NVIDIA-SMI -lgc and -rgc Usage Guide

NVIDIA’s nvidia-smi (NVIDIA System Management Interface) provides the -lgc and -rgc options, allowing users to manually control GPU clock frequencies.

Observed Behavior Without Adjusting LGC

If the LGC setting is not adjusted, the following code exhibits different behaviors:

void single_GPU(int gpuid) {
    {
        cudaSetDevice(gpuid);
        int uyvy_size = 3840 * 2160 * 2;
        uint8_t* pUyvy = new uint8_t[uyvy_size];
        uint8_t* pGPU_uyvy;
        cudaMalloc((void**)&pGPU_uyvy, uyvy_size);
        cudaEvent_t start, stop;
        cudaStream_t stream;

        cudaEventCreate(&start);
        cudaEventCreate(&stop);
        cudaStreamCreate(&stream);

        uint64_t g_loop = 0;
        float elapsedTime = 0;
        while (1) {
            cudaEventRecord(start, stream);

            cudaMemcpy(pGPU_uyvy, pUyvy, uyvy_size, cudaMemcpyHostToDevice);
            
            cudaEventRecord(stop, stream);
            cudaEventSynchronize(stop);
            cudaEventElapsedTime(&elapsedTime, start, stop);

            printf("%d>> %lld >> %2.2f\n", gpuid, g_loop++, elapsedTime);
            Sleep(1000);
            // If sleep is not added, elapsedTime remains as expected.
            // If sleep is added, elapsedTime deviates from expectations.
            // If LGC is adjusted, elapsedTime decreases.
        }
    }
}

If sleep is added without setting LGC:

0>> 0 >> 1.78
0>> 1 >> 1.87
0>> 2 >> 1.97
0>> 3 >> 1.94
0>> 4 >> 1.89
0>> 5 >> 1.97
0>> 6 >> 1.91
0>> 7 >> 10.62
0>> 8 >> 11.03
0>> 9 >> 10.97
0>> 10 >> 10.94
0>> 11 >> 10.92
0>> 12 >> 10.92
0>> 13 >> 11.06

If sleep is not added:

0>> 4661 >> 1.64
0>> 4662 >> 1.73
0>> 4663 >> 1.65
0>> 4664 >> 1.63
0>> 4665 >> 1.68
0>> 4666 >> 1.65
0>> 4667 >> 1.66
0>> 4668 >> 1.67
0>> 4669 >> 1.65
0>> 4670 >> 1.64
0>> 4671 >> 1.66

If LGC is adjusted and sleep is added:

0>> 0 >> 1.76
0>> 1 >> 1.81
0>> 2 >> 1.81
0>> 3 >> 1.81
0>> 4 >> 1.83
0>> 5 >> 1.83
0>> 6 >> 1.82
0>> 7 >> 5.52
0>> 8 >> 6.30
0>> 9 >> 5.75
0>> 10 >> 6.62
0>> 11 >> 5.75
0>> 12 >> 5.74
0>> 13 >> 5.75
0>> 14 >> 5.78

1. nvidia-smi -lgc (Lock GPU Clock Frequency)

Command Format:

nvidia-smi -lgc <min_clock>,<max_clock>

This command locks the GPU clock frequency within a specified range to prevent it from entering low-power mode or downclocking.

Example:

nvidia-smi -lgc 1000,2000

Meaning:

• Ensures the GPU frequency remains within the 1000MHz ~ 2000MHz range.
• Prevents excessive downclocking or frequency fluctuations.

Check Supported Clock Ranges:

nvidia-smi -q -d CLOCK

Example Output:

Supported Clocks for GPU 00000000:01:00.0
Memory Clocks MHz : 5001, 5500
Graphics Clocks MHz : 300, 1000, 1500, 2000

This means the GPU supports frequency ranges from 300MHz to 2000MHz.

2. nvidia-smi -rgc (Restore Default Frequency)

Command Format:

nvidia-smi -rgc

This command restores the GPU to its default dynamic clock adjustment mode, removing the -lgc restriction.

Example:

nvidia-smi -rgc

Meaning:

• Allows the GPU to dynamically adjust its frequency.
• Restores the default power-saving or performance mode settings.

3. -lgc and -rgc Use Cases

By using nvidia-smi -lgc, you can maintain stable GPU performance, and if needed, nvidia-smi -rgc restores the default settings.