为什么相同的 OpenCL 代码与 Intel Xeon CPU 和 NVIDIA GTX 1080 Ti GPU 的输出不同？
[英]Why does the same OpenCL code have different outputs from Intel Xeon CPU and NVIDIA GTX 1080 Ti GPU?
问题描述
我正在尝试使用 OpenCL 并行化 Monte Carlo 模拟。 我使用 MWC64X 作为统一随机数生成器。 该代码在不同的 Intel CPU 上运行良好，因为并行计算的输出非常接近顺序计算的输出。
Using OpenCL device: Intel(R) Xeon(R) CPU E5-2630L v3 @ 1.80GHz
Literal influence running time: 0.029048 seconds r1 seqInfl= 0.4771
Literal influence running time: 0.029762 seconds r2 seqInfl= 0.4771
Literal influence running time: 0.029742 seconds r3 seqInfl= 0.4771
Literal influence running time: 0.02971 seconds ra seqInfl= 0.4771
Literal influence running time: 0.029225 seconds trust1-57 seqInfl= 0.6001
Literal influence running time: 0.04992 seconds trust110-1 seqInfl= 0
Literal influence running time: 0.034636 seconds trust4-57 seqInfl= 0
Literal influence running time: 0.049079 seconds trust57-110 seqInfl= 0
Literal influence running time: 0.024442 seconds trust57-4 seqInfl= 0.8026
Literal influence running time: 0.04946 seconds trust33-1 seqInfl= 0
Literal influence running time: 0.049071 seconds trust57-33 seqInfl= 0
Literal influence running time: 0.053117 seconds trust4-1 seqInfl= 0.1208
Literal influence running time: 0.051642 seconds trust57-1 seqInfl= 0
Literal influence running time: 0.052052 seconds trust57-64 seqInfl= 0
Literal influence running time: 0.052118 seconds trust64-1 seqInfl= 0
Literal influence running time: 0.051998 seconds trust57-7 seqInfl= 0
Literal influence running time: 0.052069 seconds trust7-1 seqInfl= 0
Total number of literals: 17
Sequential influence running time: 0.71728 seconds
Sequential maxInfluence Literal: trust57-4 0.8026
index1= 17 size= 51 dim1_size= 6
sum0:4781 influence0:0.478100 sum2:4781 influence2:0.478100 sum6:0 influence6:0.000000 sum10:0 sum12:0 influence12:0.000000 sum7:0 influence7:0.000000 influence10:0.000000 sum4:5962 influence4:0.596200 sum8:7971 influence8:0.797100 sum1:4781 influence1:0.478100 sum3:4781 influence3:0.478100 sum13:0 influence13:0.000000 sum11:1261 influence11:0.126100 sum9:0 influence9:0.000000 sum14:0 influence14:0.000000 sum5:0 influence5:0.000000 sum15:0 influence15:0.000000 sum16:0 influence16:0.000000
Parallel influence running time: 0.054391 seconds
Parallel maxInfluence Literal: trust57-4 Infl=0.7971
但是，当我在 GeForce GTX 1080 Ti 上运行代码时，安装了 NVIDIA-SMI 430.40 和 CUDA 10.1 以及 OpenCL 1.2 CUDA，输出如下：
Using OpenCL device: GeForce GTX 1080 Ti
Influence:
Literal influence running time: 0.011119 seconds r1 seqInfl= 0.4771
Literal influence running time: 0.011238 seconds r2 seqInfl= 0.4771
Literal influence running time: 0.011408 seconds r3 seqInfl= 0.4771
Literal influence running time: 0.01109 seconds ra seqInfl= 0.4771
Literal influence running time: 0.011132 seconds trust1-57 seqInfl= 0.6001
