High-Concurrency Server I/O: epoll, Syscalls, and Zero-Copy on AWS EC2
Quick summary: C10k is solved until syscall overhead and context switches eat your Graviton cores. epoll, sendfile, and SO_REUSEPORT behaviors on EC2—and why Lambda caps concurrency differently.
Key Takeaways
- C10k is solved until syscall overhead and context switches eat your Graviton cores
- epoll, sendfile, and SO_REUSEPORT behaviors on EC2—and why Lambda caps concurrency differently
- Linux on EC2 (June 2026) serves most self-hosted APIs
- Zero-copy moves file → socket without user-space buffers—ideal for static assets behind NGINX on EC2 (often superseded by CloudFront)
- When this advice breaks - TLS termination on instance — crypto dominates; offload to ALB/CloudFront first
Table of Contents
Linux on EC2 (June 2026) serves most self-hosted APIs. Event loops use epoll (edge-triggered) to watch thousands of sockets per thread—contrast with kqueue on BSD/macOS dev laptops; production parity tests matter.
Syscall and context switch tax
Each read/write crossing user/kernel boundary costs CPU. Zero-copy sendfile() moves file → socket without user-space buffers—ideal for static assets behind NGINX on EC2 (often superseded by CloudFront).
| Pattern | Benefit on AWS |
|---|---|
| epoll + non-blocking I/O | Few threads, many connections on c7g Graviton |
| SO_REUSEPORT | Accept queue spread across workers |
| sendfile | Static file serving from EBS |
Lambda avoids your epoll tuning—concurrency is execution environment reuse with platform limits; CPU-bound work still belongs on EC2/ECS.
When this advice breaks
- TLS termination on instance — crypto dominates; offload to ALB/CloudFront first.
- Tiny payloads — syscall cost noise vs business logic.
What to do this week
- Compare
nginx/envoyworker count to CPU cores on origin ASG. - Profile with
perf topduring load test—look forsyscallhotspots. - Move static assets to S3+CloudFront; keep dynamic on epoll stack.
What this guide doesn’t cover
CPU cache false sharing—part 4 of this track.
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