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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