I have the following simple code:
#include<stdio.h>
#define BLOCKSIZE_X 32
#define BLOCKSIZE_Y 1
int iDivUp(int a, int b) { return ((a % b) != 0) ? (a / b + 1) : (a / b); }
#define gpuErrchk(ans) { gpuAssert((ans), __FILE__, __LINE__); }
inline void gpuAssert(cudaError_t code, char *file, int line, bool abort=true)
{
if (code != cudaSuccess)
{
fprintf(stderr,"GPUassert: %s %s %d\n", cudaGetErrorString(code), file, line);
if (abort) exit(code);
}
}
__global__ void kernel0(float *d_a, float *d_b, const unsigned int M, const unsigned int N)
{
const int tidx = threadIdx.x + blockIdx.x * blockDim.x;
const int tidy = threadIdx.y + blockIdx.y * blockDim.y;
if ((tidx < M)&&(tidy < N)) {
d_b[tidy * M + tidx] = d_a[tidy * M + tidx];
}
}
void main()
{
const unsigned int M = 32;
const unsigned int N = 1;
float *d_a; cudaMalloc((void**)&d_a, M*N*sizeof(float));
float *d_b; cudaMalloc((void**)&d_b, M*N*sizeof(float));
dim3 dimGrid(iDivUp(M, BLOCKSIZE_X), iDivUp(N, BLOCKSIZE_Y));
dim3 dimBlock(BLOCKSIZE_X, BLOCKSIZE_Y);
kernel0<<<dimGrid, dimBlock>>>(d_a, d_b, M, N);
gpuErrchk(cudaPeekAtLastError());
gpuErrchk(cudaDeviceSynchronize());
cudaDeviceReset();
}
which performs the assignment between two arrays 32 floats. I am trying to understand the relationship between global collaborations for accessing global memory and the efficiency of loading / storing global memory, as well as other metrics / events.
Visual Profiler shows the following indicators:
Global Memory Load Efficiency = 50%
Global Memory Store Efficiency = 100%
The value of Global Memory Load Efficiency surprises me. I would expect performance 100%in both cases, since I believe that I am performing perfectly unified memory access. So my question is:
50% - , , 100% - ?
/, :
gld_inst_32bit = 32 (Number of 32-bit global memory load transactions)
gst_inst_32bit = 32 (Number of 32-bit global memory store transactions)
, / 32 float s.
uncached global load transaction = 0 (Number of uncached global load transactions)
l1 global load miss = 2 (Number of global load misses in L1 cache)
, (, ) . l1 , 0.
gld_request = 1 (Number of executed global load instructions per warp in a SM)
gst_request = 1 (Number of executed global store instructions per warp in a SM)
, , , .
:
/*0000*/ MOV R1, c[0x1][0x100]; /* 0x2800440400005de4 */
/*0008*/ S2R R3, SR_CTAID.Y; /* 0x2c0000009800dc04 */
/*0010*/ S2R R4, SR_TID.Y; /* 0x2c00000088011c04 */
/*0018*/ IMAD R4, R3, c[0x0][0xc], R4; /* 0x2008400030311ca3 */
/*0020*/ S2R R0, SR_CTAID.X; /* 0x2c00000094001c04 */
/*0028*/ ISETP.LT.U32.AND P0, PT, R4, c[0x0][0x2c], PT; /* 0x188e4000b041dc03 */
/*0030*/ S2R R2, SR_TID.X; /* 0x2c00000084009c04 */
/*0038*/ IMAD R0, R0, c[0x0][0x8], R2; /* 0x2004400020001ca3 */
/*0040*/ ISETP.LT.U32.AND P0, PT, R0, c[0x0][0x28], P0; /* 0x18804000a001dc03 */
/*0048*/ @!P0 BRA.U 0x78; /* 0x40000000a000a1e7 */
/*0050*/ @P0 IMAD R2, R4, c[0x0][0x28], R0; /* 0x20004000a04080a3 */
/*0058*/ @P0 ISCADD R0, R2, c[0x0][0x20], 0x2; /* 0x4000400080200043 */
/*0060*/ @P0 ISCADD R2, R2, c[0x0][0x24], 0x2; /* 0x4000400090208043 */
/*0068*/ @P0 LD R0, [R0]; /* 0x8000000000000085 */
/*0070*/ @P0 ST [R2], R0; /* 0x9000000000200085 */
/*0078*/ EXIT; /* 0x8000000000001de7 */
: CUDA 6.5, GeForce GT540M, Windows 7.
M 32 64, , 100%, /:
gld_inst_32bit = 64
gst_inst_32bit = 64
uncached global load transaction = 0
l1 global load miss = 2
gld_request = 2
gst_request = 2
gld_inst_32bit, gst_inst_32bit, gld_request gst_request , 64 float 2 / . , uncached global load transaction l1 global load miss , , 100%.
K20c M=32:
Global Memory Load Efficiency = 100%
Global Memory Store Efficiency = 100%
gld_inst_32bit = 64
gst_inst_32bit = 64
gld_request = 1
gst_request = 1
uncached global load transaction = 1
l1 global load miss = 0
l1 global load hit = 0
Visual Profiler , l1.
, M BLOCKSIZE_X.
, GT540M , , 100%, 100% . 100%, .
l1, -Xptxas -dlcm=cg, @Jez, 100%, . , l1, L2.
, M,
M=32
M=64
M=96
M=128
M=160
M=192
, M , 32, .
l1, :
M=32
M=64
M=96
<411 >
- TESLA C2050
M = 32 33.3%
M = 64 28.6%
M = 96 42.9%
M = 128 57.1%
M = 160 71.4%
M = 192 85.7%
M = 224 100%
M = 256 114%
M = 288 90%
, l1, 100% .