Registers and Shared Memory Limits
How resources cap resident blocks.
Registers and Shared Memory Limits is a free CUDA Academy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the CUDA Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
The Resource Budget
Each SM owns a fixed pool of registers and shared memory. Every resident block must carve its share from these pools.
Registers Per Thread
Your kernel uses some number of registers per thread. Multiply by threads per block and you see one blocks register cost.
Registers Cap Blocks
If each thread needs many registers, fewer threads fit, so fewer blocks stay resident. Register-heavy kernels lose occupancy.
Seeing Register Use
Compile with this flag and nvcc prints registers per thread, letting you spot when a kernel is too register hungry.
nvcc -Xptxas -v vecadd.cuCapping Registers
You can force a ceiling with a launch bound so the compiler spends fewer registers and lets more warps stay resident.
__launch_bounds__(256, 4) __global__ void k() {}Shared Memory Per Block
Each block can request shared memory. The SM only fits as many blocks as its shared pool, often 48 to 100 KB, allows.
Shared Memory Caps Blocks
Ask for a large __shared__ tile and only one or two blocks fit per SM. Big tiles trade occupancy for data reuse.
The Tightest Limit Wins
The SM computes blocks allowed by registers, by shared memory, and by the warp cap. The smallest of these decides occupancy.
Register Spilling
When a thread needs more registers than allowed, extras spill to slow local memory. Spills can hurt more than low occupancy.
Tuning the Balance
Cutting registers or shared memory raises occupancy, but too aggressive a cut causes spills. The sweet spot is a balance.
Measure, Do Not Guess
Always read the actual register and shared usage from the compiler before tuning. Guessing usually picks the wrong knob.
Quick Check
Recall how the SM decides how many blocks can be resident.
Recap
You saw that registers and shared memory are fixed SM budgets, and the tightest limit caps occupancy. Watch for spills. 🧮
Frequently asked questions
Is the “Registers and Shared Memory Limits” lesson free?
Yes — the full text of “Registers and Shared Memory Limits” is free to read here on the web, and the CUDA Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the CUDA Academy course, upgrade to CoddyKit PRO.
What will I learn in “Registers and Shared Memory Limits”?
How resources cap resident blocks. You practise CUDA Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start CUDA Academy?
No prior experience is required. CUDA Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Registers and Shared Memory Limits” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this CUDA Academy lesson?
Yes. Every CUDA Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- What Occupancy Really Means
- Registers and Shared Memory Limits
- The Occupancy Calculator API
- Occupancy Is Not the Whole Story