836590 - Make the profiler (or UI) stop reporting PostTask() as sync main-thread disk IO

Status: RESOLVED INVALID

(Core :: Gecko Profiler, defect)

Description

[Chris Jones [:cjones] inactive; ni?/f?/r? if you need me]

The profiler UI has a nice annotation for main-thread disk IO, but it's flagging PostTask() as IO because it write()s to a wake-up pipe to notify the thread the task is being posted to.  That's equivalent to a futex() call, which doesn't get flagged as disk IO (correctly).

This is particularly annoying because sending IPC's always requires a PostTask(), and those get flagged as main-thread disk IO.  (Although, when I'm tracking down significant IPC's, it becomes a blessing in disguise.  But we should do that differently.)

I'm not sure which part of the system determines this.  How can we make this stop?

Comment 1

[Chris Jones [:cjones] inactive; ni?/f?/r? if you need me]

(In reply to Chris Jones [:cjones] [:warhammer] from comment #0)
> That's equivalent to a futex() call

(By which I mean, approximately equivalent in the amount of work done in the kernel.)

Comment 2

[Benoit Girard (:BenWa)]

Here's a profile showing this problem:
http://people.mozilla.com/~bgirard/cleopatra/#report=0e6042fb2a6121d84a71796bfe27ec4275c1a9db

Comment 3

[Benoit Girard (:BenWa)]

Turns out this a problem if we're not unwinding because we don't see the PostTask. We have a few options:
1) Add a sample_label to PostTask().
2) Only remove the annotation if PostTask shows up (stackwalk).
3) Ignore 1 sample long IO calls. This option is not great because it hides other fast IO steps.

Comment 4

[Benoit Girard (:BenWa)]

Bug 853363 will let the UI get rid of this false positive.

Comment 5

[Markus Stange [:mstange]]

perf.html doesn't have any stack-based heuristics for detecting main thread IO, so it doesn't have this bug.