CUDA syntax
Source code is in .cu files, which contain mixture of host (CPU) and device (GPU) code.
Declaring functions
__global__ | declares kernel, which is called on host and executed on device |
__device__ | declares device function, which is called and executed on device |
__host__ | declares host function, which is called and executed on host |
__noinline__ | to avoid inlining |
__forceinline__ | to force inlining |
Declaring variables
__device__ | declares device variable in global memory, accessible from all threads, with lifetime of application |
__constant__ | declares device variable in constant memory, accessible from all threads, with lifetime of application |
__shared__ | declares device varibale in block's shared memory, accessible from all threads within a block, with lifetime of block |
__restrict__ | standard C definition that pointers are not aliased |
Types
Most routines return an error code of type cudaError_t
.
Vector types
char1, uchar1, short1, ushort1, int1, uint1, long1, ulong1, float1
char2, uchar2, short2, ushort2, int2, uint2, long2, ulong2, float2
char3, uchar3, short3, ushort3, int3, uint3, long3, ulong3, float3
char4, uchar4, short4, ushort4, int4, uint4, long4, ulong4, float4
longlong1, ulonglong1, double1
longlong2, ulonglong2, double2
dim3
Components are accessible as variable.x,
variable.y,
variable.z,
variable.w.
Constructor is make_<type>( x, ... )
, for example:
float2 xx = make_float2( 1., 2. );
dim3 can take 1, 2, or 3 argumetns:
dim3 blocks1D( 5 );
dim3 blocks2D( 5, 5 );
dim3 blocks3D( 5, 5, 5 );
Pre-defined variables
dim3 gridDim | dimensions of grid |
dim3 blockDim | dimensions of block |
uint3 blockIdx | block index within grid |
uint3 threadIdx | thread index within block |
int warpSize | number of threads in warp |
Kernel invocation
__global__ void kernel( ... ) { ... }
dim3 blocks( nx, ny, nz );
dim3 threadsPerBlock( mx, my, mz );
kernel<<< blocks, threadsPerBlock >>>( ... );
Thread management
__threadfence_block(); | wait until memory accesses are visible to block |
__threadfence(); | wait until memory accesses are visible to block and device |
__threadfence_system(); | wait until memory accesses are visible to block and device and host (2.x) |
__syncthreads(); | wait until all threads reach sync |
Memory management
__device__ float* pointer;
cudaMalloc( (void**) &pointer, size );
cudaFree( pointer );
__constant__ float dev_data[n];
float host_data[n];
cudaMemcpyToSymbol ( dev_data, host_data, sizeof(host_data) );
cudaMemcpyFromSymbol( host_data, dev_data, sizeof(host_data) );
cudaMemcpy ( dst_pointer, src_pointer, size, direction );
cudaMemcpyAsync( dst_pointer, src_pointer, size, direction, stream );
cudaMemcpy2D ( A_dst, lda_bytes, B_src, ldb_bytes, m_bytes, n, direction );
cudaMemcpy2DAsync( A_dst, lda_bytes, B_src, ldb_bytes, m_bytes, n, direction, stream );
cublasSetVector ( n, elemSize, x_src_host, incx, y_dst_dev, incy );
cublasGetVector ( n, elemSize, x_src_dev, incx, y_dst_host, incy );
cublasSetVectorAsync( n, elemSize, x_src_host, incx, y_dst_dev, incy, stream );
cublasGetVectorAsync( n, elemSize, x_src_dev, incx, y_dst_host, incy, stream );
cublasSetMatrix ( rows, cols, elemSize, A_src_host, lda, B_dst_dev, ldb );
cublasGetMatrix ( rows, cols, elemSize, A_src_dev, lda, B_dst_host, ldb );
cublasSetMatrixAsync( rows, cols, elemSize, A_src_host, lda, B_dst_dev, ldb, stream );
cublasGetMatrixAsync( rows, cols, elemSize, A_src_dev, lda, B_dst_host, ldb, stream );
Also, malloc
and free
work inside a kernel (2.x), but memory allocated in a kernel must be deallocated in a kernel (not the host). It can be freed in a different kernel, though.
Atomic functions
old = atomicAdd ( &addr, value );
old = atomicSub ( &addr, value );
old = atomicExch( &addr, value );
old = atomicMin ( &addr, value );
old = atomicMax ( &addr, value );
old = atomicInc ( &addr, value );
old = atomicDec ( &addr, value );
old = atomicAnd ( &addr, value );
old = atomicOr ( &addr, value );
old = atomicXor ( &addr, value );
old = atomicCAS ( &addr, compare, value );
Warp vote
int __all ( predicate );
int __any ( predicate );
int __ballot( predicate );
Timer
wall clock cycle counter
clock_t clock();
Texture
can also return float2 or float4, depending on texRef.
float tex1Dfetch( texRef, ix );
float tex1D( texRef, x );
float tex2D( texRef, x, y );
float tex3D( texRef, x, y, z );
float tex1DLayered( texRef, x );
float tex2DLayered( texRef, x, y );
Low-level Driver API
#include <cuda.h>
CUdevice dev;
CUdevprop properties;
char name[n];
int major, minor;
size_t bytes;
cuInit( 0 );
cuDeviceGetCount ( &cnt );
cuDeviceGet ( &dev, index );
cuDeviceGetName ( name, sizeof(name), dev );
cuDeviceComputeCapability( &major, &minor, dev );
cuDeviceTotalMem ( &bytes, dev );
cuDeviceGetProperties ( &properties, dev );
cuBLAS
Matrices are column-major. Indices are 1-based; this affects result of i<t>amax and i<t>amin.
