我有一個運行在NVidia GTX 680上的內核,當從全局內存切換到本地內存時,執行時間增加了。OpenCL - 全局內存讀取預製比本地更好
我的內核是有限元射線追蹤器的一部分,現在在處理之前將每個元素加載到本地內存中。每個元素的數據被存儲在一個結構fastTriangle其具有以下定義:
typedef struct fastTriangle {
float cx, cy, cz, cw;
float nx, ny, nz, nd;
float ux, uy, uz, ud;
float vx, vy, vz, vd;
} fastTriangle;
我通過這些對象到內核被如下寫入的陣列(我已刪除爲了簡潔不相關碼:
__kernel void testGPU(int n_samples, const int n_objects, global const fastTriangle *objects, __local int *x_res, __global int *hits) {
// Get gid, lid, and lsize
// Set up random number generator and thread variables
// Local storage for the two triangles being processed
__local fastTriangle triangles[2];
for(int i = 0; i < n_objects; i++) { // Fire ray from each object
event_t evt = async_work_group_copy((local float*)&triangles[0], (global float*)&objects[i],sizeof(fastTriangle)/sizeof(float),0);
//Initialise local memory x_res to 0's
barrier(CLK_LOCAL_MEM_FENCE);
wait_group_events(1, &evt);
Vector wsNormal = { triangles[0].cw*triangles[0].nx, triangles[0].cw*triangles[0].ny, triangles[0].cw*triangles[0].nz};
for(int j = 0; j < n_samples; j+= 4) {
// generate a float4 of random numbers here (rands
for(int v = 0; v < 4; v++) { // For each ray in ray packet
//load the first object to be intesected
evt = async_work_group_copy((local float*)&triangles[1], (global float*)&objects[0],sizeof(fastTriangle)/sizeof(float),0);
// Some initialising code and calculate ray here
// Should have ray fully specified at this point;
for(int w = 0; w < n_objects; w++) { // Check for intersection against each ray
wait_group_events(1, &evt);
// Check for intersection against object w
float det = wsDir.x*triangles[1].nx + wsDir.y*triangles[1].ny + wsDir.z*triangles[1].nz;
float dett = triangles[1].nd - (triangles[0].cx*triangles[1].nx + triangles[0].cy*triangles[1].ny + triangles[0].cz*triangles[1].nz);
float detpx = det*triangles[0].cx + dett*wsDir.x;
float detpy = det*triangles[0].cy + dett*wsDir.y;
float detpz = det*triangles[0].cz + dett*wsDir.z;
float detu = detpx*triangles[1].ux + detpy*triangles[1].uy + detpz*triangles[1].uz + det*triangles[1].ud;
float detv = detpx*triangles[1].vx + detpy*triangles[1].vy + detpz*triangles[1].vz + det*triangles[1].vd;
// Interleaving the copy of the next triangle
evt = async_work_group_copy((local float*)&triangles[1], (global float*)&objects[w+1],sizeof(fastTriangle)/sizeof(float),0);
// Complete intersection calculations
} // end for each object intersected
if(objectNo != -1) atomic_inc(&x_res[objectNo]);
} // end for sub rays
} // end for each ray
barrier(CLK_LOCAL_MEM_FENCE);
// Add all the local x_res to global array hits
barrier(CLK_GLOBAL_MEM_FENCE);
} // end for each object
}
當我第一次寫這個內核我沒有緩衝在本地內存中的每個對象,而是剛剛訪問它形成全局內存即不是三角形的[0] .CX我會使用對象[I] .CX
當開始優化時,我切換到使用上面列出的本地內存,但觀察到執行運行時間增加了大約25%。
當使用本地內存緩衝對象而不是直接在全局內存中訪問它們時,爲什麼性能會更差?
對我來說很清楚。他正在從全球複製到本地,只使用一次並刪除數據。 Fr這個目的是更好的1次訪問。 – DarkZeros