Interrupt coalescing (also called interrupt moderation) is a technique used by NICs and storage adapters to reduce the number of CPU interrupts by grouping multiple I/O events together before notifying the processor.

🚀 Core Idea

Instead of generating an interrupt for every packet or I/O completion, the device waits for:

• A time threshold (e.g., 50 µs), or
• A packet count threshold (e.g., 32 packets)

…and then raises one interrupt for many events.

⚙️ How It Optimizes I/O Processing

1. Reduces Interrupt Rate

Without coalescing:

100,000 packets/sec → 100,000 interrupts/sec

With coalescing:

100,000 packets → ~3,000–10,000 interrupts/sec

👉 Fewer interrupts = less CPU overhead

2. Improves CPU Efficiency

• Each interrupt involves:
  • Context switch
  • Cache disruption
• Coalescing batches work → CPU processes multiple packets per interrupt

👉 Result:

• Better cache utilization
• Higher throughput per CPU core

3. Enables Batch Processing

• OS/network stack processes packets in groups instead of one-by-one
• Works well with:
  • Large receive offload (LRO)
  • TCP segmentation offload (TSO)

👉 Effect:

• Increased packets-per-second (PPS) handling capability

4. Reduces Hypervisor Overhead

In virtualized environments (e.g., IBM PowerVM):

• Fewer interrupts crossing:
  • Device → Hypervisor → VM

👉 Especially beneficial when using:

• Virtual I/O Server (VIOS)
• Shared I/O adapters

5. Prevents Interrupt Storms

• High-speed NICs (25/100 GbE) can overwhelm CPUs with interrupts
• Coalescing throttles interrupt frequency

👉 Ensures system stability under heavy load

📊 Trade-off: Latency vs Throughput

👉 Example:

• 50 µs delay adds slight latency
• But dramatically improves throughput and CPU efficiency

⚡ Real-World Impact

Without Coalescing

• High CPU usage
• Lower throughput under load
• Interrupt bottlenecks

With Coalescing

• 20–50% lower CPU usage (typical)
• Higher network/storage throughput
• Better scalability for multi-core systems

🎯 When to Tune It

Use higher coalescing for:

• Bulk data transfer
• Backup/restore
• Data lakes / analytics
• High-throughput networking

Use lower coalescing for:

• Low-latency trading systems
• Real-time apps
• Small, latency-sensitive I/O

🧠 Key Insight

Interrupt coalescing improves performance by trading a tiny amount of latency for a large gain in CPU efficiency and throughput, which is critical in modern high-speed I/O systems.