When you first look at CUDA, the terminology may be a little confusing perhaps for the beginners something like, what’s a Block? What’s a Grid? How does any of it relate to the actual hardware? But here’s the simplest way I found to think about it.
First query your GPU hardware
Before writing any CUDA code, run this to see exactly what your card supports.
#include <stdio.h>
void printGpuInfo() {
cudaDeviceProp prop;
cudaGetDeviceProperties(&prop, 0);
printf("\n========== GPU HARDWARE INFO ==========\n");
printf("GPU Name : %s\n", prop.name);
printf("Compute Capability : %d.%d\n", prop.major, prop.minor);
printf("Total Global Memory : %zu MB\n", prop.totalGlobalMem / 1024 / 1024);
printf("---------------------------------------\n");
printf("Max Threads per Block : %d\n", prop.maxThreadsPerBlock);
printf("Max Block Dim X : %d\n", prop.maxThreadsDim[0]);
printf("Max Block Dim Y : %d\n", prop.maxThreadsDim[1]);
printf("Max Block Dim Z : %d\n", prop.maxThreadsDim[2]);
printf("---------------------------------------\n");
printf("Max Grid Dim X : %d\n", prop.maxGridSize[0]);
printf("Max Grid Dim Y : %d\n", prop.maxGridSize[1]);
printf("Max Grid Dim Z : %d\n", prop.maxGridSize[2]);
printf("---------------------------------------\n");
printf("Warp Size : %d\n", prop.warpSize);
printf("Multiprocessors (SM count) : %d\n", prop.multiProcessorCount);
printf("Max Threads per SM : %d\n", prop.maxThreadsPerMultiProcessor);
printf("=======================================\n\n");
}
int main() {
printGpuInfo();
return 0;
}
Compile and run:
nvcc device_info.cu -o device_info && ./device_info
Output you get:
dp@dp-Katana-GF76-11UG:~/Desktop$ nvcc GpuInfo.cu -o device_info && ./device_info
========== GPU HARDWARE INFO ==========
GPU Name : NVIDIA GeForce RTX 3070 Laptop GPU
Compute Capability : 8.6
Total Global Memory : 7973 MB
---------------------------------------
Max Threads per Block : 1024
Max Block Dim X : 1024
Max Block Dim Y : 1024
Max Block Dim Z : 64
---------------------------------------
Max Grid Dim X : 2147483647
Max Grid Dim Y : 65535
Max Grid Dim Z : 65535
---------------------------------------
Warp Size : 32
Multiprocessors (SM count) : 40
Max Threads per SM : 1536
=======================================
dp@dp-Katana-GF76-11UG:~/Desktop$
Understand the physical hardware
Your GPU is not one giant processor. It’s a collection of smaller units nested inside each other.
GPU (For example: RTX 3070 Laptop)
│
├── SM 0
├── SM 1
├── SM 2 ← 40 Streaming Multiprocessors total
│ ... "Multiprocessors (SM count) : 40"
└── SM 39
│
├── Warp 0
├── Warp 1
│ ... ← 1536 ÷ 32 = 48 warp units per SM
└── Warp 47 "Max Threads per SM : 1536"
│ "Warp Size : 32"
│
├── Thread 0
├── Thread 1
│ ... ← always exactly 32 per warp
└── Thread 31
│
└── executes kernel → data[idx] = 1.0 / data[idx]
40 SMs × 48 warps × 32 threads = 61,440 simultaneous threads
Relate the above with the software layer (CUDA Concepts)
Grid, Block, and Warp are not new hardware. They are names for regions of the hardware tree above.
┌──────────────────────────────────────────────┐
│ GRID — your full kernel launch │
│ "Max Grid Dim X : 2,147,483,647" │
│ │
│ ┌──────────────────────────────────────┐ │
│ │ BLOCK — work assigned to one SM │ │
│ │ "Max Threads per Block : 1024" │ │
│ │ │ │
│ │ ┌──────────────────────────────┐ │ │
│ │ │ WARP — 32 threads │ │ │
│ │ │ "Warp Size : 32" │ │ │
│ │ │ │ │ │
│ │ │ T0 T1 T2 T3 ... T31 │ │ │
│ │ └──────────────────────────────┘ │ │
│ │ Warp 1 ... Warp 47 │ │
│ └──────────────────────────────────────┘ │
│ Block 1 ... Block 255 │
└──────────────────────────────────────────────┘
| concept | what it is | your number |
|---|---|---|
| Grid | entire launch — all blocks together | 256 blocks |
| Block | group of threads on one SM | 256 threads |
| Warp | 32 threads the SM runs as one unit | 8 per block |
| Thread | one element, one computation | 65,536 total |
Step 4 — What Are Dim X, Y, Z?
Your block and grid can be shaped in 1D, 2D or 3D to match the shape of your data.
1D — flat array (your program)
──────────────────────────────
data: [0.5][1.0][1.5][2.0] ...
Block Dim X = 256, Y = 1, Z = 1
Grid Dim X = 256, Y = 1, Z = 1
only X matters — one row of elements
2D — an image
─────────────
image: 1920 columns × 1080 rows
Block Dim X = 32, Y = 32, Z = 1 → 32×32 = 1024 threads per block
Grid Dim X = 60, Y = 34, Z = 1
X = columns, Y = rows, each thread handles one pixel
3D — a volume
─────────────
scan: 128 × 128 × 64 voxels
Block Dim X = 8, Y = 8, Z = 8 → 8×8×8 = 512 threads per block
Grid Dim X = 16, Y = 16, Z = 8
X = width, Y = height, Z = depth, each thread handles one voxel
The limits from my spec:
| dimension | Block max | Grid max | maps to |
|---|---|---|---|
| X | 1024 | 2,147,483,647 | columns / width |
| Y | 1024 | 65,535 | rows / height |
| Z | 64 | 65,535 | depth / layers |
** Block X × Y × Z must never exceed
Max Threads per Block(1024).
Summary
hardware (fixed silicon) software (you define this)
──────────────────────── ──────────────────────────
GPU chip → Device
40 SMs → runs your Blocks
48 warp units per SM → Warp (always 32 threads)
→ Grid (all your blocks)
→ Block (your BLOCK_SIZE)
Thread → Thread