Overlapping CPU and GPU operations using streams. Total failure. Any help?

Hi all,

I have been working on a GPU based cryptographic application performing litecoin mining. Litecoin is a craptographic currency, but unline Bitcoin it specifies the memory-hard “scrypt” algorithm for hashing.

In my implementation of scrypt, the host (CPU) performs a HMAC SHA256 message digest, and then the GPU takes over and runs 2048 rounds of memory-hard Salsa20/8 goodness. Afterwards the CPU again performs a HMAC SHA256 digest and looks at the result. The required CPU time is significantly shorter than the GPU time.

In order to get best performance I tried to use two steams, and all host computations are supposed to overlap with kernel execution. Ideally, kernel executions would be performed sequentially, with zero gaps.

What I am seeing in practice is that CPU and kernels never overlap, despite using the streams API to the best of my knowledge. This wastes about 30% of the peformance I expect from my GPUs.

I have slimmed down my code to include just some essentials (for example I removed the memory hard bits of the crypto algorithm). But I exactly reproduced the stream handling that is causing me so much grief. Would anyone have the time to look at my stream handling in host code?

This code uses the nvToolsExt.h header so that the emulated CPU workload (a Sleep of 5ms each time) shows up in the graphs. It never overlaps with any kernels, and as a result there are huge gaps between GPU work phases.

This is the pseudo-code for my stream handling loop:

Setup input for stream #0 (takes 5ms)
Copy input for stream #0 (async H2D copy)
Launch kernel for stream #0 (execution takes about 20ms)
   Copy output from stream #0 (async D2H copy)

   Setup input for stream #1 (takes 5ms)
   Copy input for stream #1 (async H2D copy)
   Launch kernel for stream #1 (execution takes about 20ms)

   Synchronize with stream #0
   Analyze output from stream #0 (takes 5ms)

   Swap roles of stream #0 and #1

I am testing on Windows 7, both 32 and 64 bit versions exhibit the same behavior. CUDA toolkit and SDK is 5.0. Driver is version 314.22. The GPUs in the system are GTX 260, GTX 460, GTX 260.

This is the code. Compiles best on Windows, Linux may need minor adjustments (provide a Sleep() function)


/* Test project which was set up to try streams for overlapping
 * host and device side computation.
 * Device + Host code.

// includes, system
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <math.h>
#include <stdint.h>
#include <windows.h> // for Sleep(time in ms)

// includes CUDA
#include <cuda_runtime.h>

// includes, project
#include <helper_cuda.h>
#include <helper_functions.h> // helper functions for SDK examples

// this comes with GPU Computing SDK (CUDA 5.0)
#include <nvToolsExt.h>

// declaration, forward
void runTest(int argc, char **argv);

// just some constants defining the work units per block, etc...
#define WU_PER_WARP 32
#define WU_PER_LAUNCH (blocks*WU_PER_BLOCK)

#define RL(a, b) (((a) << (b)) | ((a) >> (32 - (b))))

__device__ void xor_salsa8(unsigned long *B, unsigned long *C)
    uint32_t x0 = (B[ 0] ^= C[ 0]), x1 = (B[ 1] ^= C[ 1]), x2 = (B[ 2] ^= C[ 2]), x3 = (B[ 3] ^= C[ 3]);
    uint32_t x4 = (B[ 4] ^= C[ 4]), x5 = (B[ 5] ^= C[ 5]), x6 = (B[ 6] ^= C[ 6]), x7 = (B[ 7] ^= C[ 7]);
    uint32_t x8 = (B[ 8] ^= C[ 8]), x9 = (B[ 9] ^= C[ 9]), xa = (B[10] ^= C[10]), xb = (B[11] ^= C[11]);
    uint32_t xc = (B[12] ^= C[12]), xd = (B[13] ^= C[13]), xe = (B[14] ^= C[14]), xf = (B[15] ^= C[15]);
    for (int i = 0; i < 4; ++i) {