#include <cublas_v2.h>
cublasHandle_t handle;
cudaStream_t stream;
cublasCreate( &handle );
cublasDestroy( handle );
cublasGetVersion( handle, &version );
cublasSetStream( handle, stream );
cublasGetStream( handle, &stream );
cublasSetPointerMode( handle, mode );
cublasGetPointerMode( handle, &mode );
Constants
argument | constants | description (Fortran letter) |
trans | CUBLAS_OP_N | non-transposed ('N') |
| CUBLAS_OP_T | transposed ('T') |
| CUBLAS_OP_C | conjugate transposed ('C') |
|
uplo | CUBLAS_FILL_MODE_LOWER | lower part filled ('L') |
| CUBLAS_FILL_MODE_UPPER | upper part filled ('U') |
|
side | CUBLAS_SIDE_LEFT | matrix on left ('L') |
| CUBLAS_SIDE_RIGHT | matrix on right ('R') |
|
mode | CUBLAS_POINTER_MODE_HOST | alpha and beta scalars passed on host |
| CUBLAS_POINTER_MODE_DEVICE | alpha and beta scalars passed on device |
BLAS functions have cublas
prefix and first letter of usual BLAS function name is capitalized. Arguments are the same as standard BLAS, with these exceptions:
- All functions add handle as first argument.
- All functions return cublasStatus_t error code.
- Constants alpha and beta are passed by pointer. All other scalars (n, incx, etc.) are bassed by value.
- Functions that return a value, such as ddot, add result as last argument, and save value to result.
- Constants are given in table above, instead of using characters.
Examples:
cublasDdot ( handle, n, x, incx, y, incy, &result ); // result = ddot( n, x, incx, y, incy );
cublasDaxpy( handle, n, &alpha, x, incx, y, incy ); // daxpy( n, alpha, x, incx, y, incy );
Compiler
nvcc
, often found in /usr/local/cuda/bin
Defines __CUDACC__
Flags common with cc
Short flag | Long flag | Output or Description |
-c | --compile | .o object file |
-E | --preprocess | on standard output |
-M | --generate-dependencies | on standard output |
-o file | --output-file file |
-I directory | --include-path directory | header search path |
-L directory | --library-path directory | library search path |
-l lib | --library lib | link with library |
-lib |
| generate library |
-shared |
| generate shared library |
-pg | --profile | for gprof |
-g level | --debug level |
-G | --device-debug |
-O level | --optimize level |
|
Undocumented (but in sample makefiles) |
-m32 | | compile 32-bit i386 host CPU code |
-m64 | | compile 64-bit x86_64 host CPU code |
Flags specific to nvcc
-v | list compilation commands as they are executed |
-dryrun | list compilation commands, without executing |
-keep | saves intermediate files (e.g., pre-processed) for debugging |
-clean | removes output files (with same exact compiler options) |
-arch=<compute_xy> | generate PTX for capability x.y |
-code=<sm_xy> | generate binary for capability x.y, by default same as -arch |
-gencode arch=...,code=... | same as -arch and -code , but may be repeated |
Argumenents for -arch
and -code
It makes most sense (to me) to give -arch
a virtual architecture and -code
a real architecture, though both flags accept both virtual and real architectures (at times).
| Virtual architecture | Real architecture | Features |
Tesla | compute_10 | sm_10 | Basic features |
| compute_11 | sm_11 | + atomic memory ops on global memory |
| compute_12 | sm_12 | + atomic memory ops on shared memory + vote instructions |
| compute_13 | sm_13 | + double precision |
Fermi | compute_20 | sm_20 | + Fermi |
Some hardware constraints
| 1.x | 2.x |
max x- or y-dimension of block | 512 | 1024 |
max z-dimension of block | 64 | 64 |
max threads per block | 512 | 1024 |
warp size | 32 | 32 |
max blocks per MP | 8 | 8 |
max warps per MP | 32 | 48 |
max threads per MP | 1024 | 1536 |
max 32-bit registers per MP | 16k | 32k |
max shared memory per MP | 16 KB | 48 KB |
shared memory banks | 16 | 32 |
local memory per thread | 16 KB | 512 KB |
const memory | 64 KB | 64 KB |
const cache | 8 KB | 8 KB |
texture cache | 8 KB | 8 KB |
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