Date: Fri, 29 Jan 2016 17:44:56 +0100 Subject: Re: Adding a "release" setting in build.mk.sample (and some other build system questions) From: thomasmiedema@gmail.com To: voldermort@hotmail.com CC: ghc-devs@haskell.org

The default (and thus release) `SRC_HC_OPTS` uses `-H32m` (see mk/config.mk.inhttp://config.mk.in).

Maybe you want to run some tests to see if it makes a difference on the total build time?

I suggest also measuring without any `-H` flag, with just `-H` (see commit below), and with `-H1G` or some other large value.

``` commit 323950933d3260503186b93e7a5a7bdaa4822c1b Author: Simon Marlow mailto:marlowsd@gmail.com> Date: Mon Nov 30 15:18:36 2009 +0000

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.

For reference, there is some discussion in https://mail.haskell.org/pipermail/ghc-devs/2014-March/004344.html. Perhaps all of the -H options should be dropped from config.mk.in and the various build flavours, until it can be shown that some value is better than GHC's defaults for the overwhelming majority of builders? Even if I could show that some value works better for me, that doesn't mean it will work better for you or anyone else.