Tuning pocl behavior with ENV variablesΒΆ

The behavior of pocl can be controlled with multiple environment variables listed below. The variables are helpful both when using and when developing pocl.

  • POCL_BUILDING
If set, the pocl helper scripts, kernel library and headers are searched first from the pocl build directory.
  • POCL_BBVECTORIZE
If set to 1, makes the pocl kernel compiler execute the LLVM BBVectorizer in addition to the SLP vectorizer and the inner loop vectorizer. BBVectorizer has known stability issues, therefore it’s disabled by default, but it can provide performance improvements. See: https://github.com/pocl/pocl/issues/251
  • POCL_CACHE_DIR
If this is set to an existing directory, pocl uses it as the cache directory for all compilation results. This allows reusing compilation results between pocl invocations. If this env is not set, then the default cache directory will be used, which is $XDG_CACHE_DIR/pocl/kcache (if set) or $HOME/.cache/pocl/kcache/ on Unix-like systems.
  • POCL_DEBUG

Enables debug messages to stderr. This will be mostly messages from error condition checks in OpenCL API calls and Event/API timing information. Useful to e.g. distinguish between various reasons a call could return CL_INVALID_VALUE. If clock_gettime is available, messages will include a timestamp.

The old way (setting POCL_DEBUG to 1) has been updated to support categories. Using this limits the amount of debug messages produced. Current options are: general,memory,llvm,events,cache,locking,refcounts,timing,hsa,tce,all. Note: setting POCL_DEBUG to 1 still works.

  • POCL_DEBUG_LLVM_PASSES
When set to 1, enables debug output from LLVM passes during optimization.
  • POCL_DEVICES and POCL_x_PARAMETERS

POCL_DEVICES is a space separated list of the device instances to be enabled. This environment variable is used for the following devices:

  • basic A minimalistic example device driver for executing

    kernels on the host CPU. No multithreading.

  • pthread Native kernel execution on the host CPU with

    threaded execution of work groups using pthreads.

  • ttasim Device that simulates a TTA device using the

    TCE’s ttasim library. Enabled only if TCE libraries installed.

  • hsa* Uses HSA Runtime API to control HSA-compliant

    kernel agents that support HSAIL finalization.

If POCL_DEVICES is not set, one pthread device will be used. To specify parameters for drivers, the POCL_<drivername><instance>_PARAMETERS environment variable can be specified (where drivername is in uppercase). Example:

export POCL_DEVICES="pthread ttasim ttasim"
export POCL_TTASIM0_PARAMETERS="/path/to/my/machine0.adf"
export POCL_TTASIM1_PARAMETERS="/path/to/my/machine1.adf"

Creates three devices, one CPU device with pthread multithreading and two TTA device simulated with the ttasim. The ttasim devices gets a path to the architecture description file of the tta to simulate as a parameter. POCL_TTASIM0_PARAMETERS will be passed to the first ttasim driver instantiated and POCL_TTASIM1_PARAMETERS to the second one.

  • POCL_IMPLICIT_FINISH
Add an implicit call to clFinish afer every clEnqueue* call. Useful mostly for pocl internal development, and is enabled only if pocl is configured with --enable-debug.
  • POCL_KERNEL_CACHE
If this is set to 0 at runtime, kernel compilation files will be deleted at clReleaseProgram(). Note that it’s currently not possible for pocl to avoid interacting with LLVM via on-disk files, so pocl requires some disk space at least temporarily (at runtime).
  • POCL_KERNEL_COMPILER_OPT_SWITCH
Override the default “-O3” that is passed to the LLVM opt as a final optimization switch.
  • POCL_LEAVE_KERNEL_COMPILER_TEMP_FILES
If this is set to 1, the kernel compiler cache/temporary directory that contains all the intermediate compiler files are left as it is. This will be handy for debugging
  • POCL_MAX_PTHREAD_COUNT
The maximum number of threads created for work group execution in the pthread device driver. The default is to determine this from the number of hardware threads available in the CPU.
  • POCL_MAX_WORK_GROUP_SIZE
Forces the maximum WG size returned by the device or kernel work group queries to be at most this number.
  • POCL_OFFLINE_COMPILE
Bool. When enabled(==1), some drivers will create virtual devices which are only good for creating pocl binaries. Requires those drivers to be compiled with support for compilation for those devices.
  • POCL_VECTORIZER_REMARKS
When set to 1, prints out remarks produced by the loop vectorizer of LLVM during kernel compilation.
  • POCL_VERBOSE
If set to 1, output the LLVM commands as they are executed to compile and run kernels.
  • POCL_WORK_GROUP_METHOD

The kernel compiler method to produce the work group functions from multiple work items. Legal values:

auto – Choose the best available method depending on the
kernel and the work group size. Use POCL_FULL_REPLICATION_THRESHOLD=N to set the maximum local size for a work group to be replicated fully with ‘repl’. Otherwise, ‘loops’ is used.
loops – Create for-loops that execute the work items

(under stabilization). The drawback is the need to save the thread contexts in arrays.

The loops will be unrolled a certain number of times of which maximum can be controlled with POCL_WILOOPS_MAX_UNROLL_COUNT=N environment variable (default is to not perform unrolling).

loopvec – Create work-item for-loops (see ‘loops’) and execute
the LLVM LoopVectorizer. The loops are not unrolled but the unrolling decision is left to the generic LLVM passes (the default).
repl – Replicate and chain all work items. This results
in more easily scalarizable private variables, thus might avoid storing work-item context to memory. However, the code bloat is increased with larger WG sizes.
  • POCL_TRACE_EVENT, POCL_TRACE_EVENT_OPT and POCL_TRACE_EVENT_FILTER

If POCL_TRACE_EVENT is set to some tracer name, then all events will be traced automatically. Depending on the backend, traces may be output in different formats. POCL_TRACE_EVENT_FILTER is a comma separated list of string to indicate which event status should be filtered. For instance to trace complete and running events POCL_TRACE_EVENT_FILTER should be set to “complete,running”. Default behavior is to trace all events.

text – Basic text logger for each events state
Use POCL_TRACE_EVENT_OPT=<file> to set the output file. If not specified, it defaults to pocl_trace_event.log
lttng – LTTNG tracepoint support. When activated, a lttng session
must be started. The following tracepoints are available:
  • pocl_trace:ndrange_kernel -> Kernel execution
  • pocl_trace:read_buffer -> Read buffer
  • pocl_trace:write_buffer -> Write buffer
  • pocl_trace:copy_buffer -> Copy buffer
  • pocl_trace:map -> Map image/buffer
  • pocl_trace:command -> other commands

For more information, please see lttng documentation: http://lttng.org/docs/#doc-tracing-your-own-user-application

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