The Surprising gRPC Client Bottleneck in Low-Latency Networks

The Surprising gRPC Client Bottleneck in Low-Latency Networks(blog.ydb.tech)

76 points by eivanov89 301 days ago | 21 comments

yuliyp 301 days ago |

If you have a single TCP connection, all the data flows through that connection, ultimately serializing at least some of the processing. Given that the workers are just responding with OK, no matter how many CPU cores you give to that you're still bound by the throughput of the IO thread (well by the minimum of the client and server IO thread). If you want more than 1 IO thread to share the load, you need more than one TCP connection.

atombender 301 days ago | |

When I started out using the gRPC SDK in Go, I was really surprised to find that it created just one connection per client. You'd think someone had made a multiplexing client wrapper, but I haven't been able to find one, so inevitably I've just written lightweight pooling myself. (Actually, I did come across one, but the code was very low quality.)

lacop 301 days ago |

Somewhat related, I'm running into a gRPC latency issue in https://github.com/grpc/grpc-go/issues/8436

If request payload exceeds certain size the response latency goes from network RTT to double that, or triple.

Definitely something wrong with either TCP or HTTP/2 windowing as it doesn't send the full request without getting ACK from server first. But none of the gRPC windowing config options nor linux tcp_wmem/rmem settings work. Sending one byte request every few hundred milliseconds fixes it by keeping the gRPC channel / TCP connection active. Nagle / slow start is disabled.

littlecranky67 301 days ago | |

sounds like classic tcp congestion window scaling delay. Sounds like your payload exceeds 10x initcwnd.

lacop 301 days ago | | |

Doesn't initcwnd only apply as the initial value? I don't care that the first request on the gRPC channel is slow, but subsequent requests on the same channel reuse the TCP connection and should have larger window size. This works as long as the channel is actively being used, but after short inactivity (few hundred ms, unsure exactly) something appears to revert back.

eivanov89 301 days ago | |

That's indeed interesting, thank you for sharing.

ltbarcly3 301 days ago |

gRPC is a very badly implemented system. I have gotten 25%-30%+ improvements in throughput just by monkeypatching client libraries for google cloud to force json api endpoint usage.

At least try something else besides gRPC when building systems so you have a baseline performance understanding. gRPC is OFTEN introducing performance breakdowns that goes unnoticed.

stock_toaster 301 days ago | |

Have you done any comparisons with connect-rpc?

xtoilette 301 days ago |

classic case of head of line blocking!

yuliyp 301 days ago | |

I don't think this is head-of-line blocking. That is, it's not like a single slow request causes starvation of other requests. The IO thread for the connection is grabbing and dispatching data to workers as fast as it can. All the requests are uniform, so it's not like one request would be bigger/harder to handle for that thread.

otterley 301 days ago | | |

> First, we checked the number of TCP connections using lsof -i TCP:2137 and found that only a single TCP connection was used regardless of in-flight count.

It's head-of-line blocking. When requests are serialized, the queue will grow as long as the time to service a request is longer than the interval between arriving requests. Queue growth is bad if sufficient capacity exists to service requests in parallel.