Packager very slow on first run

[ywongweb]

Description

When I runreact-native run-ios, the package takes a very long time(2-3 minutes on a brand new init project) to run /bin/sh -c /Users/username/dev/myapp/ios/build/Build/Intermediates/myapp.build/Debug-iphonesimulator/myapp.build/Script-F8DA82EA1DF9468900363441.sh.

Observations

• The slowdown only happens when I don't have the packager running already. If it's running, subsequent react-native run-ios is very fast.
• There is no slowdown if I run the packager and reset it's cache node_modules/react-native/packager/packager.sh --reset-cache. But I have do the reset every time, also I need to manually refresh the iOS simulator twice to see the app.
• The slow down is happening even with a newly init react-native project (react-native 0.39.2), also happens on projects with older versions of react-native.
• I've tried removing node_modules, cleaning npm cache with npm cache clean, reinstalling watchman and react-native cli. Non of these work.

Additional Information

content of Script-F8DA82EA1DF9468900363441.sh

#!/bin/sh
if [ -z "${RCT_NO_LAUNCH_PACKAGER+xxx}" ] ; then
  if nc -w 5 -z localhost 8081 ; then
    if ! curl -s "http://localhost:8081/status" | grep -q "packager-status:running" ; then
      echo "Port 8081 already in use, packager is either not running or not running correctly"
      exit 2
    fi
  else
    open "$SRCROOT/../packager/launchPackager.command" || echo "Can't start packager automatically"
  fi
fi

[ericvicenti]

Yeah, it is known that the packager is slower when it is run for the first time, because it has to do a lot more work. The main priority is to keep the packager fast while developing.

I'm going to close this for now because it isn't super actionable. If you have any suggestions to make the packager go faster, feel free to open an issue describing your proposed change. Or you can post on product pains. If the community upvotes the issue, the RN team will know to prioritize it.

[ywongweb]

I managed to found a work around for this problem base on the suggestions in this post #11291

Steps

1. Manually start the packager in the SAME directory as the project you want to run:
   node_modules/react-native/packager/packager.sh

2. run react-native run-xxx

=============

Looks like the long wait at start up was caused by this line in the shell script: nc -w 5 -z localhost 8081 waiting for the packager to start. Still have no clues on why my packager takes 2-3 minutes to launch as manually launching it before hand seem to work fine.

[license2e]

@ericvicenti I just started getting this issue, and changing the code to use nc -w 5 -z 127.0.0.1 8081 launches the simulator almost instantly when built, and a lot faster when running the first time.

I submitted a PR: #13565

[AggarwalAnkit]

@license2e packager is still taking time to star. Now emulator opens up very fast after changing localhost to 127.0.0.1:8081

[punksta]

Why react use --reset-cache in release builds? What issues can we face if we use cache in release build?