        /* Operate on columns. */
        x4 ^= RL(x0 + xc,  7);  x9 ^= RL(x5 + x1,  7); xe ^= RL(xa + x6,  7);  x3 ^= RL(xf + xb,  7);
        x8 ^= RL(x4 + x0,  9);  xd ^= RL(x9 + x5,  9); x2 ^= RL(xe + xa,  9);  x7 ^= RL(x3 + xf,  9);
        xc ^= RL(x8 + x4, 13);  x1 ^= RL(xd + x9, 13); x6 ^= RL(x2 + xe, 13);  xb ^= RL(x7 + x3, 13);
        x0 ^= RL(xc + x8, 18);  x5 ^= RL(x1 + xd, 18); xa ^= RL(x6 + x2, 18);  xf ^= RL(xb + x7, 18);
        /* Operate on rows. */
        x1 ^= RL(x0 + x3,  7);  x6 ^= RL(x5 + x4,  7); xb ^= RL(xa + x9,  7);  xc ^= RL(xf + xe,  7);
        x2 ^= RL(x1 + x0,  9);  x7 ^= RL(x6 + x5,  9); x8 ^= RL(xb + xa,  9);  xd ^= RL(xc + xf,  9);
        x3 ^= RL(x2 + x1, 13);  x4 ^= RL(x7 + x6, 13); x9 ^= RL(x8 + xb, 13);  xe ^= RL(xd + xc, 13);
        x0 ^= RL(x3 + x2, 18);  x5 ^= RL(x4 + x7, 18); xa ^= RL(x9 + x8, 18);  xf ^= RL(xe + xd, 18);
    B[ 0] += x0; B[ 1] += x1; B[ 2] += x2; B[ 3] += x3; B[ 4] += x4; B[ 5] += x5; B[ 6] += x6; B[ 7] += x7;
    B[ 8] += x8; B[ 9] += x9; B[10] += xa; B[11] += xb; B[12] += xc; B[13] += xd; B[14] += xe; B[15] += xf;

//! Running the Salsa 20/8 core "loops" times per work unit.
//! @param g_idata  input data in global memory
//! @param g_odata  output data in global memory
__global__ void
salsaKernel(uint32_t*g_idata, uint32_t *g_odata, int loops)
    __shared__ unsigned long X[WARPS_PER_BLOCK][WU_PER_WARP][32+1]; // +1 to resolve bank conflicts
    volatile int warpIdx    = threadIdx.x / warpSize;
    volatile int warpThread = threadIdx.x % warpSize;
    g_idata += 32      * (blockIdx.x * WU_PER_BLOCK + warpIdx * WU_PER_WARP);
    g_odata += 32      * (blockIdx.x * WU_PER_BLOCK + warpIdx * WU_PER_WARP);
#pragma unroll WU_PER_WARP
    for (int wu=0; wu < WU_PER_WARP; ++wu) X[warpIdx][wu][warpThread] = g_idata[32*wu+warpThread];
    for (int i = 0; i < loops; i++)
        { xor_salsa8(&X[warpIdx][warpThread][0], &X[warpIdx][warpThread][16]);
          xor_salsa8(&X[warpIdx][warpThread][16], &X[warpIdx][warpThread][0]); }
#pragma unroll WU_PER_WARP
    for (int wu=0; wu < WU_PER_WARP; ++wu) g_odata[32*wu+warpThread] = X[warpIdx][wu][warpThread];

// Program main
main(int argc, char **argv)
    runTest(argc, argv);

//! Run a simple streams test for CUDA
runTest(int argc, char **argv)
    printf("%s Starting...\n\n", argv[0]);

    // use command-line specified CUDA device, otherwise use device with highest Gflops/s
    int devID = findCudaDevice(argc, (const char **)argv);

    // get device properties
    struct cudaDeviceProp prop;
    checkCudaErrors(cudaGetDeviceProperties(&prop, devID));

    // initialize with enough blocks to fill the device nicely
    int blocks = 16 * prop.multiProcessorCount;
    int mem_size = 32 * sizeof(uint32_t) * WU_PER_LAUNCH;
    int loops = 2048;

    // create two streams to operate on
    cudaStream_t streamHandle[2];

