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Andy Wingo 2022-05-11 22:29:37 +02:00
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@ -39,13 +39,16 @@ in the mark byte table.
You might think this is a bad tradeoff, and perhaps it is: I don't know
yet. If your granule size is two pointers, then one mark byte per
granule is 6.25% overhead on 64-bit, or 12.5% on 32-bit. Especially on
32-bit, it's a lot! On the other hand, you don't need GC bits in the
object itself, and you get a number of other benefits from the mark byte
table -- you can also stuff other per-object data there, such as pinning
bits, nursery and remset bits, multiple mark colors for concurrent
marking, and you can also use the mark byte (which is now a metadata
byte) to record the object end, so that finding holes in a block can
just read the mark table and can avoid looking at object memory.
32-bit, it's a lot! On the other hand, instead of the worst case of one
survivor object wasting a line (or two, in the case of conservative line
marking), granule-size-is-line-size instead wastes nothing. Also, you
don't need GC bits in the object itself, and you get a number of other
benefits from the mark byte table -- you can also stuff other per-object
data there, such as pinning bits, nursery and remset bits, multiple mark
colors for concurrent marking, and you can also use the mark byte (which
is now a metadata byte) to record the object end, so that finding holes
in a block can just read the mark table and can avoid looking at object
memory.
Other ideas in whippet: