8.2.1-rc2 upgrade report

[Simon Peyton Jones]

Thanks for the report.

Going from 67G to 56G allocation is a very worthwhile improvement in runtime!  Hurrah.

However, trebling compile time is very bad.  It is (I think) far from typical: generally 8.2 is faster at compiling than 8.0 so you must be hitting something weird.  Anything you can do to make a reproducible case would be helpful.  -dshow-passes shows the size of each intermediate form, which at least sometimes shows where the big changes are.

Simon

From: Glasgow-haskell-users [mailto:glasgow-haskell-users-bounces at haskell.org] On Behalf Of Alberto Valverde
Sent: 06 June 2017 12:39
To: GHC users <glasgow-haskell-users at haskell.org>
Subject: 8.2.1-rc2 upgrade report

Hi,

I've finally managed to upgrade all the dependencies of the proprietary app I mentioned some days ago in this list and there are good and bad differences I've noticed between 8.0.2 that I'd like to share.

The bad
-----------

* An optimized cold build (-O2)  is about 3 times slower (~53s vs. ~2m55s) and consumes more memory (~2Gb vs. ~7Gb) at it's peak.

The good
-------------

* An un-optimized cold build (-O0) takes about the same time (~21s, phew! :) It's maybe even slightly faster with 8.2 (too few and badly taken measurements to really know, though)
* The optimized executable is slightly faster and allocates less memory. For this app it makes up for the performance regression of the optimized build (which is almost always done by CI), IMHO.

I did only a couple of runs and only wrote down [1] the last run results (which were similar to the previous results) so take these observations with a grain of salt (except maybe the optimized build slowdown, which doesn't have much margin for variance to be skewing the results). I also measured the peak memory usage by observing "top".

In case gives a clue: The app is a multi-threaded 2D spread simulator which deals with many mmapped Storable mutable vectors and has been pretty optimized for countless hours (I mean by this that it has (too) many INLINE pragmas. Mostly on polymorphic functions to aid in their specialization). I think some of this information can be deduced from the results I'm linking at the footer. I believe the INLINEs are playing a big part of the slowdown since the slowest modules to compile are the "Main" ones which put everything together, along with the typical lens-th-heavy "Types" ones.

I'd like to help by producing a reproducible and isolated benchmark or a better analysis or ... so someone more knowledgeable than me on GHC internals can someday hopefully attack the regression. Any pointers on what would help and where can I learn to do it?

Thanks!


[1] https://gist.github.com/albertov/46fbb13d940f67a569f9a25c1cb8154c<https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgist.github.com%2Falbertov%2F46fbb13d940f67a569f9a25c1cb8154c&data=02%7C01%7Csimonpj%40microsoft.com%7Cce8170b01de84bacdab308d4acd0aef3%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636323459695695689&sdata=iRWChz2fzNFuXBAhiYABO5OepSXYBLxvvIQ8CAiABYg%3D&reserved=0>
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