    // allocate page-locked host memory (separate buffers for streams)
    uint32_t *h_idata[2];
    checkCudaErrors(cudaMallocHost(&h_idata[0], mem_size));
    checkCudaErrors(cudaMallocHost(&h_idata[1], mem_size));
    uint32_t *h_odata[2];
    checkCudaErrors(cudaMallocHost(&h_odata[0], mem_size));
    checkCudaErrors(cudaMallocHost(&h_odata[1], mem_size));

    // allocate device memory (separate buffers for streams)
    uint32_t *d_idata[2];
    checkCudaErrors(cudaMalloc((void **) &d_idata[0], mem_size));
    checkCudaErrors(cudaMalloc((void **) &d_idata[1], mem_size));
    uint32_t *d_odata[2];
    checkCudaErrors(cudaMalloc((void **) &d_odata[0], mem_size));
    checkCudaErrors(cudaMalloc((void **) &d_odata[1], mem_size));

    // setup execution parameters
    dim3 threads = dim3(WU_PER_BLOCK, 1, 1);
    dim3 grid = dim3(blocks, 1, 1);
    printf("grid.x is %d, threads.x is %d\n", grid.x, threads.x);

    // do some timing tests. Find a block configuration where execution time exceeds 20ms
    StopWatchInterface *timer = 0;
    printf("Tuning kernel run time for 20ms...\n");
    for (int i=8; i < 16384; i += 8)
        loops = i;

        salsaKernel<<< grid, threads, 0>>>(d_idata[0], d_odata[0], loops);
        // check if kernel execution generated and error
        getLastCudaError("Kernel execution failed");
        double time = sdkGetTimerValue(&timer);
        printf("loops=%d, Processing time: %f (ms)\n", loops, time);

        if (time > 20.0) break;

    printf("\nunning dual-stream operation (hopefully overlapping...or not!?!)\n");

    // stream points to current stream, other points to the other stream.
    int stream = 0, other = 1;

     // simulate the CPU time required for input preparation work
    char tmp[50]; sprintf(tmp, "Prep #%d\n", stream); nvtxRangeId_t rng1 = nvtxRangeStartA(tmp);

    // copy H2D
    checkCudaErrors(cudaMemcpyAsync(d_idata[stream], h_idata[stream], mem_size, cudaMemcpyHostToDevice, streamHandle[stream]));
    // execute kernel on the first stream
    salsaKernel<<< grid, threads, 0, streamHandle[stream] >>>(d_idata[stream], d_odata[stream], loops);

    int iterations = 0;
        // copy D2H on the current stream
        checkCudaErrors(cudaMemcpyAsync(h_odata[stream], d_odata[stream], mem_size, cudaMemcpyDeviceToHost, streamHandle[stream]));

         // simulate the CPU time required for input preparation work
        sprintf(tmp, "Prep #%d\n", other); nvtxRangeId_t rng2 = nvtxRangeStartA(tmp);

        // copy H2D on the other stream
        checkCudaErrors(cudaMemcpyAsync(d_idata[other], h_idata[other], mem_size, cudaMemcpyHostToDevice, streamHandle[other]));
        // execute the kernel on the other stream
        salsaKernel<<< grid, threads, 0, streamHandle[other] >>>(d_idata[other], d_odata[other], loops);

        // wait for this stream's execution to complete

        // simulate the CPU time required for result processing work
        sprintf(tmp, "Analyze #%d\n", other); nvtxRangeId_t rng3 = nvtxRangeStartA(tmp);

        stream = (stream+1)&1;
        other  = (other +1)&1;
    } while (++iterations < 10);


Some spelling corrections. The editing feature for the above post is broken (won’t work for me in Firefox or IE10). Strangely enough, it works in this reply. Duh?

s/craptographic/cryptographic <-- oops.

Cudaprof screenshot follows. Ignore the left quarter which has the initial setup and timing calibration. I would really want the green (host side) bits to overlap the blue bits (kernel launches), and no gaps to exist between the blue bits.

Why are my streams named 2, 7, 8?

Could this be part of the problem (pg.29)?