Literal influence running time: 0.018978 seconds trust110-1 seqInfl= 0
Literal influence running time: 0.013093 seconds trust4-57 seqInfl= 0
Literal influence running time: 0.018968 seconds trust57-110 seqInfl= 0
Literal influence running time: 0.009105 seconds trust57-4 seqInfl= 0.8026
Literal influence running time: 0.018753 seconds trust33-1 seqInfl= 0
Literal influence running time: 0.018583 seconds trust57-33 seqInfl= 0
Literal influence running time: 0.02005 seconds trust4-1 seqInfl= 0.1208
Literal influence running time: 0.01957 seconds trust57-1 seqInfl= 0
Literal influence running time: 0.019686 seconds trust57-64 seqInfl= 0
Literal influence running time: 0.019632 seconds trust64-1 seqInfl= 0
Literal influence running time: 0.019687 seconds trust57-7 seqInfl= 0
Literal influence running time: 0.019859 seconds trust7-1 seqInfl= 0
Total number of literals: 17
Sequential influence running time: 0.272032 seconds
Sequential maxInfluence Literal: trust57-4 0.8026
index1= 17 size= 51 dim1_size= 6
sum0:10000 sum1:10000 sum2:10000 sum3:10000 sum4:10000 sum5:0 sum6:0 sum7:0 sum8:10000 sum9:0 sum10:0 sum11:0 sum12:0 sum13:0 sum14:0 sum15:0 sum16:0
Parallel influence running time: 0.193581 seconds
“影响”值等于sum*1.0/10000 ，因此并行影响仅由 1 和 0 组成，这是不正确的（在 GPU 运行中）并且在 Intel CPU 上并行化时不会发生。
当我检查随机数生成器的输出时if(flag==0) printf("randint=%u",randint); ，似乎 GPU 上的输出都为零。 下面是clinfo和.cl代码：
Device Name GeForce GTX 1080 Ti
Device Vendor NVIDIA Corporation
Device Vendor ID 0x10de
Device Version OpenCL 1.2 CUDA
Driver Version 430.40
Device OpenCL C Version OpenCL C 1.2
Device Type GPU
Device Topology (NV) PCI-E, 68:00.0
Device Profile FULL_PROFILE
Device Available Yes
Compiler Available Yes
Linker Available Yes
Max compute units 28
Max clock frequency 1721MHz
Compute Capability (NV) 6.1
Device Partition (core)
Max number of sub-devices 1
Supported partition types None
Max work item dimensions 3
Max work item sizes 1024x1024x64
Max work group size 1024
Preferred work group size multiple 32
Warp size (NV) 32
Preferred / native vector sizes
char 1 / 1
short 1 / 1
int 1 / 1
long 1 / 1
half 0 / 0 (n/a)
float 1 / 1
double 1 / 1 (cl_khr_fp64)
Half-precision Floating-point support (n/a)
Single-precision Floating-point support (core)
Denormals Yes
Infinity and NANs Yes
Round to nearest Yes
Round to zero Yes
Round to infinity Yes
IEEE754-2008 fused multiply-add Yes
Support is emulated in software No
Correctly-rounded divide and sqrt operations Yes
Double-precision Floating-point support (cl_khr_fp64)
Denormals Yes
Infinity and NANs Yes
Round to nearest Yes
Round to zero Yes
Round to infinity Yes
IEEE754-2008 fused multiply-add Yes
Support is emulated in software No
Address bits 64, Little-Endian
Global memory size 11720130560 (10.92GiB)
Error Correction support No
Max memory allocation 2930032640 (2.729GiB)
