device, in Level Zero plugin for device to device copy operations. The default is 0. This option is experimental and will be removed once heuristics are added to make a decision about use of copy engine for device to device copy operations.
SYCL_PI_LEVEL_ZERO_TRACK_INDIRECT_ACCESS_MEMORY Any(*) Enable support of the kernels with indirect access and corresponding deferred release of memory allocations in the Level Zero plugin.
SYCL_PARALLEL_FOR_RANGE_ROUNDING_TRACE Any(*) Enables tracing of parallel_for invocations with rounded-up ranges.
SYCL_DISABLE_PARALLEL_FOR_RANGE_ROUNDING Any(*) Disables automatic rounding-up of parallel_for invocation ranges.
SYCL_ENABLE_PCI Integer When set to 1, enables obtaining the GPU PCI address when using the Level Zero backend. The default is 0.
SYCL_HOST_UNIFIED_MEMORY Integer Enforce host unified memory support or lack of it for the execution graph builder. If set to 0, it is enforced as not supported by all devices. If set to 1, it is enforced as supported by all devices.
SYCL_CACHE_DIR Path Path to persistent cache root directory. Default values are %AppData%\libsycl_cache for Windows and $XDG_CACHE_HOME/libsycl_cache on Linux, if XDG_CACHE_HOME is not set then $HOME/.cache/libsycl_cache. When none of the environment variables are set SYCL persistent cache is disabled.
SYCL_CACHE_TRACE Any(*) If the variable is set, messages are sent to std::cerr when caching events or non-blocking failures happen (e.g. unable to access cache item file).
SYCL_CACHE_DISABLE_PERSISTENT (deprecated) Any(*) Has no effect.
SYCL_CACHE_PERSISTENT Integer Controls persistent device compiled code cache. Turns it on if set to '1' and turns it off if set to '0'. When cache is enabled SYCL runtime will try to cache and reuse JIT-compiled binaries. Default is off.
SYCL_CACHE_EVICTION_DISABLE Any(*) Switches cache eviction off when the variable is set.
SYCL_CACHE_MAX_SIZE Positive integer Cache eviction is triggered once total size of cached images exceeds the value in megabytes (default - 8 192 for 8 GB). Set to 0 to disable size-based cache eviction.
SYCL_CACHE_THRESHOLD Positive integer Cache eviction threshold in days (default value is 7 for 1 week). Set to 0 for disabling time-based cache eviction.
SYCL_CACHE_MIN_DEVICE_IMAGE_SIZE Positive integer Minimum size of device code image in bytes which is reasonable to cache on disk because disk access operation may take more time than do JIT compilation for it. Default value is 0 to cache all images.
SYCL_CACHE_MAX_DEVICE_IMAGE_SIZE Positive integer Maximum size of device image in bytes which is cached. Too big kernels may overload disk too fast. Default value is 1 GB.

(*) Note: Any means this environment variable is effective when set to any non-null value.


This environment variable limits the SYCL RT to use only a subset of the system’s devices. Setting this environment variable affects all of the device query functions (platform::get_devices() and platform::get_platforms()) and all of the device selectors.

The value of this environment variable is a comma separated list of filters, where each filter is a triple of the form “backend:device_type:device_num” (without the quotes). Each element of the triple is optional, but each filter must have at least one value. Possible values of “backend” are:

  • host

  • level_zero

  • opencl

  • cuda

  • *

Possible values of “device_type” are:

  • host

  • cpu

  • gpu

  • acc

  • *

Device_num is an integer that indexes the enumeration of devices from the sycl-ls utility tool, where the first device in that enumeration has index zero in each backend. For example, SYCL_DEVICE_FILTER=2 will return all devices with index ‘2’ from all different backends. If multiple devices satisfy this device number (e.g., GPU and CPU devices can be assigned device number ‘2’), then default_selector will choose the device with the highest heuristic point.

Assuming a filter has all three elements of the triple, it selects only those devices that come from the given backend, have the specified device type, AND have the given device index. If more than one filter is specified, the RT is restricted to the union of devices selected by all filters. The RT does not include the “host” backend and the host device automatically unless one of the filters explicitly specifies the “host” device type. Therefore, SYCL_DEVICE_FILTER=host should be set to enforce SYCL to use the host device only.

Note that all device selectors will throw an exception if the filtered list of devices does not include a device that satisfies the selector. For instance, SYCL_DEVICE_FILTER=cpu,level_zero will cause host_selector() to throw an exception. SYCL_DEVICE_FILTER also limits loading only specified plugins into the SYCL RT. In particular, SYCL_DEVICE_FILTER=level_zero will cause the cpu_selector to throw an exception since SYCL RT will only load the level_zero backend which does not support any CPU devices at this time. When multiple devices satisfy the filter (e..g, SYCL_DEVICE_FILTER=gpu), only one of them will be selected.


SYCL_PRINT_EXECUTION_GRAPH can accept one or more comma separated values from the table below

Option Description
before_addCG print graph before addCG method
after_addCG print graph after addCG method
before_addCopyBack print graph before addCopyBack method
after_addCopyBack print graph after addCopyBack method
before_addHostAcc print graph before addHostAccessor method
after_addHostAcc print graph after addHostAccessor method
always print graph before and after each of the above methods


SYCL_PI_TRACE accepts a bit-mask. Supported tracing levels are in the table below

Option Description
1 Enable basic tracing, which is tracing of PI plugins/devices discovery
2 Enable tracing of the PI calls
-1 Enable all levels of tracing