3.2 OpenCL平台模型
OpenCL平台需要包含一个主处理器和一个或多个OpenCL设备。平台模型定义了host和device的角色，并且为device提供了一种抽象的硬件模型。一个device可以被划分成一个或多个计算单元，这些计算单元在之后能被分成一个或多个“处理单元”(processing elements)。具体的关系可见图3.1。
图3.1 OpenCL平台具有多个计算设备。每个计算设备都具有一个或多个计算单元。一个计算单元又由一个或多个处理元素(PEs)构成。系统中可以同时具有多个平台。例如，在一个系统中可以既有AMD的平台和Intel的平台。
平台模型是应用开发的重点，其保证了OpenCL代码的可移植性(在具有OpenCL能力的系统间)。即使只在一个系统中，这个系统也可以具有多个不同的OpenCL平台，这些平台可以被不同的应用所使用。平台模型的API允许一个OpenCL应用能够适应和选择对应的平台和计算设备，从而在相应平台和设备上运行应用。
应用可以使用OpenCL运行时API，选择对应提供商提供的对应平台。不过，平台上能指定和互动的设备，也只限于供应商提供的那些设备。例如，如果选择了A公司的平台，那么就不能使用B公司的GPU。不过，平台硬件并不需要由供应商独家提供。例如，AMD和Intel的实现可以使用其他公司的x86 CPU作为设备。
编程者写编写OpenCL C代码时，设备架构会被抽象成平台模型。供应商只需要将抽象的架构映射到对应的物理硬件上即可。平台模型定义了具有一组计算单元的设备，且每个计算单元的功能都是独立的。计算单元也可以划分成更多个处理单元。图3.1展示的就是这样的一种层级模型。举个例子，AMD Radeon R9 290X图形卡(device)包含44个向量处理器(计算单元)。每个计算单元都由4个16通道SIMD引擎，一共就有64个SIMD通道(处理单元)。Radeon R9 290X上每个SIMD通道都能处理一个标量指令。运行GPU设备能同时执行44x16x4=2816条指令。
3.2.1 平台和设备
clGetPlatformIDs()这个API就是查找制定系统上的可用OpenCL平台的集合。在具体的OpenCL程序中，这个API一般会调用两次，用来查询和获取到对应的平台信息。第一次调用这个API需要传入num_platforms作为数量参数，传入NULL作为平台参数。这样就能获取在该系统上有多少个平台可供使用。编程者可以开辟对应大小的空间(指针命名为platforms)，来存放对应的平台对象(类型为 cl_platform_id)。第二次调用该API是，就可将platforms传入来获取对应数量的平台对象。平台查找完成后，使用clGetPlatformInfo()API可以查询对应供应商所提供的平台，然后决定使用哪个平台进行运行OpenCL程序。clGetPlatformIDs()这个API需要在其他API之前调用，3.6节中可以从矢量相加的源码中进一步了解。
cl_int
clGetPlatformIDs(
cl_uint num_entries,
cl_platform_id *platforms,
cl_uint *num_platforms)当平台确定好之后，下一步就是要查询平台上可用的设备了。clGetDeviceIDs()API就是用来做这件事的，并且在使用流程上和clGetPlatformIDs()很类似。clGetDeviceIDs()多了平台对象和设备类型作为入参，不过也需要简单的三步就能创建device：第一，查询设备的数量；第二，分配对应数量的空间来存放设备对象；第三，选择期望使用的设备(确定设备对象)。device_type参数可以将设备限定为GPU(CL_DEVICE_TYPE_GPU)，限定为CPU(CL_DEVICE_TYPE_CPU)，或所有设备(CL_DEVICE_TYPE_ALL)，当然还有其他选项。这些参数都必须传递给clGetDeviceIDs()。相较于平台的查询API，clGetDeviceInfo()API可用来查询每个设备的名称、类型和供应商。
cl_int
clGetDeviceIDs(
cl_platform_id platform,
cl_device_type device_type,
cl_uint num_entries,
cl_device_id *devices,
cl_uint *num_devices)AMD的并行加速处理软件开发工具(APP SDK)中有一个clinfo的程序，其使用clGetPlatformInfo()和clGetDeviceInfo()来获取对应系统中的平台和设备信息。硬件信息，比如内存总量和总线带宽也是可以用该程序获取。在了解其他OpenCL特性之前，我们先休息一下，了解一下clinfo的输入，如图3.2。
译者机器的clinfo显示，译者和原书使用的AMD APP SDK版本不大一样。从观察上来看，原书应该隐藏了一些硬件显示。
Number of platforms: 3
Platform Profile: FULL_PROFILE
Platform Version: OpenCL 1.2 CUDA 8.0.0
Platform Name: NVIDIA CUDA
Platform Vendor: NVIDIA Corporation
Platform Extensions:
cl_khr_global_int32_base_atomics
cl_khr_global_int32_extended_atomics
cl_khr_local_int32_base_atomics
cl_khr_local_int32_extended_atomics
cl_khr_fp64
cl_khr_byte_addressable_store
cl_khr_icd cl_khr_gl_sharing
cl_nv_compiler_options
cl_nv_device_attribute_query
cl_nv_pragma_unroll
cl_nv_d3d10_sharing
cl_khr_d3d10_sharing
cl_nv_d3d11_sharing
cl_nv_copy_opts
Platform Profile: FULL_PROFILE
Platform Version: OpenCL 1.2
Platform Name: Intel(R) OpenCL
Platform Vendor: Intel(R) Corporation
Platform Extensions:
cl_intel_dx9_media_sharing
cl_khr_3d_image_writes
cl_khr_byte_addressable_store
cl_khr_d3d11_sharing
cl_khr_depth_images
cl_khr_dx9_media_sharing
cl_khr_gl_sharing
cl_khr_global_int32_base_atomics
cl_khr_global_int32_extended_atomics
cl_khr_icd cl_khr_local_int32_base_atomics
cl_khr_local_int32_extended_atomics
