FWIW, apparently the UI is in the devtools profiler UI, not in the profiler 
https://profiler.firefox.com/ still tells users to install the addon from there.

I was told that one can get the button similar to the addon by enabling
devtools.performance.popup.enabled boolean pref and then using 'Customize...'

Anyhow, great stuff. Seeing IPC messages in the profiles can be really handy.
(and so far I've been positively surprised that we don't seem to send that many 
IPC messages)


On 10/30/19 10:14 PM, Jim Porter wrote:
Recently, we landed a new feature for the Firefox Profiler: the ability to record IPC messages for monitored threads. This should be useful for evaluating IPC-related performance issues as we make progress on Project Fission. To enable this feature, just check the "IPC Messages" feature in the profiler popup and collect a profile! Then, IPC messages on all monitored threads will be recorded to the profile.

For an example of what this looks like, see this profile of a user (me) opening 
mozilla.org in a new tab: <https://perfht.ml/2Wtkgx1>.

Since IPC messages are (obviously) cross-process, each IPC message is actually comprised of two profiler markers: one for the sending thread and one for the receiving thread. The profiler frontend then examines all the collected IPC markers and correlates the sending and receiving sides. After correlating each side, we can then determine the latency of the IPC message: this is defined to be the time between when the message is sent (i.e. when `SendMessage` or similar is called) and when it's received (i.e. once the recipient thread has constructed a `Message` object).

Sometimes, IPC messages will have an unknown duration. This means that the profiler marker for the other side of the IPC call wasn't recorded (either the thread wasn't profiled at all or the other side occurred outside of the time range of the profile).

As you can probably see from the example profile, the user interface is fairly basic for now: each thread just has a new timeline track to display its IPC messages, with outgoing messages in teal and incoming messages in purple. Of course, there's lots of room for improvement here, so if you have ideas for a visualization that would be useful to you, just file a bug and CC me on it!

Happy profiling!
- Jim

P.S.: For those who are curious about how we correlate each side of an IPC message, we compare the source and destination PIDs, the message's type, and its seqno. This is enough to uniquely identify any IPC message, though it does mean that reply messages are considered a separate message. If people find it useful, it should be straightforward to correlate initial and reply messages with each other as well.

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