When running layout tests on swarming, the shards take roughly 3.5 minutes each. Thus this ~50 seconds is a large part of the time run time.

jeffcarp@ do you know anything about this?

dpranke@ with qyearsley out until 8th of May, is there a good person to look into this?

This might be unfairly blaming wpt and there is something which isn't logging that is taking up the time.

Yes, I wrote the CL to add this functionality: https://crrev.com/c/447959/

We realized a downside is that manifest generation time would go up if WPT_BASE_MANIFEST.json got out of date. However, imports (which update that file) have been happening regularly, so it'd be good to verify on a current local checkout to see if this is the cause.

I'm currently troopering today and tomorrow but I can check this soon when I get time.

How do you decide when to do the generation?

It runs as part of WPT import at 6am PT. If an import is successful, the base manifest gets updated. The last few imports have been successful:
https://build.chromium.org/p/chromium.infra.cron/builders/wpt-importer

Today I'll try reproducing this slowness on my workstation.

On ToT on my workstation the manifest generation step takes between 2-10 seconds. On a machine with less resources I could see it taking longer, but 50 seconds is a big jump.

We could add some timing instrumentation code to manifest generation to help diagnose this problem on swarming. Does that sound like a good next step?

Let's start by putting in a log call after it finishes to see if it is actually the culprit...

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a46809a3fbc319f5679b0c55102c870602119ac7

commit a46809a3fbc319f5679b0c55102c870602119ac7
Author: jeffcarp <jeffcarp@chromium.org>
Date: Wed May 03 03:39:20 2017

Add WPTManifest log line to diagnose slow manifest generation

BUG= 714503 

Review-Url: https://codereview.chromium.org/2848353002
Cr-Commit-Position: refs/heads/master@{#468884}

[modify] https://crrev.com/a46809a3fbc319f5679b0c55102c870602119ac7/third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/controllers/manager.py
[modify] https://crrev.com/a46809a3fbc319f5679b0c55102c870602119ac7/third_party/WebKit/Tools/Scripts/webkitpy/w3c/wpt_manifest.py

Looking at a sample of logs of shards from recent webkit_layout_test jobs on https://luci-milo.appspot.com/buildbot/chromium.fyi/WebKit%20Linux%20-%20RandomOrder/, it seems like the manifest generation is taking about 5-10 seconds recently.

Maybe it's possible that WPT_BASE_MANIFEST.json was just out of date on April 24, and as long as it's kept up to date we can expect a time of around 5 seconds for updating the manifest?

Closing since this seems to no longer be an issue.