什么是银行冲突? (进行 Cuda/OpenCL 编程)

我一直在阅读 CUDA 和 OpenCL 的编程指南，但我不知道什么是银行冲突。他们只是深入研究如何解决这个问题，而没有详细说明这个主题本身。有人能帮我理解一下吗？如果帮助是在 CUDA/OpenCL 的背景下，或者只是计算机科学中的一般银行冲突，我没有偏好。

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The shared memory that can be accessed in parallel is divided into modules (also called banks). If two memory locations (addresses) occur in the same bank, then you get a bank conflict during which the access is done serially, losing the advantages of parallel access.

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最佳答案

For nvidia (and amd for that matter) gpus the local memory is divided into memorybanks. Each bank can only address one dataset at a time, so if a halfwarp tries to load/store data from/to the same bank the access has to be serialized (this is a bank conflict). For gt200 gpus there are 16 banks (32banks for fermi), 16 or 32 banks for AMD gpus (57xx or higher: 32, everything below: 16)), which are interleaved with a granuity of 32bit (so byte 0-3 are in bank 1, 4-7 in bank 2, ..., 64-69 in bank 1 and so on). For a better visualization it basically looks like this:

Bank    |      1      |      2      |      3      |...
Address |  0  1  2  3 |  4  5  6  7 |  8  9 10 11 |...
Address | 64 65 66 67 | 68 69 70 71 | 72 73 74 75 |...
...

So if each thread in a halfwarp accesses successive 32bit values there are no bank conflicts. An exception from this rule (every thread must access its own bank) are broadcasts: If all threads access the same address, the value is only read once and broadcasted to all threads (for GT200 it has to be all threads in the halfwarp accessing the same address, iirc fermi and AMD gpus can do this for any number of threads accessing the same value).

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In simple words, bank conflict is a case when any memory access pattern fails to distribute IO across banks available in the memory system. The following examples elaborates the concept:-

Let us suppose we have two dimensional 512x512 array of integers and our DRAM or memory system has 512 banks in it. By default the array data will be layout in a way that arr[0][0] goes to bank 0, arr[0][1] goes to bank 1, arr[0][2] to bank 2 ....arr[0][511] goes to bank 511. To generalize arr[x][y] occupies bank number y. Now some code (as shown below) start accessing data in column major fashion ie. changing x while keeping y constant, then the end result will be that all consecutive memory access will hit the same bank--hence bank conflict.

int arr[512][512];
for ( j = 0; j < 512; j++ ) // outer loop
for ( i = 0; i < 512; i++ ) // inner loop
arr[i][j] = 2 * arr[i][j]; // column major processing

Such problems, usually, are avoided by compilers by buffering the array or using prime number of elements in the array.

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http://en.wikipedia.org/wiki/Memory_bank
and http://mprc.pku.cn/mentors/training/ISCAreading/1989/p380-weiss/p380-weiss.pdf

from this page, you can find the detail about memory bank. but it is a little different from what is said by @Grizzly. in this page, the bank is like this

bank 1 2 3

address|0, 3, 6...| |1, 4, 7...| | 2, 5,8...|

hope this would help

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(CUDA Bank Conflict) I hope this will help.. this is very good explaination ...

http://www.youtube.com/watch?v=CZgM3DEBplE


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