Bug 157945 – Web Inspector: retained size for typed arrays does not count native backing store

• Status: RESOLVED
• Resolution: FIXED
• Component: Web Inspector

[Blaze Burg]

STEPS TO REPRODUCE

1. Go to http://bl.ocks.org/syntagmatic/6c149c08fc9cde682635
2. Capture a recording long enough that a really big GC happens
3. Compare heap snapshots before and after a big GC

EXPECTED

It lists a Float32Array and ArrayBuffer with 10's of thousands of elements, so it should be MB's of data

ACTUAL

Retained size is 32 bytes regardless of typed array size

[Radar WebKit Bug Importer]

<rdar://problem/26392238>

[Blaze Burg]

Created attachment 279483 [details]
[SCREENSHOT] Bad timeline data

[Joseph Pecoraro]

I guess this is a specific type of TypedArray that we do not get the size correctly.

For a basic:

    new Uint32Array(5000);

I'm seeing it get a size of 19.56kb.

So there must be something non-trivial about the arrays on that page.

[Joseph Pecoraro]

Ahh, I see the issue when using a Buffer and View separately:

    window.lists = [];
    console.takeHeapSnapshot("before");
    var buffer = new ArrayBuffer(4*5000);
    var view = new Float32Array(buffer);
    window.lists.push(view);
    console.takeHeapSnapshot("after");    

Neither the ArrayBuffer or the Float32Array claiming the data!

[Joseph Pecoraro]

Looks like JSArrayBuffer doesn't report an estimated size. I missed it because ArrayBuffer's have their own unique reporting of their size to the Heap.

[Joseph Pecoraro]

Created attachment 279501 [details]
[PATCH] Proposed Fix

Note, with this change, the memory will be associated with the ArrayBuffer.

There currently does not exist a way to know if the Float32Array is the only retainer of the ArrayBuffer, so the Float32Array's will still appear small. That means they are just a view on an ArrayBuffer.

[Geoffrey Garen]

Comment on attachment 279501 [details]
[PATCH] Proposed Fix

r=me

[WebKit Commit Bot]

Comment on attachment 279501 [details]
[PATCH] Proposed Fix

Clearing flags on attachment: 279501

Committed r201226: <http://trac.webkit.org/changeset/201226>

[WebKit Commit Bot]

All reviewed patches have been landed.  Closing bug.