Implement a new heap-tuning option: -H
-H alone causes the RTS to use a larger nursery, but without exceeding the amount of memory that the application is already using. It trades off GC time against locality: the default setting is to use a fixed-size 512k nursery, but this is sometimes worse than using a very large nursery despite the worse locality. Not all programs get faster, but some programs that use large heaps do much better with -H. e.g. this helps a lot with #3061 (binary-trees), though not as much as specifying -H<large>. Typically using -H<large> is better than plain -H, because the runtime doesn't know ahead of time how much memory you want to use. Should -H be on by default? I'm not sure, it makes some programs go slower, but others go faster.
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