cl_khr_spir
Platform Profile: FULL_PROFILE
Platform Version: OpenCL 2.0 AMD-APP (1800.8)
Platform Name: AMD Accelerated Parallel Processing
Platform Vendor: Advanced Micro Devices, Inc.
Platform Extensions:
cl_khr_icd
cl_khr_d3d10_sharing
cl_khr_d3d11_sharing
cl_khr_dx9_media_sharing
cl_amd_event_callback
cl_amd_offline_devices
Platform Name: NVIDIA CUDA
Number of devices: 1
Device Type: CL_DEVICE_TYPE_GPU
Vendor ID: 10deh
Max compute units: 4
Max work items dimensions: 3
Max work items[0]: 1024
Max work items[1]: 1024
Max work items[2]: 64
Max work group size: 1024
Preferred vector width char: 1
Preferred vector width short: 1
Preferred vector width int: 1
Preferred vector width long: 1
Preferred vector width float: 1
Preferred vector width double: 1
Native vector width char: 1
Native vector width short: 1
Native vector width int: 1
Native vector width long: 1
Native vector width float: 1
Native vector width double: 1
Max clock frequency: 862Mhz
Address bits: 64
Max memory allocation: 536870912
Image support: Yes
Max number of images read arguments: 256
Max number of images write arguments: 16
Max image 2D width: 16384
Max image 2D height: 16384
Max image 3D width: 4096
Max image 3D height: 4096
Max image 3D depth: 4096
Max samplers within kernel: 32
Max size of kernel argument: 4352
Alignment (bits) of base address: 4096
Minimum alignment (bytes) for any datatype: 128
Single precision floating point capability
Denorms: Yes
Quiet NaNs: Yes
Round to nearest even: Yes
Round to zero: Yes
Round to +ve and infinity: Yes
IEEE754-2008 fused multiply-add: Yes
Cache type: Read/Write
Cache line size: 128
Cache size: 65536
Global memory size: 2147483648
Constant buffer size: 65536
Max number of constant args: 9
Local memory type: Scratchpad
Local memory size: 49152
Kernel Preferred work group size multiple: 32
Error correction support: 0
Unified memory for Host and Device: 0
Profiling timer resolution: 1000
Device endianess: Little
Available: Yes
Compiler available: Yes
Execution capabilities:
Execute OpenCL kernels: Yes
Execute native function: No
Queue on Host properties:
Out-of-Order: Yes
Profiling : Yes
Platform ID: 000002D3A374DC10
Name: GeForce GTX 765M
Vendor: NVIDIA Corporation
Device OpenCL C version: OpenCL C 1.2
Driver version: 375.95
Profile: FULL_PROFILE
Version: OpenCL 1.2 CUDA
Extensions:
cl_khr_global_int32_base_atomics
cl_khr_global_int32_extended_atomics
cl_khr_local_int32_base_atomics
cl_khr_local_int32_extended_atomics
cl_khr_fp64
cl_khr_byte_addressable_store
cl_khr_icd
cl_khr_gl_sharing
cl_nv_compiler_options
cl_nv_device_attribute_query
cl_nv_pragma_unroll
cl_nv_d3d10_sharing
cl_khr_d3d10_sharing
cl_nv_d3d11_sharing
cl_nv_copy_opts
Platform Name: Intel(R) OpenCL
Number of devices: 2
Device Type: CL_DEVICE_TYPE_GPU
Vendor ID: 8086h
Max compute units: 20
Max work items dimensions: 3
Max work items[0]: 512
Max work items[1]: 512
Max work items[2]: 512
Max work group size: 512
Preferred vector width char: 1
Preferred vector width short: 1
Preferred vector width int: 1
Preferred vector width long: 1
Preferred vector width float: 1
