939439 - Creating Blob from Uint8Array doubles memory use

Status: RESOLVED INACTIVE

(Core :: JavaScript Engine, enhancement)

Description

[Nicolas George]

Using the following minimal HTML file:

<script>
var test_array;
var test_blob;
function create_array() {
  test_array = new Uint8Array(512 * 1024 * 1024);
  for (var i = 0; i < test_array.length; i++)
    test_array[i] = i;
  alert("array created, length = " + test_array.length);
}
function create_blob() {
  test_blob = Blob([ test_array ], { type: "application/octet-stream" });
  alert("blob created, size = " + test_blob.size);
}
</script>
<p onclick="create_array()">create array</p>
<p onclick="create_blob()">create blob</p>

Observe (using ps) memory use before and after creating the array and the blob.

Result:

Before: VSZ = 295476 / RSS = 75544
After create array: VSZ = 838292 RSS = 601180 (+ ~513 M)
After create blob: VSZ = 1362588 RSS = 1122992 (+ ~510 M)

Memory increase after create array is normal.

When creating the Blob, sharing the memory buffers (using some kind of copy-on-write) should allow to avoid the increase of memory use.

Since the L in "blob" usually means "large", it can be a real issue. The problem was initially spotted when using a blob to download, decrypt and offer to save a 500 M file and the array → blob step caused an out-of-memory condition in the browser.

Comment 1

[Boris Zbarsky [:bzbarsky]]

> sharing the memory buffers (using some kind of copy-on-write)

Would mean that all writes to typed arrays end up getting slower, generally... :(