The hundred steps of a heap ceiling

Any script in a Chromium browser can read the ceiling on the memory it may hold. Nothing asks the person for permission and nothing on the page shows that it happened.

The value is worth reading for one reason: it does not describe the page. It is arithmetic done on the memory of the machine underneath.

The rounded answer is not rounding by eye but a place in a fixed table. We computed that table with a verbatim copy of the engine source and counted what came out.

Each step keeps three significant digits, and neighbours stand about six percent apart. A number that falls between two of them never reaches the page as it was.

One step is handed to everyone whose real ceiling lands inside it. At the top of the table that band is 220 000 000 bytes wide, and very different machines answer identically.

An exact byte count works the other way round. It is the arithmetic of one machine and one build, it stays the same from visit to visit, and that is the kind of value worth collecting.

77 of the hundred steps lie below a billion bytes. The table is finer at the bottom than at the top, so a small machine is described more closely than a large one.

The ceiling is two parts added together: a long lived part sized from memory, and a short lived part that comes from the build.

A desktop profile is owed half of its declared memory as the long lived part and a phone profile a quarter. Neither one goes below 256 MB or above 4 GB.

Only the long lived part is replaced, and the number the declared memory calls for takes its place. The short lived part is carried over as it is: it belongs to the build, and writing it by hand would make a fresh tell out of nothing.

When the declared memory already agrees with the machine underneath, the value is not touched at all.

1. 1

   Declared memory sets the long part

   Half of it on a desktop profile and a quarter on a phone.

2. 2

   The short part is carried over

   It comes from the build and stays as it was.

3. 3

   The shape of the answer is kept

   A step stays a step and an exact count stays exact.

The acceptance run reads this value on a built engine instead of trusting the arithmetic. On the Windows build it came back as 4 192 MB against the 4 096 MB of long lived part the profile was owed.

The difference is the short lived part of that build. That number is not one of the hundred steps either, which is the browser answering the second way.

In a Chromium browser, open the developer console, read performance.memory.jsHeapSizeLimit and divide it by 1024 twice for megabytes.

A rounded answer is easy to recognise: three digits and then zeros all the way down. Anything above 3 760 000 000 never came out of the table.