Sun's HotSpot

Hans-Martin Mosner hm.mosner at
Thu Oct 8 19:13:31 UTC 1998

Markus Kohler wrote:
> Hi all,
> There's now some information about Sun's new HotSpot Java Compiler at
> The paper says they use a two word object header. That's 8 Bytes right ?
> How does this compare to Squeak ?

Most objects (small instances of so-called 'compact classes') use a 4
byte header which includes flags, a hash value, size bits and a class
Objects whose class is not in the compact class table use 8 bytes (small
hdr + class pointer), and objects whose size is more than 255 bytes use
12 bytes (small hdr + class pointer + size word).

I don't have exact numbers, but I would guess that on average, a Squeak
object has less than 8 header bytes.


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