NCA Performance Optimization & Debugging 4

Question 1

What is warp divergence and how does it impact GPU performance?

Accepted Answer

Threads in a warp take different code paths, forcing serial execution of each path

Answer

When threads in a warp branch to different code paths due to conditional logic, the GPU must execute each path serially with inactive threads masked off, reducing effective parallelism.

Question 2

A CUDA developer enables Unified Memory (cudaMallocManaged). What is the main performance risk if not managed carefully?

Accepted Answer

Page faults trigger on-demand migration, stalling the GPU until pages are transferred

Answer

Without prefetching hints, Unified Memory causes GPU page faults that stall kernel execution while pages are migrated from CPU to GPU on demand, creating latency spikes.

Question 3

Which CUDA-GDB command pauses execution when a specific CUDA thread encounters a condition?

Accepted Answer

break with a conditional expression targeting thread coordinates

Answer

CUDA-GDB supports conditional breakpoints using thread/block index expressions like `break kernel.cu:42 if threadIdx.x == 0 && blockIdx.x == 1`.

Question 4

In a multi-GPU setup, which API call is used to enable direct peer-to-peer memory access between two CUDA devices?

Accepted Answer

cudaDeviceEnablePeerAccess()

Answer

cudaDeviceEnablePeerAccess() enables one GPU to directly read/write another GPU's memory over NVLink or PCIe without routing through host memory, reducing latency and improving bandwidth.

Question 5

What is the effect of using too much shared memory per block on SM occupancy?

Accepted Answer

Fewer blocks can fit on the SM simultaneously, reducing occupancy

Answer

Shared memory is partitioned among concurrent blocks on an SM; allocating more per block means fewer blocks fit simultaneously, potentially reducing occupancy and latency-hiding ability.

(NCA) NVIDIA-Certified Associate Practice Test

(NCA) NVIDIA-Certified Associate Practice Test

NCA Performance Optimization & Debugging 4