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The research of Urs Hölzle
By Simon Hem Pedersen | July 2, 2009
I look forward to this years JAOO and especially to meet and experience Urs Hölzle. Urs Hölzle is, like David Ungar, one of my Garbage Collection research heroes.
When my companion and I wrote our Master’s Thesis “Garbage Collection in a gbeta Virtual Machine with the Train Algorithm” we read one of Urs Hölzle’s articles – “A Fast Write Barrier for Generational Garbage Collectors”. The article described how to implement an efficient write barrier, more specifically how to use a card-marking write barrier scheme. It was a short article without a lot of complex theory and it treated the subject in a clear way making it easy to understand and inspiring to read – like candy for the brain 
.
The purpose of a write barrier is to ensure that the garbage collector keeps track of the object heap each time a pointer is updated.
The write barrier in a generational garbage collector ensures that the remembered set of each generation is updated (in the train algorithm the remembered set is used between cars (mini generations within a train) and between trains (macro generations)). A remembered set (proposed by David Ungar) keeps track of which objects of a younger generations is referenced (kept alive) from an object residing in an older generation.
The card-marking write barrier scheme does this by dividing the heap into cards of size 2kwords. Every card has an associated bit in a separate bit vector. A store check simply marks the bit corresponding to the location being updated. At garbage collection time, the collector scans the bit vector and, whenever it finds a marked bit, examines all pointers in the corresponding card in the heap.
At JAOO Urs Hölzle will be speaking about “Energy-efficient cloud computing” and telling us how Google made their infrastructure scalable, cost effective and energy efficient.
