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function timing - averages (100 samples).tsv
function-timing (with netty) - 100 samples.tsv


Trying to figure out why first request using WebClient takes so much time.


When we started using webclient in our library and we've noticed that the CI builds failed because of a 500ms timeout. We narrowed it down to first WebClient request and started digging what's happening.

Testing environment

  os : Darwin / 17.5.0 / x64
 cpu : Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz × 8


If you run restTemplateRequest() test in WebClientFirstRequestTest. An example output is:

First took: 46 ms
Second took: 5 ms
Difference in time: 40 ms

Which means that the first request to a WireMock server took 46 ms and the second request took 5 ms. This test uses RestTemplate to perform the request.

If you run webclientRequest() the first request takes 316 ms and the second 6 ms.

Creating a new WebClient instance doesn't mean that the first request in the new instance will be slow. Running webclientRequestFromDifferentInstances should produce a similar result:

block(): 319 ms
block(): 8 ms
block(): 6 ms
block(): 6 ms

This means that the time is mainly spent in creating shared state, meaning static fields initialization.

I wanted to profile this code to see which functions take the most time, it's only done on the first call so it cannot be run in a loop and profiled with something like async-profiler.

In order to measure call time I wrote a java agent (based on callspy). It instruments all the methods and inserts timing information. See CallSpy and Timer.

That produces an output in which > means entering a function. < means returning from function. A bigger indentation means that the function is higher on the stack. That is followed by function name and time it took to run it. You can see first_call.txt, second_call.txt and diff.html (this is just a convenience file - you can do your diff manually) to see the differences between first and second WebClient call.

You can find sorted results in a file function timing - averages (100 samples).tsv.

Bear in mind that the time of a function lower on the stack is a sum of all the functions above it. For example:

 <b() 50ms
<a() 51ms

means that function a() probably* took ~1ms and the heavy function might* be b() (* - I found that javasisst can't instrument all the functions so there might be somewhat in between calls);

After analyzing the diff, function stacks and trying to remove functions that are just* "wrappers" I think that the most time consuming functions are:

  • ReactorClientHttpConnector.adaptRequest
  • ClientRequest.create
  • BodyExtractors.readWithMessageReaders (which is a part of bodyToMono)
  • AbstractDataBufferDecoder.decodeToMono (which is a part of bodyToMono)
  • DefaultUriBuilderFactory$DefaultUriBuilder.createUri
  • HierarchicalUriComponents.encode(java.nio.charset.Charset)

I tried to mitigate the long times by creating a preload method that uses some of the classes but this had only limited effect, I was able to reduce firsts request time by ~80 ms. Try running webclientRequestWithPreload and webclientRequest test to see the effects. On my machine the times are around 345 ms and 257 ms.


Is there a way to work around this issue?

Should I just perform a request to a mocked server before starting the test suite?

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