peria-san: Would you take a look at this?

This looks like it might be extensions, does creating the isolated world involve calling that twice?

Ah, that's plausible. Yes, WindowProxy::initialize needs to be called per window (i.e., per isolated world).

I tried it on some environment, and it reproduced only on Google corp machines' Chrome.  Newly built Chromium on the same machine does not reproduce it.

Arguments of ParseHTML events, that are the parents of the second WindowProxy::initialize(), are

beginData : {frame: "0x39c7014424f8", startLine: 0, url: "about:blank"}
endData	  : {endLine: -1}

Do they mean that some extensions/apps try to open "about:blank" in iframes?

Extensions need to opt in to loading in about:blank I think? A clean build of Chrome wouldn't have any extensions so it wouldn't show this. You should be able to copy PasswordAlert out of Chrome and use the "Load unpacked extension" mode in chrome://extensions to see this.

We definitely need to figure out how to make all of this faster. Does every extension result in another call to WindowProxy::initialize for the new isolated world? If that's true it means extensions are crazy expensive at the iframe level.

WindowProxy::initialize is called lazily. I guess your extension is proactively running a content script that accesses window in some way. Then we cannot avoid instantiating the WindowProxy.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8607ed4014092c8fea4db5a35cea0cf3fd96dd03

commit 8607ed4014092c8fea4db5a35cea0cf3fd96dd03
Author: peria <peria@chromium.org>
Date: Fri Nov 04 07:33:23 2016

Expose a function messageHanlderInMainThread in V8Initializer.cpp
as a static method of V8Initializer to be visible from other
compile units.

This function will be referred from V8 in its context serialization.

BUG= 636159 

Review-Url: https://codereview.chromium.org/2480583003
Cr-Commit-Position: refs/heads/master@{#429825}

[modify] https://crrev.com/8607ed4014092c8fea4db5a35cea0cf3fd96dd03/third_party/WebKit/Source/bindings/core/v8/V8Initializer.cpp
[modify] https://crrev.com/8607ed4014092c8fea4db5a35cea0cf3fd96dd03/third_party/WebKit/Source/bindings/core/v8/V8Initializer.h

This issue has been Available for over a year. If it's no longer important or seems unlikely to be fixed, please consider closing it out. If it is important, please re-triage the issue.

Sorry for the inconvenience if the bug really should have been left as Available. If you change it back, also remove the "Hotlist-Recharge-Cold" label.

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peria@, is this something we can address?

If #7 is true (i.e. only limited extensions create their contexts proactively and most extensions do not), then this might not be a big problem.  Does the initialization happen on all extensions or limited extensions?

According to #6, an extension PasswordAlert creates a context on every iframe, and it makes sense.
So I think we have nothing to do to reduce the number of calls.

Let me close as WontFix.