Unified memory for Host and Device No
Integrated memory (NV) No
Minimum alignment for any data type 128 bytes
Alignment of base address 4096 bits (512 bytes)
Global Memory cache type Read/Write
Global Memory cache size 458752 (448KiB)
Global Memory cache line size 128 bytes
Image support Yes
Max number of samplers per kernel 32
Max size for 1D images from buffer 134217728 pixels
Max 1D or 2D image array size 2048 images
Max 2D image size 16384x32768 pixels
Max 3D image size 16384x16384x16384 pixels
Max number of read image args 256
Max number of write image args 16
Local memory type Local
Local memory size 49152 (48KiB)
Registers per block (NV) 65536
Max number of constant args 9
Max constant buffer size 65536 (64KiB)
Max size of kernel argument 4352 (4.25KiB)
Queue properties
Out-of-order execution Yes
Profiling Yes
Prefer user sync for interop No
Profiling timer resolution 1000ns
Execution capabilities
Run OpenCL kernels Yes
Run native kernels No
Kernel execution timeout (NV) Yes
Concurrent copy and kernel execution (NV) Yes
Number of async copy engines 2
printf() buffer size 1048576 (1024KiB)
#define N 70 // N > index, which is the total number of literals
#define BASE 4294967296UL
//! Represents the state of a particular generator
typedef struct{ uint x; uint c; } mwc64x_state_t;
enum{ MWC64X_A = 4294883355U };
enum{ MWC64X_M = 18446383549859758079UL };
void MWC64X_Step(mwc64x_state_t *s)
{
uint X=s->x, C=s->c;
uint Xn=MWC64X_A*X+C;
uint carry=(uint)(Xn<C); // The (Xn<C) will be zero or one for scalar
uint Cn=mad_hi(MWC64X_A,X,carry);
s->x=Xn;
s->c=Cn;
}
//! Return a 32-bit integer in the range [0..2^32)
uint MWC64X_NextUint(mwc64x_state_t *s)
{
uint res=s->x ^ s->c;
MWC64X_Step(s);
return res;
}
__kernel void setInfluence(const int literals, const int size, const int dim1_size, __global int* lambdas, __global float* lambdap, __global int* dim2_size, __global float* influence){
int flag=get_global_id(0);
int sum=0;
int count=10000;
int assignment[N];
//or try to get newlambda like original version does
if(flag < literals){
mwc64x_state_t rng;
for(int i=0; i<count; i++){
for(int j=0; j<size; j++){
uint randint=MWC64X_NextUint(&rng);
float rand=randint*1.0/BASE;
//if(flag==0)
// printf("randint=%u",randint);
if(lambdap[j]<rand)
assignment[lambdas[j]]=0;
else
assignment[lambdas[j]]=1;
}
//the true case
assignment[flag]=1;
int valuet=0;
int index=0;
for(int m=0; m<dim1_size; m++){
int valueMono=1;
for(int n=0; n<dim2_size[m]; n++){
if(assignment[lambdas[index+n]]==0){
valueMono=0;
index+=dim2_size[m];
break;
}
}
if(valueMono==1){
valuet=1;
break;
}
}
//the false case
assignment[flag]=0;
int valuef=0;
index=0;
for(int m=0; m<dim1_size; m++){
int valueMono=1;
for(int n=0; n<dim2_size[m]; n++){
if(assignment[lambdas[index+n]]==0){
valueMono=0;
index+=dim2_size[m];
break;
}
}