Kernel launches are synchronous in the following cases:

  • The application is run via a debugger or memory checker (cuda-gdb, cudamemcheck, Nsight) on a device of compute capability 1.x; [*] Hardware counters are collected via a profiler (Nsight, Visual Profiler).
  • [ BTW, “craptographic” is a great name! ]

    Edit: I compiled and ran your example on sm12/sm21/sm30 and sm35 devices on Win7/x64. Only the sm35 device (w/a TCC driver) shows an overlap of the simulated host range with the kernels. The rest of the devices generate Visual Profiler timelines similar to what you have above.

    Sorry, I don’t have time to look into this in details, so just a quick general note. Please note that GPUs prior to Kepler provide a single physical input queue. This can cause “false dependencies” between independent CUDA stream, i.e. the code does not behave as one would expect under a pure data dependency model. A colleague of mine created a clarifying write-up on this in the context of CUDA Fortran, but the scheduling strageties explained in his whitepaper apply to CUDA C in the same fashion:


    One additional thought: Does checkCudaErrors() possibly include some synchronization API call?

    Second thought: I may misunderstand the pseudo code, but what operations do you expect to overlap in the loop’s steady state? Each stage seems to be dependent on the previous stage, with the exception of the stages in lines 5 and 7, and these cannot be performed in parallel because the GPUs listed only have a single copy engine (so no simultaneous upload and download is possible). Why do you need the stream synchronization? Can you get rid of it?

    On WDDM (WinVista/7/8) the overhead to submit a kernel or a memory copy is significantly higher than through the TCC or Linux driver. As such the CUDA user mode driver queues work. In your case the work is sitting in the software queue in the CUDA driver. If you flush this call it will fix your overlap issue.

    salsaKernel<<< grid, threads, 0, streamHandle[other] >>>(d_idata[other], d_odata[other], loops);
        // flush streamHandle[other] to start the kernel

    In Nsight Visual Studio Edition you can identify this issue by enabling the Driver Queue Latency counters as follows:

    In the Analysis Activity Trace Settings

    • Expand CUDA
    • Enable Driver Queue Latency

    When you run the activity the timeline and launches page will now contain additional columns for queue time and submit time. In the timeline you will see the new row

    Processes[Process][CUDA][Context]\Counters\Queue/Submit Delta

    This chart has two area graphs

    The red positive area graph is the software queue depth. This height increases when launches, memory copies, and memory sets are pushed into the CUDA driver software queue. The height decreases when work is submitted to the kernel mode driver. This occurs when work is flushed (cuSynchronize, synchronous API calls, cuQuery, …).

    The blue negative area graph is the kernel mode driver and hardware queue. The height increases (large negative value) when work is submitted from the user mode driver. The decreases (approaches 0) when work is started by the GPU.

    The Device%, Host to Device, Device to Host, and Host to Host graphs under the queue graph show you when the GPU is actually executing commands.

    If this is not clear please let me know and I’ll find a reasonable way to post comparison screenshots with and without the cudaStreamQuery().

    In my screenshot the Analyze #1/#0 and Prep #0/#1 range overlap with the first half of the salsaKernel for each iteration providing about 45% concurrency between CPU and the kernel. The rest of the CPU time is spent in cudaStreamSynchronize.

    It would appear that Greg@NV 's problem description and proposed workaround is spot on.

    So with WDDM drivers you need to flush your work queues. If you don’t flush, it will clog. This was not very apparent from any documentation on streams that I read so far.

    NVIDIA still loses out to ATI/AMD big time in brute force cryptography attacks, but my newer CUDA code closes the gap slightly. The existing OpenCL miners applications are all not particularly well suited to nVidia devices.

    That “craptocurrency” called litecoin shot up from 0.5 USD per coin to something like 4 USD per coin in less than a week. Talk about volatility! Similarly, bitcoin went up like crazy. Pure speculation and mayhem.

    I am doing this for educational purposes only, others do it for a living. And those who have hoarded LTC or BTC previously will now be able to buy a shiny new car or something. But I got to check out the CUDA streams and events API.