1709703 - Requests blocked despite connection maximum not reached

Status: RESOLVED DUPLICATE of bug 1526974

(DevTools :: Netmonitor, defect)

Description

[Paul Menzel]

Attachment: nightly-blocked-connection-lwn.net.png

User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:90.0) Gecko/20100101 Firefox/90.0

Steps to reproduce:

With Nightly 90.0a1 (20210504092024), open several Web sites, and analyze the request timings in the developer tools.

Actual results:

Several requests are shown to be blocked. From section Request Timing from the Web page Network request details:

Time spent in a queue waiting for a network connection.

The browser imposes a limit on the number of simultaneous connections that
can be made to a single server. In Firefox this defaults to 6, but can be
changed using the network.http.max-persistent-connections-per-server
preference. If all connections are in use, the browser can't download more
resources until a connection is released.

Expected results:

As the number of connections is not depleted (just for checking I increased them to 64, and no difference), there is something fishy going on.

Comment 1

[Paul Menzel]

Attachment: Output of `about:support`

Comment 2

[BugBot [:suhaib / :marco/ :calixte]]

The Bugbug bot thinks this bug should belong to the 'DevTools::Netmonitor' component, and is moving the bug to that component. Please revert this change in case you think the bot is wrong.

Comment 3

[Jan Honza Odvarko [:Honza] (always need-info? me)]

Thanks for the report, Paul.

It looks like some requests are marked as blocked from no reason (unexpectedly)?

Can you please provide a test page (attach it to this bug report or just provide an URL), so we can try to reproduce the problem on our machines?
I don't see the problem when loading e.g. google.com

Thank you!

Comment 4

[Paul Menzel]

Attachment: Request timings with https://www.google.com/

The screenshot was done with https://lwn.net, and https://blog.fefe.de, which both use HTTP/1.1. But, it’s also happening with https://www.google.de and https://www.mozilla.com.

Comment 5

[Paul Menzel]

Attachment: Request timings with https://www.mozilla.com/de/

Comment 6

[Jan Honza Odvarko [:Honza] (always need-info? me)]

Attachment: image.png

Thanks for the screenshots. So, the problem is about "Blocking time", not entirely blocked requests (due to CORS, etc.)

My network.http.max-persistent-connections-per-server is 6 and I am seeing the following (see the attached screenshot) when loading https://lwn.net/

The top one is correct shows that the 7th request is blocked
The bottom one is also correct, the 7th request starts after all the previous ones are finished, so no blocking is needed.

But, the screenshot attached to the original report shows weird state.
Obviously, depends also how net connection is fast, so might be harder to repro.

Bomsy, can you reproduce the issue on your machine?

Comment 7

[Jan Honza Odvarko [:Honza] (always need-info? me)]

Paul, I am marking this as duplicate of bug 1526974
Feel free to reopen if you don't think it's a dup.

Thank you.

Comment 8

[Paul Menzel]

Hmm, not sure. In my examples, the first request is already blocked, isn’t it?

Comment 9

[Jan Honza Odvarko [:Honza] (always need-info? me)]

I am reading this comment:
https://bugzilla.mozilla.org/show_bug.cgi?id=1526974#c33
"I have a new log, where every new connection is blocked"

I am waiting for the HAR file (or a screenshot)

The underlying problem seems to be the same (we should keep in mind various flavors of it when testing it)

Comment 10

[Paul Menzel]

Comment 33. You are right. Thank you for digging through it. ;-)

Comment 11

[Paul Menzel]

http.max-persistent-connections-per-proxy = 32 in my setup.