> We introduce a novel experimental methodology that lets us quan-
tify the performance of precise garbage collection versus explicit
memory management. Our system allows us to treat unaltered Java
programs as if they used explicit memory management by relying
on oracles to insert calls to free. These oracles are generated
from profile information gathered in earlier application runs.
If you dig into the paper, on page 3 they find out that their null oracle approach (ie without actually freeing the memory) increases run times erratically by 12 to 33%. They then mention that their simulated approach should handle that case but it seems unlikely to me that their stats aren't affected. Also they disable multi-threading – again for repeatability – but that will obviously have a performance impact.
> We introduce a novel experimental methodology that lets us quan- tify the performance of precise garbage collection versus explicit memory management. Our system allows us to treat unaltered Java programs as if they used explicit memory management by relying on oracles to insert calls to free. These oracles are generated from profile information gathered in earlier application runs.