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uvloop is the classic “install and go faster” story for asyncio. On bare echo servers it looks magical; on ECS services blocked on RDS, Redis, or Bedrock HTTP it often disappears into noise — or worse, conflicts with libraries that assume the default loop policy. Seniors install uvloop only after a benchmark on the real critical path, with a kill switch.
⚡ TL;DR: Benchmark p50/p99 with and without uvloop on production-like fan-out. Expect wins on many small sockets; expect little when wait time is downstream. Watch for loop-policy clashes, uvloop+Windows CI gaps, and libraries that call get_event_loop() badly. Gate enablement behind an env flag.
uvloop With asyncio: production guidance
python
# app/loop.py
import asyncio, os
def maybe_install_uvloop() -> str:
if os.getenv("ENABLE_UVLOOP", "0") != "1":
return "asyncio"
try:
import uvloop
uvloop.install() # ✅ before asyncio.run / framework startup
return "uvloop"
except Exception as e:
# ❌ Never crash boot because the speedup layer failed
print(f"uvloop_unavailable:{e}")
return "asyncio"
if __name__ == "__main__":
kind = maybe_install_uvloop()
asyncio.run(main())
Benchmark the workload you actually run
python
# bench/fanout.py
import asyncio, time, statistics as stats
async def fake_downstream(latency_ms: float):
await asyncio.sleep(latency_ms / 1000)
async def handle():
await asyncio.gather(*[fake_downstream(5) for _ in range(50)])
async def run(n=200):
samples = []
for _ in range(n):
t0 = time.perf_counter()
await handle()
samples.append((time.perf_counter() - t0) * 1000)
return {
"p50": stats.median(samples),
"p99": sorted(samples)[int(0.99 * (len(samples) - 1))],
}
| Workload | Typical uvloop effect |
|---|---|
| Many concurrent short TCP calls | Noticeable |
| Few long Postgres queries | Negligible |
| CPU-bound JSON in-event-loop | None (fix the CPU path) |
| Already using httptools/uvicorn defaults | Often already on |
When the drop-in stops helping
- Downstream-bound services — RDS round trips dominate; loop policy is not your bottleneck.
- Library incompatibilities — some native bindings and older loop-getters misbehave; pin tests on Linux.
- Duplicate install sites — frameworks that create loops early before
uvloop.install()silently ignore you. - Observability skew — comparing laptop Mac (no uvloop) to Linux prod without labeling metrics.
python
# ✅ Emit loop implementation as a metric dimension
import asyncio
loop = asyncio.get_running_loop()
loop_name = type(loop).__module__ + "." + type(loop).__name__
metrics.gauge("event_loop_info", 1, tags=[f"loop:{loop_name}"])
Closing checklist
✅ Dos – ✅ Gate withENABLE_UVLOOP and default off until proven
– ✅ Benchmark p99 on staging with production fan-out
– ✅ Install before the framework creates a loop
– ✅ Label metrics with loop implementation
– ✅ Keep CI green on platforms without uvloop wheels
❌ Don’ts
– ❌ Don’t expect uvloop to fix blocking ORM calls
– ❌ Don’t crash process startup if uvloop import fails
– ❌ Don’t A/B without fixing CPU work on the loop thread
– ❌ Don’t assume Mac local results match Linux ECS
– ❌ Don’t stack conflicting loop policies across workers
Related reading
- asyncio TaskGroups: Structured Concurrency That Cancels Cleanly Under Load
- Node undici Dispatcher Pools: Kill Keep-Alive Storms Under Spikes
- OpenTelemetry Sampling for Node: Stay Useful at Fifty Thousand RPS
- Python Pool Timeouts: Connection Checkout Bugs That Look Like 500s
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