커널 내부의 printf

Believe it or not, you can call printf right inside a kernel. It is the simplest way to peek at what your threads are doing. 👀

Remember the kernel runs in every thread, so a single printf line fires once per thread. Launch 256 threads and you get 256 lines.

Include the threadIdx in your message so you can tell threads apart. Otherwise the output is just a wall of identical lines.

Threads run in parallel, so the printed order is unpredictable. Do not rely on lines arriving in index sequence.

Device printf supports the usual format specifiers like %d, %f, and %s. It feels just like host printf.

Device output is staged in a GPU buffer and flushed to your console later, not the instant printf runs. That is normal.

Because the launch is async, you will not see prints until the GPU finishes. Call cudaDeviceSynchronize to flush them.

Printing from millions of threads floods the buffer. Guard it so only thread 0 prints, or only a few do.

printf is your fastest debugging tool: drop one in to check an index or a value, confirm the bug, then remove it.

Heavy printing hurts performance badly. Use it to find a problem, then delete it before you measure real speed.

Device printf needs a reasonably modern compute capability (2.0 and up). Almost every current GPU supports it just fine.

Check what you know about device printf.

Use printf to peek inside threads, add threadIdx to tell them apart, sync to flush, and remove it before timing. Handy tool! 🎉