Preferred vector width double: 0
Native vector width char: 1
Native vector width short: 1
Native vector width int: 1
Native vector width long: 1
Native vector width float: 1
Native vector width double: 0
Max clock frequency: 1150Mhz
Address bits: 64
Max memory allocation: 427189862
Image support: Yes
Max number of images read arguments: 128
Max number of images write arguments: 128
Max image 2D width: 16384
Max image 2D height: 16384
Max image 3D width: 2048
Max image 3D height: 2048
Max image 3D depth: 2048
Max samplers within kernel: 16
Max size of kernel argument: 1024
Alignment (bits) of base address: 1024
Minimum alignment (bytes) for any datatype: 128
Single precision floating point capability
Denorms: No
Quiet NaNs: Yes
Round to nearest even: Yes
Round to zero: Yes
Round to +ve and infinity: Yes
IEEE754-2008 fused multiply-add: No
Cache type: Read/Write
Cache line size: 64
Cache size: 262144
Global memory size: 1708759450
Constant buffer size: 65536
Max number of constant args: 8
Local memory type: Scratchpad
Local memory size: 65536
Kernel Preferred work group size multiple: 32
Error correction support: 0
Unified memory for Host and Device: 1
Profiling timer resolution: 80
Device endianess: Little
Available: Yes
Compiler available: Yes
Execution capabilities:
Execute OpenCL kernels: Yes
Execute native function: No
Queue on Host properties:
Out-of-Order: No
Profiling : Yes
Platform ID: 000002D3A374C760
Name: Intel(R) HD Graphics 4600
Vendor: Intel(R) Corporation
Device OpenCL C version: OpenCL C 1.2
Driver version: 20.19.15.4531
Profile: FULL_PROFILE
Version: OpenCL 1.2
Extensions:
cl_intel_accelerator
cl_intel_advanced_motion_estimation
cl_intel_ctz
cl_intel_d3d11_nv12_media_sharing
cl_intel_dx9_media_sharing
cl_intel_motion_estimation
cl_intel_simultaneous_sharing
cl_intel_subgroups
cl_khr_3d_image_writes
cl_khr_byte_addressable_store
cl_khr_d3d10_sharing
cl_khr_d3d11_sharing
cl_khr_depth_images
cl_khr_dx9_media_sharing
cl_khr_gl_depth_images
cl_khr_gl_event
cl_khr_gl_msaa_sharing
cl_khr_global_int32_base_atomics
cl_khr_global_int32_extended_atomics
cl_khr_gl_sharing
cl_khr_icd
cl_khr_image2d_from_buffer
cl_khr_local_int32_base_atomics
cl_khr_local_int32_extended_atomics
cl_khr_spir
Device Type: CL_DEVICE_TYPE_CPU
Vendor ID: 8086h
Max compute units: 8
Max work items dimensions: 3
Max work items[0]: 8192
Max work items[1]: 8192
Max work items[2]: 8192
Max work group size: 8192
Preferred vector width char: 1
Preferred vector width short: 1
Preferred vector width int: 1
Preferred vector width long: 1
Preferred vector width float: 1
Preferred vector width double: 1
Native vector width char: 32
Native vector width short: 16
Native vector width int: 8
Native vector width long: 4
Native vector width float: 8
Native vector width double: 4
Max clock frequency: 2400Mhz
Address bits: 64
Max memory allocation: 2126515200
Image support: Yes
Max number of images read arguments: 480
Max number of images write arguments: 480
Max image 2D width: 16384
Max image 2D height: 16384
Max image 3D width: 2048
Max image 3D height: 2048
Max image 3D depth: 2048
