On 8/14/19 9:21 PM, Gil Tene wrote: > The fact that they visit and parse dead objects in order to identify > recycle-able memory is what makes them > sweepers. Other techniques that don't visit dead objects do not perform a > sweep.
I am trying to understand what is your central argument here. This seems to be it. Are you saying that "sweeping" is when you visit dead objects, and non-"sweeping" is when you do not? >> I would leave "sweep" as something that reclaims the storage of the objects >> (i.e. "sweeps away" the >> garbage), which makes M-C and M-S classes more clearly defined and easier to >> reason about. > > When M-C implies "also does a full linear traversal that visits all dead > objects" (aka "a sweep"), that> terminology fails. Examples like C4, G1GC, > Shenandoah, and ZGC are all Mark-Compact (NO Sweep) > collectors, while ParallelGC and Serial GC are Mark-Compact (WITH Sweep) > collectors. Since the math > behind those varies dramatically in wasy that effect everyday choices, using > "M-C" as a classification > that covers both leads people to misunderstand the interactions between e.g. > heap size and GC work. M-C does not imply "full linear traversal that visits all dead objects". As the counter-example, it is very practical to use off-side marking bitmaps to walk only live objects of the heap. Careful design of such the marking bitmap would yield dramatically better results than using whatever self-parsable-heap walk. You can even make it multi-level and quite dense to skip over large chunks of sparse heap. When the marking phase is a separate phase, it stands to reason that the subsequent phases would have to process the marking results somehow. That would naturally do some sort of walk over the data structure that handles marks. If we call that walk "sweeping", then every Mark-* algorithm is necessarily "sweeping" too. So we have to break this somehow to make the definition viable. Is walking _dead_ objects the discriminator for "sweeping" then? So in your book, if we take the same Lisp2 collector, and compute-new-addresses and adjust-pointers steps are walking the self-parsable heap (visiting dead objects along the way), it is M-S-C? But if it uses marking bitmap (thus only visiting live objects), it becomes M-C? [This would be a weird flex, but okay]. -Aleksey -- You received this message because you are subscribed to the Google Groups "mechanical-sympathy" group. To unsubscribe from this group and stop receiving emails from it, send an email to mechanical-sympathy+unsubscr...@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/mechanical-sympathy/1de1d683-164c-7163-7138-ee2f12f66638%40gmail.com.
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