High Latency on First Request to localhost

[julmb]

I have this code:

main :: IO ()
main = do
    request <- parseRequest "http://localhost:3000/test"
    getCurrentTime >>= print
    httpBS request >>= T.putStr . T.decodeUtf8Lenient . getResponseBody
    getCurrentTime >>= print
    httpBS request >>= T.putStr . T.decodeUtf8Lenient . getResponseBody
    getCurrentTime >>= print

With a simple servant service at http://localhost:3000/test, I get:

2024-09-05 03:47:47.018037777 UTC
Hello, world!
2024-09-05 03:47:47.271253156 UTC
Hello, world!
2024-09-05 03:47:47.272148168 UTC

Given that this is all local, I would have expected these requests to finish very fast. However, the first request takes about 250ms, only subsequent ones are fast.

I did some investigating, and it seems that getAddrInfo returns [::1]:3000 and 127.0.0.1:3000, in that order. The function firstSuccessful then attempts to connect to each of these, staggered by 250ms, which only succeeds for the second address.

Not sure if there is an easy/elegant solution to this, but it is very unfortunate that a simple connection to localhost incurs such a high latency on the first request.