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net/http: unexpectedly slow initialization #50511

KAction opened this issue Jan 8, 2022 · 4 comments

net/http: unexpectedly slow initialization #50511

KAction opened this issue Jan 8, 2022 · 4 comments


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@KAction KAction commented Jan 8, 2022

Here is trivial program that makes GET request:

import (

func main() {
        resp, _ := http.Get("")
        defer resp.Body.Close()
        body, err := ioutil.ReadAll(resp.Body)
        if err != nil {


When compiled and run on Darwin system (using binary distribution), it have more or less expected performance -- twice as slow compared to curl.

$ /usr/bin/time ./hello        0.25 real         0.05 user         0.01 sys
$ /usr/bin/time curl        0.12 real         0.01 user         0.01 sys

But when I compile and run it on my Linux box (Linux void-live 5.13.13_1 #1 SMP Fri Aug 27 13:28:13 UTC 2021 x86_64 GNU/Linux),
also using binary distribution, I get following results:

$ time ./hello
real      0m 1.03s
user    0m 1.30s
sys     0m 0.14s
$ time curl
real      0m 0.16s
user    0m 0.06s
sys     0m 0.00s

Difference is much bigger. This time seems to come from some one-time initialization -- second request made in same program is fast. Here is annotated strace. It looks like it is doing many "rt_sigreturn" calls, but this is just my guess.

I have access to another linux box, with unusual kernel (lacking madvise(2), for example), which have more of the same problem: first request takes around 4 seconds, any subsequent work as expected.

@KAction KAction changed the title net/http is unexpectedly slow compared to curl(1) net/http has unexpectedly slow initialization Jan 8, 2022
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@bcmills bcmills commented Jan 10, 2022

The rt_sigreturn calls are in response to SIGURG, which the runtime uses for asynchronous preemption.

That seems to imply that the latency is going into something CPU-bound, and the fact that the reported user time exceeds the reported real time corroborates that at least part of the program is spending non-trivial CPU on something.

If you add a pprof.StartCPUProfile at the beginning of func main and pprof.StopCPUProfile at the end, does it show anything interesting?

@bcmills bcmills added this to the Backlog milestone Jan 10, 2022
@bcmills bcmills changed the title net/http has unexpectedly slow initialization net/http: unexpectedly slow initialization Jan 10, 2022
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@KAction KAction commented Jan 10, 2022

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@mengzhuo mengzhuo commented Jan 11, 2022

Take a look at the strace log, it seems stalled after reading a large host file (14M) .

2022-01-08 09:58:49.823669500 read(6, " sourcehut.local\n10.0.2."..., 65536) = 65536
....after 7 seconds? still reading...
2022-01-08 09:58:56.787029500 read(6, "\n0.0.0.0 22"..., 65522) = 65522

I've tested on my Linux box, parsing 19M host file(655360 lines with same host name) takes 800ms for the first time and 3ms for second time.

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@mengzhuo mengzhuo commented Jan 11, 2022

@bcmills maybe we can change title to: "net: takes too long to parse *nix host file" ?

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