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From Peter Crowther <>
Subject Re: Setting the Right Amount of Memory
Date Sun, 20 Jun 2010 16:03:39 GMT
On 20 June 2010 16:01, Caldarale, Charles R <>wrote:

> All modern GC algorithms are variations of mark-sweep-compact.  The
> basic operation consists of following the object reference graph from
> a set of known roots (eg, thread stack frames), marking each
> discovered object with a found flag, and copying the found objects to
> an unoccupied area.  Unreachable (dead) objects are never encountered,
> so their number or size does not come into play.  The space occupied
> by dead objects is automatically reclaimed as live objects are copied
> over them.
> Note that in some variants, the cost of a compact *does* vary with the
number of dead objects.  In Squeak (Smalltalk), for example, each object
header contains the object size, and hence by implication the address of the
next object header.  Squeak's GC compacts (after the mark phase) by starting
at the bottom of object memory and iterating through each object, copying
down the live ones and updating pointers on the way. In Java, this would
only ever be necessary in OldSpace - with a generational GC where you're
collecting anything other than the oldest generation, you just copy the live
objects out as you encounter them and update any pointers to other objects
in the same generation, after which you know the space is empty.  Typically,
the speed of such a compact is limited by the speed of access to memory.
Most modern memory architectures will treat such sequential read access
specially and start to read-ahead, so the memory overhead isn't as bad as it
might be.

My GC knowledge is a little out of date, so if any of the current GC experts
wish to correct me I'll be interested!

- Peter

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