WebAssembly Cold Start Comparison FAQ & Answers

WASM cold start factors: (1) Module size (<1MB ideal, each MB adds ~1-3ms), (2) Import count (each import adds parsing overhead), (3) Linear memory size (initial pages allocation), (4) Initialization code (module start function), (5) Host bindings (WASI calls, host functions). Optimize: minimize imports, lazy initialization, use WAT optimizer.

Cloudflare optimizations: (1) V8 isolates (not full VMs, 5ms startup), (2) Module caching (pre-compiled WASM cached globally), (3) No filesystem/network init, (4) Streaming compilation (start execution before full download), (5) Per-request isolation without OS process overhead. Result: median 5ms P50, 15ms P99 cold start (2024).

Benchmark (2024): WASM (Cloudflare Workers) = 5ms P50, 15ms P99. Container (Lambda x86) = 250ms P50, 800ms P99. Container (Lambda ARM Graviton2) = 200ms P50, 600ms P99. Container (Cloud Run) = 400ms P50, 1500ms P99. WASM advantage: 40-100x faster. Container advantage: language ecosystem, filesystem, complex dependencies.

Fastly Compute@Edge: <1ms cold start using Lucet (ahead-of-time WASM compiler). Pre-compiles WASM to native code during deployment. Runtime: native code execution without JIT. Memory isolation via MPK (Memory Protection Keys). Initialization: instant module load from pre-compiled cache. Trade-off: longer build time, faster runtime.

Runtime cold start benchmarks (2024-2025): (1) Wasmer: 2-5ms (LLVM backend), (2) Wasmtime: 3-8ms (Cranelift JIT), (3) WasmEdge: 1-3ms (AOT compilation), (4) WAMR: 5-10ms (interpreter mode), (5) V8: 5-15ms (TurboFan JIT). Choose based on: deployment environment, compilation model (JIT vs AOT), memory constraints.

Instantiation patterns: (1) Lazy: defer imports until first use (fastest start, slower first call), (2) Eager: resolve all imports upfront (slower start, consistent performance), (3) Streaming: compile while downloading (overlaps network + compile), (4) Pre-initialization: warm instance pool (eliminates cold start, higher memory). Edge platforms use streaming + lazy for optimal cold start.

WASI (WebAssembly System Interface) adds 1-5ms overhead for cold starts. Overhead from: filesystem namespace setup, capability-based security checks, preopened directories. Minimal WASI (no filesystem): <1ms. Full WASI (filesystem, environment, networking): 3-5ms. Edge platforms (Cloudflare, Fastly) use restricted WASI subset for faster cold starts.

Cold start vs module size (2024): <100KB = 1-2ms, 100KB-500KB = 2-5ms, 500KB-1MB = 5-10ms, 1MB-5MB = 10-30ms, >5MB = 30-100ms. Size optimization: (1) wasm-opt --Oz (aggressive size reduction, 20-40% smaller), (2) Strip debug info, (3) Tree shaking/dead code elimination, (4) Code splitting (load core + lazy load features).

Language cold start overhead (beyond base WASM): Rust = +0-2ms (minimal runtime), Go = +5-15ms (GC initialization), AssemblyScript = +1-3ms (lightweight), C/C++ = +0-1ms (no runtime). Avoid: Python/Ruby WASM (100-300ms startup for interpreter). Edge recommendation: Rust/C/C++/AssemblyScript for <10ms total cold start.

Hybrid: WASM runtime inside container (e.g., Lambda function running Wasmtime). Cold start: container overhead (200ms) + WASM init (5ms) = 205ms total. No advantage over pure container for cold start. Use for: gradual migration (existing container infrastructure), mixed workloads (some container, some WASM), specific runtimes unavailable on pure WASM platforms.

WASM cold start factors: (1) Module size (<1MB ideal, each MB adds ~1-3ms), (2) Import count (each import adds parsing overhead), (3) Linear memory size (initial pages allocation), (4) Initialization code (module start function), (5) Host bindings (WASI calls, host functions). Optimize: minimize imports, lazy initialization, use WAT optimizer.

Cloudflare optimizations: (1) V8 isolates (not full VMs, 5ms startup), (2) Module caching (pre-compiled WASM cached globally), (3) No filesystem/network init, (4) Streaming compilation (start execution before full download), (5) Per-request isolation without OS process overhead. Result: median 5ms P50, 15ms P99 cold start (2024).

Benchmark (2024): WASM (Cloudflare Workers) = 5ms P50, 15ms P99. Container (Lambda x86) = 250ms P50, 800ms P99. Container (Lambda ARM Graviton2) = 200ms P50, 600ms P99. Container (Cloud Run) = 400ms P50, 1500ms P99. WASM advantage: 40-100x faster. Container advantage: language ecosystem, filesystem, complex dependencies.

Fastly Compute@Edge: <1ms cold start using Lucet (ahead-of-time WASM compiler). Pre-compiles WASM to native code during deployment. Runtime: native code execution without JIT. Memory isolation via MPK (Memory Protection Keys). Initialization: instant module load from pre-compiled cache. Trade-off: longer build time, faster runtime.

Runtime cold start benchmarks (2024-2025): (1) Wasmer: 2-5ms (LLVM backend), (2) Wasmtime: 3-8ms (Cranelift JIT), (3) WasmEdge: 1-3ms (AOT compilation), (4) WAMR: 5-10ms (interpreter mode), (5) V8: 5-15ms (TurboFan JIT). Choose based on: deployment environment, compilation model (JIT vs AOT), memory constraints.

Instantiation patterns: (1) Lazy: defer imports until first use (fastest start, slower first call), (2) Eager: resolve all imports upfront (slower start, consistent performance), (3) Streaming: compile while downloading (overlaps network + compile), (4) Pre-initialization: warm instance pool (eliminates cold start, higher memory). Edge platforms use streaming + lazy for optimal cold start.

WASI (WebAssembly System Interface) adds 1-5ms overhead for cold starts. Overhead from: filesystem namespace setup, capability-based security checks, preopened directories. Minimal WASI (no filesystem): <1ms. Full WASI (filesystem, environment, networking): 3-5ms. Edge platforms (Cloudflare, Fastly) use restricted WASI subset for faster cold starts.

Cold start vs module size (2024): <100KB = 1-2ms, 100KB-500KB = 2-5ms, 500KB-1MB = 5-10ms, 1MB-5MB = 10-30ms, >5MB = 30-100ms. Size optimization: (1) wasm-opt --Oz (aggressive size reduction, 20-40% smaller), (2) Strip debug info, (3) Tree shaking/dead code elimination, (4) Code splitting (load core + lazy load features).

Language cold start overhead (beyond base WASM): Rust = +0-2ms (minimal runtime), Go = +5-15ms (GC initialization), AssemblyScript = +1-3ms (lightweight), C/C++ = +0-1ms (no runtime). Avoid: Python/Ruby WASM (100-300ms startup for interpreter). Edge recommendation: Rust/C/C++/AssemblyScript for <10ms total cold start.

Hybrid: WASM runtime inside container (e.g., Lambda function running Wasmtime). Cold start: container overhead (200ms) + WASM init (5ms) = 205ms total. No advantage over pure container for cold start. Use for: gradual migration (existing container infrastructure), mixed workloads (some container, some WASM), specific runtimes unavailable on pure WASM platforms.