1554242 - Sending a blob to a content process does main thread I/O

Status: RESOLVED FIXED

(Core :: DOM: File, defect, P3)

Description

[Florian Quèze [:florian]]

See this profile: https://perfht.ml/2EuYXmJ

Some activity stream code is sending a PNG thumbnail as a blob to its privileged content process. This causes main thread I/O in the parent process: nsresult mozilla::dom::IPCBlobInputStream::Close calls nsFileInputStream::Close which indirectly calls nsFileStreamBase::DoOpen (through calling nsFileStreamBase::Tell)

Comment 1

[Denis Palmeiro [:denispal]]

Marking this as p1:pageload since it may occur frequently during every new tab. Florian, could you please confirm whether this occurs on every new tab, or only the startup new tab page? If it's the latter, we'll probably end up marking it p2 instead.

Comment 2

[Florian Quèze [:florian]]

(In reply to Denis Palmeiro [:denispal] from comment #1)

Marking this as p1:pageload since it may occur frequently during every new tab. Florian, could you please confirm whether this occurs on every new tab, or only the startup new tab page?

This profile of opening a few new tabs show the main thread I/O markers to the same .png images 3 times: https://perfht.ml/2ZvPg05
So yes, I confirm it happens every time about:newtab is loaded.

Comment 3

[Alex Thayer [:alexical] (she/her)]

Taking a look at this to see if anything can be done.

Comment 4

[Alex Thayer [:alexical] (she/her)]

Attachment: Bug 1554242 - Don't open file just to ftell it r?baku

In bug 1554242 we're noticing main thread file IO sending a PNG
thumbnail as a blob over IPC. The stack is roughly
IPCBlobInputStream::Close() -> nsFileInputStream::Close() ->
nsFileStreamBase::DoOpen(). The file in question is always a
deferred open file, and we didn't open it in append mode, so there
is no way for ftell to be anything other than 0 (?). This was
verified with an assertion on a try run.

Comment 5

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/7261371d7f5e
Don't open file just to ftell it r=baku

Comment 6

[Alexandru Michis [:malexandru]]

https://hg.mozilla.org/mozilla-central/rev/7261371d7f5e