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dv/dt prof: OpenCL API profiler

The dv/dt prof profiler (libprof.so) intercepts some OpenCL API calls and logs their arguments before invoking the underlying OpenCL implementation.

As of v0.2, the profiler supports two modes:

  1. The ostream mode logs to stdout blocks of text like the following:
[dv/dt] clEnqueueNDRangeKernel
[dv/dt] clEnqueueNDRangeKernel name im2col_float
[dv/dt] clEnqueueNDRangeKernel queue 0x5d3240
[dv/dt] clEnqueueNDRangeKernel kernel 0xbb0300
[dv/dt] clEnqueueNDRangeKernel gwo 0 0 0
[dv/dt] clEnqueueNDRangeKernel gws 16384 1 1
[dv/dt] clEnqueueNDRangeKernel lws 128 1 1
[dv/dt] clEnqueueNDRangeKernel event_wait_list
[dv/dt] clEnqueueNDRangeKernel event 0
[dv/dt] clEnqueueNDRangeKernel start 2016-10-11T20:41:18.041468
[dv/dt] clEnqueueNDRangeKernel profiling 52910121520869 52910121595577 52910130751092 52910132647472
[dv/dt] clEnqueueNDRangeKernel end 2016-10-11T20:41:18.054802
[dv/dt] clEnqueueNDRangeKernel errcode 0

In an offline post-processing step, the Python parser (prof_parser.py) converts the profiler's output into JSON as the following:

{
  "kernel": "0x7f8700",
  "profiling": {
    "start": 46559873667079,
    "end": 46559875636796,
    "queued": 46559863661412,
    "submit": 46559863742203
  },
  "name": "im2col_float",
  "lws": [
    128,
    1,
    1
  ],
  "gwo": [
    0,
    0,
    0
  ],
  "errcode": 0,
  "queue": "0x1cd240",
  "call": "clEnqueueNDRangeKernel",
  "gws": [
    16384,
    1,
    1
  ],
  "timestamp": {
    "start": "2016-10-11T15:50:45.883538",
    "end": "2016-10-11T15:50:45.897364"
  },
  "enqueue_id": 24,
  "event_wait_list": [],
  "event": "0"
}
  1. The cjson mode uses the cJSON library to build JSON online, which then gets logged to stdout. In an offline post-processing step, the Python parser (prof_parser.py) simply loads JSON between the [dv/dt] << and [dv/dt] >> markers.

Effect on execution time

Using the profiler can slow down the program for several reasons:

  • To make parsing robust, the profiler uses formatted printing which is relatively expensive.

  • To time non-blocking calls accurately, the profiler makes them blocking.

  • Optionally, the profiler can alter the program behaviour in other ways, for example, by changing the local work size for one or more kernels in the program. This functionality requires keeping additional state.

The good news is that the kernel execution time and memory copy time are not affected.

OpenCL page at the Khronos Group

Installing the profiler

NB: The easiest way to install the profiler is by using CK-Caffe packages:

$ ck pull repo:ck-caffe --url=https://github.com/dividiti/ck-caffe
$ ck install ck-caffe:package:tool-dvdt-prof
$ ck install ck-caffe:package:tool-dvdt-prof-cjson

Prerequisites

  • CMake 3.0.

  • OpenCL headers and library.

Building the profiler.

Place the source into ${SRC_DIR}. Create ${BUILD_DIR}.

$ cd ${BUILD_DIR}
$ cmake ${SRC_DIR}
$ make prof

${SRC_DIR}/lib now contains libprof.so.

To build and run tests:

$ make check

${SRC_DIR}/lib now contains libprof_test.so which is only useful for testing.

Build options.

By default, the profiler uses boost::chrono to measure wall-clock time. This can be disabled by setting the WALLCLOCK option as follows:

cmake ${SRC_DIR} -DWALLCLOCK=timeofday

(This is particularly handy for Android platforms.)

Alternative mechanisms like gettimeofday() are not supported at the moment.

Using the profiler

Collecting runtime information

$ LD_PRELOAD=<path to profiler e.g. "${BUILD_DIR}/lib/libprof.so"> <path to program to be profiled>

Changing the program behaviour

Several environment variables can be defined when launching the program.

DVDT_PROF_LWS

DVDT_PROF_LWS specifies changes that should be made to the local work size when launching one or more kernels in the program.

DVDT_PROF_LWS="kernel_A:lws_A0,lws_A1,lws_A2 kernel_B:lws_B0,lws_B1,lws_B1 ..."

For example:

DVDT_PROF_LWS="transpose:8,8 gemm:4,16" LD_PRELOAD=<path to profiler> <path to program>

Namely, the per-kernel list elements are separated by spaces; the kernel names (strings) are separated from the local work size tuple by colons; the tuple elements (unsigned integers) are delimited by commas. The number of elements in a tuple must match the number of work-group dimensions as specified in the program or start with the value of 0 to use NULL as the local work size for this kernel.

Note that the profiler cannot check the correctness of any given specification. In particular, the usual execution constraints hold: the global work size dimensions must be divisible by the local work size dimensions; the total work-group size (the product of all the dimensions) cannot exceed CL_KERNEL_WORK_GROUP_SIZE.

DVDT_PROF_LWS_NULL

For convenience, if the environment defines DVDT_PROF_LWS_NULL then NULL is used when launching any kernel in the program. (In this case, DVDT_PROF_LWS gets ignored.)