title: CUDA: What is the threads per multiprocessor and threads per block distinction?
body: <p>We have a workstation with two Nvidia Quadro FX 5800 cards installed. Running the deviceQuery CUDA sample reveals that the maximum threads per multiprocessor (SM) is 1024, while the maximum threads per block is 512. </p>
  
  <p>Given that only one block can be executed on each SM at a time, why is max threads / processor double the max threads / block? How do we utilise the other 512 threads per SM?</p>
  
  <pre><code>Device 1: "Quadro FX 5800"
    CUDA Driver Version / Runtime Version          5.0 / 5.0
    CUDA Capability Major/Minor version number:    1.3
    Total amount of global memory:                 4096 MBytes (4294770688 bytes)
    (30) Multiprocessors x (  8) CUDA Cores/MP:    240 CUDA Cores
    GPU Clock rate:                                1296 MHz (1.30 GHz)
    Memory Clock rate:                             800 Mhz
    Memory Bus Width:                              512-bit
    Max Texture Dimension Size (x,y,z)             1D=(8192), 2D=(65536,32768), 3D=(2048,2048,2048)
    Max Layered Texture Size (dim) x layers        1D=(8192) x 512, 2D=(8192,8192) x 512
    Total amount of constant memory:               65536 bytes
    Total amount of shared memory per block:       16384 bytes
    Total number of registers available per block: 16384
    Warp size:                                     32
    Maximum number of threads per multiprocessor:  1024
    Maximum number of threads per block:           512
    Maximum sizes of each dimension of a block:    512 x 512 x 64
    Maximum sizes of each dimension of a grid:     65535 x 65535 x 1
    Maximum memory pitch:                          2147483647 bytes
    Texture alignment:                             256 bytes
    Concurrent copy and kernel execution:          Yes with 1 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):      No
    Device PCI Bus ID / PCI location ID:           4 / 0
    Compute Mode:
       &lt; Default (multiple host threads can use ::cudaSetDevice() with device simultaneously) &gt;
  </code></pre>
  
  <p>Cheers,
  James. </p>