Max samplers within kernel: 480
Max size of kernel argument: 3840
Alignment (bits) of base address: 1024
Minimum alignment (bytes) for any datatype: 128
Single precision floating point capability
Denorms: Yes
Quiet NaNs: Yes
Round to nearest even: Yes
Round to zero: No
Round to +ve and infinity: No
IEEE754-2008 fused multiply-add: No
Cache type: Read/Write
Cache line size: 64
Cache size: 262144
Global memory size: 8506060800
Constant buffer size: 131072
Max number of constant args: 480
Local memory type: Global
Local memory size: 32768
Kernel Preferred work group size multiple: 128
Error correction support: 0
Unified memory for Host and Device: 1
Profiling timer resolution: 427
Device endianess: Little
Available: Yes
Compiler available: Yes
Execution capabilities:
Execute OpenCL kernels: Yes
Execute native function: Yes
Queue on Host properties:
Out-of-Order: Yes
Profiling : Yes
Platform ID: 000002D3A374C760
Name: Intel(R) Core(TM) i7-4700MQ CPU @ 2.40GHz
Vendor: Intel(R) Corporation
Device OpenCL C version: OpenCL C 1.2
Driver version: 5.2.0.10094
Profile: FULL_PROFILE
Version: OpenCL 1.2 (Build 10094)
Extensions:
cl_khr_icd
cl_khr_global_int32_base_atomics
cl_khr_global_int32_extended_atomics
cl_khr_local_int32_base_atomics
cl_khr_local_int32_extended_atomics
cl_khr_byte_addressable_store
cl_khr_depth_images
cl_khr_3d_image_writes
cl_intel_exec_by_local_thread
cl_khr_spir
cl_khr_dx9_media_sharing
cl_intel_dx9_media_sharing
cl_khr_d3d11_sharing
cl_khr_gl_sharing
cl_khr_fp64
Platform Name: AMD Accelerated Parallel Processing
Number of devices: 1
Device Type: CL_DEVICE_TYPE_CPU
Vendor ID: 1002h
Board name:
Max compute units: 8
Max work items dimensions: 3
Max work items[0]: 1024
Max work items[1]: 1024
Max work items[2]: 1024
Max work group size: 1024
Preferred vector width char: 16
Preferred vector width short: 8
Preferred vector width int: 4
Preferred vector width long: 2
Preferred vector width float: 8
Preferred vector width double: 4
Native vector width char: 16
Native vector width short: 8
Native vector width int: 4
Native vector width long: 2
Native vector width float: 8
Native vector width double: 4
Max clock frequency: 2394Mhz
Address bits: 64
Max memory allocation: 2147483648
Image support: Yes
Max number of images read arguments: 128
Max number of images write arguments: 64
Max image 2D width: 8192
Max image 2D height: 8192
Max image 3D width: 2048
Max image 3D height: 2048
Max image 3D depth: 2048
Max samplers within kernel: 16
Max size of kernel argument: 4096
Alignment (bits) of base address: 1024
Minimum alignment (bytes) for any datatype: 128
Single precision floating point capability
Denorms: Yes
Quiet NaNs: Yes
Round to nearest even: Yes
Round to zero: Yes
Round to +ve and infinity: Yes
IEEE754-2008 fused multiply-add: Yes
Cache type: Read/Write
Cache line size: 64
Cache size: 32768
Global memory size: 8506060800
Constant buffer size: 65536
Max number of constant args: 8
Local memory type: Global
Local memory size: 32768
Max pipe arguments: 16
Max pipe active reservations: 16
Max pipe packet size: 2147483648
Max global variable size: 1879048192