if(valueMono==1){
valuef=1;
break;
}
}
sum += valuet-valuef;
}
influence[flag] = 1.0*sum/count;
printf("sum%d:%d\t", flag, sum);
}
}
在 GPU 上运行代码时可能会出现什么问题？ 是MWC64X吗？ 据其作者称，它可以在 NVIDIA GPU 上表现良好。 如果是这样，我该如何解决； 如果没有，可能是什么问题？
2 个解决方案
解决方案1
1 已采纳 2019-08-27 15:46:08
（这是作为评论开始的，事实证明这是问题的根源，所以我将其变成了答案。）
你没有初始化你的mwc64x_state_t rng; 变量之前读取它，所以任何结果都将是未定义的：
mwc64x_state_t rng;
for(int i=0; i<count; i++){
for(int j=0; j<size; j++){
uint randint=MWC64X_NextUint(&rng);
其中MWC64X_NextUint()在更新之前立即从 rng 状态读取：
uint MWC64X_NextUint(mwc64x_state_t *s)
{
uint res=s->x ^ s->c;
请注意，您可能希望为每个工作项以不同的方式为 RNG 设置种子，否则您将在结果中获得令人讨厌的相关性伪影。
解决方案2
0 2019-08-27 15:31:17
伪随机数的所有用例都是真正[PARALLEL]计算平台（不是语言、平台）中的下一级挑战。
任何一个，
有一些随机源，一旦大规模并行请求以真正的[PARALLEL]方式得到公平处理（在这里，硬件资源可能会有所帮助，但代价是无法在同一个平台的“外部”重现相同的行为（并且及时，如果这样的源不是通过某些种子注入功能进行软件操作的，则可能会设置“只是”伪随机算法，该算法会创建纯[SERIAL]序列产生的“只是”伪随机数 ) )
或者，
有一些伪随机数的“共享”生成器，它享有更高级别的系统范围的熵级别（这有利于产生伪随机性的“质量”），但代价是纯- 串行依赖性（不可能并行执行，串行序列以顺序方式一个接一个地提供）并且可重复运行的机会几乎为零（可重复科学的必需品）提供可重复的相同序列，需要测试和方法验证案件。
恢复 ：
该代码可以采用工作本期特价货品“私人”的伪随机生成功能（S）（隐私是产生这些伪为并行代码执行和相互独立（非介入处理两者的缘故必须的）随机数） ，但每个实例都必须a) 独立初始化，以便提供可在并行代码运行中实现的预期随机性水平和b)任何此类初始化都应以可重复的方式执行，以便运行不同时间的测试，通常使用不同的 OpenCL 目标计算平台。
对于不依赖于特定硬件随机源的__kernel -s，满足条件a && b将足以接收可重复重现的（相同）结果以进行“体外”测试，从而提供合理的随机方法来生成在通用生产级用例代码“体内”运行期间的结果。
网络运行时间的比较（上面进行了基准测试）似乎表明，阿姆达尔定律附加开销成本加上工作原子性的尾端效应最终决定了网络运行时间快了~ 3.6x在至强上与 GPU 相比：
index1 = 17
size = 51
dim1_size = 6
sum0: 4781 influence0: 0.478100
sum2: 4781 influence2: 0.478100
sum6: 0 influence6: 0.000000
sum10: 0 influence10: 0.000000
sum12: 0 influence12: 0.000000
sum7: 0 influence7: 0.000000
sum4: 5962 influence4: 0.596200
sum8: 7971 influence8: 0.797100
sum1: 4781 influence1: 0.478100
sum3: 4781 influence3: 0.478100
sum13: 0 influence13: 0.000000
sum11: 1261 influence11: 0.126100
sum9: 0 influence9: 0.000000
sum14: 0 influence14: 0.000000
sum5: 0 influence5: 0.000000
sum15: 0 influence15: 0.000000
sum16: 0 influence16: 0.000000
Parallel influence running time: 0.054391 seconds on XEON E5-2630L v3 @ 1.80GHz using OpenCL
|....
index1 = 17 |....
size = 51 |....
dim1_size = 6 |....
sum0: 10000 |....
sum1: 10000 |....
sum2: 10000 |....
sum3: 10000 |....
sum4: 10000 |....
sum5: 0 |....
sum6: 0 |....
sum7: 0 |....
sum8: 10000 |....
sum9: 0 |....
sum10: 0 |....
sum11: 0 |....
sum12: 0 |....
sum13: 0 |....
sum14: 0 |....
sum15: 0 |....
sum16: 0 |....
Parallel influence running time: 0.193581 seconds on GeForce GTX 1080 Ti using OpenCL
我是否需要为OpenCL安装Nvidia的SDK（CUDA）来检测Nvidia GPU？
[英]Do i need to install Nvidia's SDK(CUDA) for OpenCL to detect Nvidia GPU?
MQL5中的OpenCL代码不会将分布式作业发送到每个GPU核心
[英]An OpenCL code in MQL5 does not get distributed jobs to each GPU core
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[英]OpenCL code on Nvidia GEFORCE using Visual Studio 2010
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