Where do I start if I would like help improve GHC compilation times?

Alfredo Di Napoli alfredo.dinapoli at gmail.com
Tue Apr 18 10:17:29 UTC 2017

Hey Simon,

thanks for chiming in. I had the same suspect as well, and in fact my first
temptation was to use dlists to avoid costly concatenation (I guess a
Builder shares pretty much the same idea, which is avoid right-appends). I
eventually bailed out as that Pretty module had some functions like sepX or
fillNBE (I might be wrong, going by memory only here) which needed to
*inspect* the current [Doc] we were carrying around, and thus dlists
couldn’t accomplish this in O(1), but forcing it back to a list was
necessary. Am I right in thinking that using a Builder here will suffer the
same malady? Ideally I would like constant time for both appends, left
concat & inspection (as in pattern-matching inspection) but I don’t think
what I’m asking exists, at least not in its functional declination anyway ;)

That’s why I was thinking to give Deques (or more generally, Sequences) a
go: they don’t have O(1) appends but at least can be inspected in O(1).
Question has to be answered whether this will yield improvement or not,
which I guess depends upon the ratio of inspection / appending we do in the
pretty printing. In that case using dlists or builders might be better.
Last but not least, although the current implementation doesn’t backtrack
anyway, I’m wondering wether switching to a different representation for a
Doc (namely a huge function composition of Token(s), as described in the
paper) could be beneficial as well.

Do you guys have any opinions? Yesterday I extracted Pretty.hs from the
sourcetree and I’m now planning to produce a criterion benchmark and
compare different implementations, althought it’s a bit hard to predict the
real world usage as I don’t have access to a representative Doc document as
produced by GHC, so my benchs could be all ill-founded.


On 18 April 2017 at 12:01, Simon Marlow <marlowsd at gmail.com> wrote:

> Pretty-printing the asm is a likely contender for optimisation, however
> the problem is not the pretty-printing per se.  We don't actually use any
> of the backtracking stuff when printing asm, since there's no point nicely
> indenting things or wrapping lines.  The overhead is probably all in the
> layer of data structure that we generate in Pretty before it gets dumped
> into raw bytes.  Using a ByteString Builder instead might yield some
> improvement.
> Cheers
> Simon
> On 17 April 2017 at 18:44, Alfredo Di Napoli <alfredo.dinapoli at gmail.com>
> wrote:
>> Dear all,
>> after sprinkling (ehm, littering) GHC source code with cost centres, I
>> was not surprised to see that roughly 20% of the compilation time (as in
>> .prof) was spent in the core gen/simplification process (10% of the total
>> time) and on the asm code gen (another 10%).
>> I have almost immediately abandoned the idea of try optimising some
>> modules in simplCore (considering my 0-knowledge of GHC internals anyway..)
>> but I have been dwelling on the following: Outputable.hs and Pretty.hs
>> seems to be have been implemented making deliberate use of lists and
>> concatenations between them, which left me wondering if there was room for
>> optimisation there. I have found this interesting paper on the topic:
>> https://www.cs.kent.ac.uk/pubs/2005/2062/content.pdf
>> Now, it’s totally possible that this has been already tried (with no
>> success) but judging from the original copyright of Pretty.hs (dated 2001),
>> it seems it was written prior to the work of Olaf Chitil (the author of the
>> paper).
>> TL;DR I was thinking (even just as a fun exercise to learn more about GHC
>> internals) to leverage the ideas of that paper and switch to a different
>> implementation for `Doc` coupled with the use of lazy dequeues, which
>> *might* increase the performances of the codegen and thus of the compiler
>> overall. Am I fighting a strawman (or flogging a dead horse, pick your
>> rethorical figure :D ) or is there even a tiny chance of this being
>> actually useful?
>> Have a nice evening,
>> Alfredo
>> On 11 April 2017 at 00:47, Ben Gamari <ben at smart-cactus.org> wrote:
>>> Alfredo Di Napoli <alfredo.dinapoli at gmail.com> writes:
>>> > Hey Ben,
>>> >
>>> Hi Alfredo,
>>> Sorry for the late response! The email queue from the weekend was a bit
>>> longer than I would like.
>>> > as promised I’m back to you with something more articulated and
>>> hopefully
>>> > meaningful. I do hear you perfectly — probably trying to dive
>>> head-first
>>> > into this without at least a rough understanding of the performance
>>> > hotspots or the GHC overall architecture is going to do me more harm
>>> than
>>> > good (I get the overall picture and I’m aware of the different stages
>>> of
>>> > the GHC compilation pipeline, but it’s far from saying I’m proficient
>>> with
>>> > the architecture as whole). I have also read a couple of years ago the
>>> GHC
>>> > chapter on the “Architeture of Open Source Applications” book, but I
>>> don’t
>>> > know how much that is still relevant. If it is, I guess I should
>>> refresh my
>>> > memory.
>>> >
>>> It sounds like you have done a good amount of reading. That's great.
>>> Perhaps skimming the AOSA chapter again wouldn't hurt, but otherwise
>>> it's likely worthwhile diving in.
>>> > I’m currently trying to move on 2 fronts — please advice if I’m a fool
>>> > flogging a dead horse or if I have any hope of getting anything done ;)
>>> >
>>> > 1. I’m trying to treat indeed the compiler as a black block (as you
>>> > adviced) trying to build a sufficiently large program where GHC is not
>>> “as
>>> > fast as I would like” (I know that’s a very lame definition of “slow”,
>>> > hehe). In particular, I have built the stage2 compiler with the “prof”
>>> > flavour as you suggested, and I have chosen 2 examples as a reference
>>> > “benchmark” for performance; DynFlags.hs (which seems to have been
>>> > mentioned multiple times as a GHC perf killer) and the
>>> highlighting-kate
>>> > package as posted here: https://ghc.haskell.org/trac/ghc/ticket/9221 .
>>> Indeed, #9221 would be a very interesting ticket to look at. The
>>> highlighting-kate package is interesting in the context of that ticket
>>> as it has a very large amount of parallelism available.
>>> If you do want to look at #9221, note that the cost centre profiler may
>>> not provide the whole story. In particular, it has been speculated that
>>> the scaling issues may be due to either,
>>>  * threads hitting a blackhole, resulting in blocking
>>>  * the usual scaling limitations of GHC's stop-the-world GC
>>> The eventlog may be quite useful for characterising these.
>>> > The idea would be to compile those with -v +RTS -p -hc -RTS enabled,
>>> > look at the output from the .prof file AND the `-v` flag, find any
>>> > hotspot, try to change something, recompile, observe diff, rinse and
>>> > repeat. Do you think I have any hope of making progress this way? In
>>> > particular, I think compiling DynFlags.hs is a bit of a dead-end; I
>>> > whipped up this buggy script which
>>> > escalated into a Behemoth which is compiling pretty much half of the
>>> > compiler once again :D
>>> >
>>> > ```
>>> > #!/usr/bin/env bash
>>> >
>>> > ../ghc/inplace/bin/ghc-stage2 --make -j8 -v +RTS -A256M -qb0 -p -h \
>>> > -RTS -DSTAGE=2 -I../ghc/includes -I../ghc/compiler
>>> -I../ghc/compiler/stage2
>>> > \
>>> > -I../ghc/compiler/stage2/build \
>>> > -i../ghc/compiler/utils:../ghc/compiler/types:../ghc/compile
>>> r/typecheck:../ghc/compiler/basicTypes
>>> > \
>>> > -i../ghc/compiler/main:../ghc/compiler/profiling:../ghc/comp
>>> iler/coreSyn:../ghc/compiler/iface:../ghc/compiler/prelude
>>> > \
>>> > -i../ghc/compiler/stage2/build:../ghc/compiler/simplStg:../g
>>> hc/compiler/cmm:../ghc/compiler/parser:../ghc/compiler/hsSyn
>>> > \
>>> > -i../ghc/compiler/ghci:../ghc/compiler/deSugar:../ghc/compil
>>> er/simplCore:../ghc/compile/specialise
>>> > \
>>> > -fforce-recomp -c $@
>>> > ```
>>> >
>>> > I’m running it with `./dynflags.sh ../ghc/compiler/main/DynFlags.hs`
>>> but
>>> > it’s taking a lot to compile (20+ mins on my 2014 mac Pro) because it’s
>>> > pulling in half of the compiler anyway :D I tried to reuse the .hi
>>> files
>>> > from my stage2 compilation but I failed (GHC was complaining about
>>> > interface file mismatch). Short story short, I don’t think it will be a
>>> > very agile way to proceed. Am I right? Do you have any recommendation
>>> in
>>> > such sense? Do I have any hope to compile DynFlags.hs in a way which
>>> would
>>> > make this perf investigation feasible?
>>> >
>>> What I usually do in this case is just take the relevant `ghc` command
>>> line directly from the `make` output and execute it manually. I would
>>> imagine your debug cycle would look something like,
>>>  * instrument the compiler
>>>  * build stage1
>>>  * use stage2 to build DynFlags using the stage1 compiler (using a saved
>>> command line)
>>>  * think
>>>  * repeat
>>> This should only take a few minutes per iteration.
>>> > The second example (the highlighting-kate package) seems much more
>>> > promising. It takes maybe 1-2 mins on my machine, which is enough to
>>> take a
>>> > look at the perf output. Do you think I should follow this second
>>> lead? In
>>> > principle any 50+ modules package I think would do (better if with a
>>> lot of
>>> > TH ;) ) but this seems like a low-entry barrier start.
>>> >
>>> > 2. The second path I’m exploring is simply to take a less holistic
>>> approach
>>> > and try to dive in into a performance ticket like the ones listed here:
>>> > https://www.reddit.com/r/haskell/comments/45q90s/is_anything
>>> _being_done_to_remedy_the_soul/czzq6an/
>>> > Maybe some are very specific, but it seems like fixing small things and
>>> > move forward could help giving me understanding of different sub-parts
>>> of
>>> > GHC, which seems less intimidating than the black-box approach.
>>> >
>>> Do you have any specific tickets from these lists that you found
>>> interesting?
>>> > In conclusion, what do you think is the best approach, 1 or 2, both or
>>> > none? ;)
>>> I would say that it largely depends upon what you feel most comfortable
>>> with. If you feel up for it, I think #9221 would be a nice, fairly
>>> self-contained, yet high-impact ticket which would be worth spending a
>>> few days diving further into.
>>> Cheers,
>>> - Ben
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