Max global variable preferred total size: 1879048192
Max read/write image args: 64
Max on device events: 0
Queue on device max size: 0
Max on device queues: 0
Queue on device preferred size: 0
SVM capabilities:
Coarse grain buffer: No
Fine grain buffer: No
Fine grain system: No
Atomics: No
Preferred platform atomic alignment: 0
Preferred global atomic alignment: 0
Preferred local atomic alignment: 0
Kernel Preferred work group size multiple: 1
Error correction support: 0
Unified memory for Host and Device: 1
Profiling timer resolution: 427
Device endianess: Little
Available: Yes
Compiler available: Yes
Execution capabilities:
Execute OpenCL kernels: Yes
Execute native function: Yes
Queue on Host properties:
Out-of-Order: No
Profiling : Yes
Queue on Device properties:
Out-of-Order: No
Profiling : No
Platform ID: 00007FFB80F36D30
Name: Intel(R) Core(TM) i7-4700MQ CPU @ 2.40GHz
Vendor: GenuineIntel
Device OpenCL C version: OpenCL C 1.2
Driver version: 1800.8 (sse2,avx)
Profile: FULL_PROFILE
Version: OpenCL 1.2 AMD-APP (1800.8)
Extensions:
cl_khr_fp64
cl_amd_fp64
cl_khr_global_int32_base_atomics
cl_khr_global_int32_extended_atomics
cl_khr_local_int32_base_atomics
cl_khr_local_int32_extended_atomics
cl_khr_int64_base_atomics
cl_khr_int64_extended_atomics
cl_khr_3d_image_writes
cl_khr_byte_addressable_store
cl_khr_gl_sharing
cl_ext_device_fission
cl_amd_device_attribute_query
cl_amd_vec3
cl_amd_printf
cl_amd_media_ops
cl_amd_media_ops2
cl_amd_popcnt
cl_khr_d3d10_sharing
cl_khr_spir
cl_khr_gl_event原书clinfo信息
Number of platforms: 1
Platform Profile: FULL_PROFILE
Platform Version: OpenCL 2.0 AMD-APP (1642.5)
Platform Name: AMD Accelerated Parallel Processing
Platform Vendor: Advanced Micro Devices, Inc.
Platform Extensions:
cl_khr_icd
cl_khr_d3d10_sharing
cl_khr_icd
cl_amd_event_callback
cl_amd_offline_devices
Platform Name: AMD Accelerated Parallel Processing
Number of devices: 2
Vendor ID: 1002h
Device Type: CL_DEVICE_TYPE_GPU
Board name: AMD Radeon R9 200 Series
Device Topology: PCI[B#1, D#0, F#0]
Max compute units: 40
Max work group size: 256
Native vector width int: 1
Max clock frequency: 1000Mhz
Max memory allocation: 2505572352
Image support: Yes
Max image 3D width: 2048
Cache line size: 64
Global memory size: 3901751296
Platform ID: 0x7f54fb22cfd0
Name: Hawaii
Vendor: Advanced Micro Devices, Inc.
Device OpenCL C version: OpenCL C 2.0
Driver version: 1642.5(VM)
Profile: FULL_PROFILE
Version: OpenCL 2.0 AMD-APP (1642.5)
Extensions:
cl_khr_fp64_cl_amd_fp64
cl_khr_global_int32_base_atomics
cl_khr_global_int32_extended_atomics
cl_khr_local_int32_base_atomics
Device Type: CL_DEVICE_TYPE_CPU
Vendor ID: 1002h
Board name:
Max compute units: 8
Max work items dimensions: 3
Max work items[0]: 1024
Max work items[1]: 1024
Name: AMD FX(tm)-8120 Eight-Core Processor
Vendor: AuthenticAMD
Device OpenCL C version: OpenCL C 1.2
Driver version: 1642.5(sse2, avx, fma4)
Profile: FULL_PROFILE
Version: OpenCL 1.2 (Build 10094)图3.2 通过clinfo程序输出一些OpenCL平台和设备信息。我们能看到AMD平台上有两个设备(一个CPU和一个GPU)。这些信息都能通过平台API查询到。
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