CUDA 8 Unified Memory on Pascal Titan X / GP102

Question

This article says that CUDA 8 improved Unified Memory support on Pascal GPUs so that "on supporting platforms, memory allocated with the default OS allocator (e.g. ‘malloc’ or ‘new’) can be accessed from both GPU code and CPU code using the same pointer".

I was excited about this and wrote a small test program to see if my system support this:

#include <stdio.h>

#define CUDA_CHECK( call ) {\
    cudaError_t code = ( call );\
    if ( code != cudaSuccess ) {\
        const char* msg = cudaGetErrorString( code );\
        printf( "%s #%d: %s\n", __FILE__, __LINE__, msg );\
    }\
}

#define N 10

__global__
void test_unified_memory( int* input, int* output )
{
    output[ threadIdx.x ] = input[ threadIdx.x ] * 2;
}

int main()
{
    int* input = (int*) malloc( N );
    int* output = (int*) malloc( N );

    for ( int i = 0; i < N; ++i ) input[ i ] = i;

    test_unified_memory <<< 1, N >>>( input, output );
    CUDA_CHECK( cudaDeviceSynchronize() );

    for ( int i = 0; i < N; ++i ) printf( "%d, ", output[ i ] );

    free( input );
    free( output );
}

But it didn't work.

I am wondering what does "supporting platform" means. Here are my system configurations:

$uname -r
3.10.0-327.el7.x86_64

$nvidia-smi
Tue Jan 10 14:46:11 2017       
+-----------------------------------------------------------------------------+
| NVIDIA-SMI 375.26                 Driver Version: 375.26                    |
|-------------------------------+----------------------+----------------------+
| GPU  Name        Persistence-M| Bus-Id        Disp.A | Volatile Uncorr. ECC |
| Fan  Temp  Perf  Pwr:Usage/Cap|         Memory-Usage | GPU-Util  Compute M. |
|===============================+======================+======================|
|   0  TITAN X (Pascal)    Off  | 0000:01:00.0     Off |                  N/A |
| 36%   61C    P0    88W / 250W |      2MiB / 12189MiB |    100%      Default |
+-------------------------------+----------------------+----------------------+

+-----------------------------------------------------------------------------+
| Processes:                                                       GPU Memory |
|  GPU       PID  Type  Process name                               Usage      |
|=============================================================================|
|  No running processes found                                                 |
+-----------------------------------------------------------------------------+

$deviceQuery
NVIDIA_CUDA-7.5_Samples/bin/x86_64/linux/release/deviceQuery Starting...

 CUDA Device Query (Runtime API) version (CUDART static linking)

Detected 1 CUDA Capable device(s)

Device 0: "TITAN X (Pascal)"
  CUDA Driver Version / Runtime Version          8.0 / 7.5
  CUDA Capability Major/Minor version number:    6.1
  Total amount of global memory:                 12189 MBytes (12781551616 bytes)
MapSMtoCores for SM 6.1 is undefined.  Default to use 128 Cores/SM
MapSMtoCores for SM 6.1 is undefined.  Default to use 128 Cores/SM
  (28) Multiprocessors, (128) CUDA Cores/MP:     3584 CUDA Cores
  GPU Max Clock rate:                            1531 MHz (1.53 GHz)
  Memory Clock rate:                             5005 Mhz
  Memory Bus Width:                              384-bit
  L2 Cache Size:                                 3145728 bytes
  Maximum Texture Dimension Size (x,y,z)         1D=(131072), 2D=(131072, 65536), 3D=(16384, 16384, 16384)
  Maximum Layered 1D Texture Size, (num) layers  1D=(32768), 2048 layers
  Maximum Layered 2D Texture Size, (num) layers  2D=(32768, 32768), 2048 layers
  Total amount of constant memory:               65536 bytes
  Total amount of shared memory per block:       49152 bytes
  Total number of registers available per block: 65536
  Warp size:                                     32
  Maximum number of threads per multiprocessor:  2048
  Maximum number of threads per block:           1024
  Max dimension size of a thread block (x,y,z): (1024, 1024, 64)
  Max dimension size of a grid size    (x,y,z): (2147483647, 65535, 65535)
  Maximum memory pitch:                          2147483647 bytes
  Texture alignment:                             512 bytes
  Concurrent copy and kernel execution:          Yes with 2 copy engine(s)
  Run time limit on kernels:                     No
  Integrated GPU sharing Host Memory:            No
  Support host page-locked memory mapping:       Yes
  Alignment requirement for Surfaces:            Yes
  Device has ECC support:                        Disabled
  Device supports Unified Addressing (UVA):      Yes
  Device PCI Domain ID / Bus ID / location ID:   0 / 1 / 0
  Compute Mode:
     < Default (multiple host threads can use ::cudaSetDevice() with device simultaneously) >

deviceQuery, CUDA Driver = CUDART, CUDA Driver Version = 8.0, CUDA Runtime Version = 7.5, NumDevs = 1, Device0 = TITAN X (Pascal)
Result = PASS

The answer may simply be that Titan X / GP102 does not support this feature. However I could not find any information / documentation on this. Could anyone please let me know whether or not it is supported on my configuration, and point me to the reference of such information? Thank you.

As talonmies suggested in the comment, it may related to the host OS. Then, what is the requirements on the host, and how to check / fix them?

Answer 1

It appears that this new unified memory feature requires an experimental Linux kernel patch which is not yet integrated into any mainline kernel trees. It should be regarded as a future feature rather than something which can be used now.

EDIT to add that, as noted in comments, you are also using CUDA 7.5, and irrespective of the host kernel issue, you would need to use CUDA 8 for this feature.