Hi Simon.
Issue 1: should the maximum heap size be unbounded by default? Currently the maximum heap size is bounded at 64M. Arguments for: this stops programs with a space leak eating all your swap space. Arguments against: it's annoying to have to raise the limit when you legitimately need more space.
Options:
Remove the default limit altogether - because you don't always know how much data a temporo-spatially remote end user of a finished product might need to use.
(any others?)
with the option to set a limit during debugging in the event that a space leak is beginning to be annoying and troublesome to find during development.
Issue 2: Should -M be renamed to -H, and -H renamed to something else? The argument for this change is that GHC's -M option is closer to the traditional meaning of -H.
I suggest remove -H and -M to avoid legacy semantic confusion and introduce something like "--minimum-heap" (-Hmin) and "--maximum-heap" (-Hmax) to get away from the unintuitive "M".
Issue 3: (suggestion from Julian S.) Perhaps there should be two options to specify "optimise for memory use" or "optimise for performance", which have the effect of setting the defaults for various GC options to appropriate values. Optimising for memory use might enable the compacting collector all the time, for instance. Optimising for performance is hard - we may be able to change some of the defaults to trade space for time, but it's unlikely to be entirely reliable (eg. turning on the compacting collector sometimes improves performance, sometimes reduces it).
Sounds sensible.
How about:
* Renaming current -M to -H, and current -H to -HS.
Don't like this because it's not intuitive and could cause legacy mixup problems.
* Fixing up the sizing calculations a bit so that the max heap size is more closely observed.
Depends on time cost and perceived run-time GC activity - anything which minimises runtime pauses is best.
Also having an auto-fallback to compacting collection when heap gets full. Overall aim is to reduce, ideally to zero, the number of flags users have to give to the RTS in order to get reasonable performance yet efficient use of memory. People simply won't use the compacting collector if you have to ask for it specially.
Agree with all of this paragraph. Cheers Mike Thomas.