Performance drop in host_to_device_memcpy_sm with large buffers

[yuyuankang]

I am using Ubuntu 22.04 and testing the performance of an NVIDIA A100 GPU with the nvbandwidth tool. I observed that as the buffer size increases, the reported throughput decreases:

Test case: host_to_device_memcpy_sm

• 512 MiB: 25.13 GB/s
• 1 GiB: 25.13 GB/s
• 10 GiB: 18.05 GB/s
• 20 GiB: 16.50 GB/s

Below is the output:

$ ./nvbandwidth -t host_to_device_memcpy_sm -b 512
nvbandwidth Version: v0.8
Built from Git version: v0.8

CUDA Runtime Version: 12060
CUDA Driver Version: 12060
Driver Version: 560.35.05

Device 0: NVIDIA A100-SXM4-40GB (00000000:99:00)

Running host_to_device_memcpy_sm.
memcpy SM CPU(row) -> GPU(column) bandwidth (GB/s)
           0
 0     25.13

SUM host_to_device_memcpy_sm 25.13

NOTE: The reported results may not reflect the full capabilities of the platform.
Performance can vary with software drivers, hardware clocks, and system topology.

$ ./nvbandwidth -t host_to_device_memcpy_sm -b 1024
nvbandwidth Version: v0.8
Built from Git version: v0.8

CUDA Runtime Version: 12060
CUDA Driver Version: 12060
Driver Version: 560.35.05

Device 0: NVIDIA A100-SXM4-40GB (00000000:99:00)

Running host_to_device_memcpy_sm.
memcpy SM CPU(row) -> GPU(column) bandwidth (GB/s)
           0
 0     25.13

SUM host_to_device_memcpy_sm 25.13

NOTE: The reported results may not reflect the full capabilities of the platform.
Performance can vary with software drivers, hardware clocks, and system topology.

$ ./nvbandwidth -t host_to_device_memcpy_sm -b 10240
nvbandwidth Version: v0.8
Built from Git version: v0.8

CUDA Runtime Version: 12060
CUDA Driver Version: 12060
Driver Version: 560.35.05

Device 0: NVIDIA A100-SXM4-40GB (00000000:99:00)

Running host_to_device_memcpy_sm.
memcpy SM CPU(row) -> GPU(column) bandwidth (GB/s)
           0
 0     18.05

SUM host_to_device_memcpy_sm 18.05

NOTE: The reported results may not reflect the full capabilities of the platform.
Performance can vary with software drivers, hardware clocks, and system topology.

$ ./nvbandwidth -t host_to_device_memcpy_sm -b 20480
nvbandwidth Version: v0.8
Built from Git version: v0.8

CUDA Runtime Version: 12060
CUDA Driver Version: 12060
Driver Version: 560.35.05

Device 0: NVIDIA A100-SXM4-40GB (00000000:99:00)

Running host_to_device_memcpy_sm.
memcpy SM CPU(row) -> GPU(column) bandwidth (GB/s)
           0
 0     16.50

SUM host_to_device_memcpy_sm 16.50

NOTE: The reported results may not reflect the full capabilities of the platform.
Performance can vary with software drivers, hardware clocks, and system topology.