592308 - IsLowMemory() is terribly slow and there is nothing to do about it.

Status: RESOLVED FIXED

(Core :: XPCOM, defect)

Description

[Doug Turner (:dougt)]

The IsLowMemory predicate allows a piece of code test to see if we are in a low memory situation.  We only do this sort of test in two major places:

1) allocation of image data
2) during js callbacks

Testing for low memory requires us to read proc files, or acquire a kernal lock (like when we have to call into GlobalMemoryStatus on windows).  On Windows CE we disabled this check all together because of the cost.

(1) doesn't really matter that much in terms of perf, but (2) really does.

In addition, some memory allocators do reservations and commit on write.  In this case, the test would not actually do what you wanted.

So, I think we should move away from using IsLowMemory.  Instead, we should continue broadcasting memory-pressure notifications when we can and if the OS has such a feature.  For example, on osso, we get a notification that our process is running low on memory.  We can also detect this on Android.

If this is agreeable, we can remove the IsLowMemory() callers, make the call return false, and add documentation about deprecating this method.

Comment 1

[Doug Turner (:dougt)]

Attachment: patch v.1

Comment 2

[:Felipe Gomes (needinfo for replies!)]

http://hg.mozilla.org/mozilla-central/rev/a5d1b45234b8

Comment 3

[Eric Shepherd [:sheppy]]

Documented this change:

https://developer.mozilla.org/en/XPCOM_Interface_Reference/nsIMemory/isLowMemory

And mentioned it here as well:

https://developer.mozilla.org/en/XPCOM_Interface_Reference/nsIMemory

and of course on Fx4 for developers.