On 8 June 2018 at 00:33, Evan Laforge <qdunkan@gmail.com> wrote:

On Thu, Jun 7, 2018 at 2:48 PM, Bartosz Nitka <niteria@gmail.com> wrote:
> What version of GHC are you using?
> There have been some significant improvements like
> https://phabricator.haskell.org/rGHCb8fec6950ad99cbf11cd22698b8d5ab35afb828f,
> that only just made it into GHC 8.4.

I did in fact notice a very nice speedup in 8.4, this explains it.
Finally I know who to thank for it! Thank you very much for that fix,
it really makes a difference.

Are there more goodies in the 8.0.2 facebook branch, or have they all
made it into 8.4?

As loaded modules seem to consume a lot of memory, I've considered
trying GHC.Compact on them, but haven't looked into what that would
entail. Have you considered something like that?

I think I looked into this and found that it wasn't going to be easy, but I forget exactly why. Off the top of my head:

- you can't compact mutable things: perhaps the FastString table would give us problems here

- there is lots of deliberate laziness to support demand-loading of interface files, compaction would force all of it

- you can't compact functions, so if there are any functions in ModIface or ModDetails we would have to avoid compacting those parts of the structure somehow

- there are cycles and sharing in these structures so we would need to use the more expensive compaction method that keeps a hash table, which is 10x slower than cheap compaction

Probably worth looking into to find out exactly what the problems are though.

Cheers

Simon

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