[poppler] [RFC] Replace GooHash by std::unordered_map

Albert Astals Cid aacid at kde.org
Sat Mar 3 19:25:32 UTC 2018


El dimecres, 21 de febrer de 2018, a les 7:13:57 CET, Adam Reichold va 
escriure:
> Hello again,
> 
> Am 21.02.2018 um 00:31 schrieb Albert Astals Cid:
> > El dimarts, 20 de febrer de 2018, a les 8:58:24 CET, Adam Reichold va
> > 
> > escriure:
> >> Hello again,
> >> 
> >> Am 18.02.2018 um 23:23 schrieb Adam Reichold:
> >>> Am 18.02.2018 um 23:08 schrieb Albert Astals Cid:
> >>>> El diumenge, 18 de febrer de 2018, a les 16:55:37 CET, Adam Reichold va
> >>>> 
> >>>> escriure:
> >>>>> Hello,
> >>>>> 
> >>>>> the attached patch replaced Poppler's internal hash table
> >>>>> implementation
> >>>>> GooHash by std::unordered_map found in the C++ standard library since
> >>>>> C++11. This continues Poppler's slow drift towards standard library
> >>>>> containers and removes one of the home-grown data structures with the
> >>>>> main goals of reducing code size and improving the long term
> >>>>> maintainability of the code base.
> >>>> 
> >>>> Do you have any benchmarks on "rendering" speed and code size?
> >>> 
> >>> Sorry, not at hand. I will try to prepare them during the week.
> >> 
> >> I did run Splash rendering benchmarks of this branch against master with
> >> the result of rendering the circa 2400 documents of the TeXLive
> > 
> >> documentation present on my machine being:
> > I'm wondering if those 2400 documents are diverse enough, which they may
> > not be given they are all coming from "the same place".
> 
> They seem pretty diverse w.r.t. content, some being text heavy and some
> graphics rich. But I guess they are definitely not diverse technically
> as all are prepared using TeX itself.
> 
> The main problem on my side is that I have failed to find my DVD copy of
> the Poppler regtest document collection until now. :-\ In any case, I am
> reasonably confident on the zero sum result since GooHash does not seem
> to live in any performance critical code path.
> 
> >> Cumulative run time:
> >>         Result: 90.95 min ∓ 1.1 %
> >>         Reference: 91.57 min ∓ 1.2 %
> >>         Deviation: -0.0 %
> >> 
> >> Cumulative memory usage:
> >>         Result: 37.2 MB ∓ 0.7 %
> >>         Reference: 37.0 MB ∓ 0.7 %
> >>         Deviation: +0.0 %
> >> 
> >> (Where result is this patch and the reference is master.) (The
> >> measurement was taken using the perftest script which I proposed here
> >> some time ago for which I'll attach the patch again for
> >> reproduceability.)
> > 
> > Did you really attach this before? i really don't remember it :D
> 
> This was a very long-winded thread ending on 2016-01-01 centered around
> the regtest framework. It went from custom driver using QTest's
> benchmark functionality to custom backend in the regtest framework to
> separate Python script. The script still has problems to reliably
> measure really short things like running "pdftotext" for which a custom
> C++ driver might be needed, but for long-running stuff like "pdftoppm",
> the results seem reasonable.
> 
> > Anyhow it seems the change is mostly a zero-sum runtime wise.
> 
> Indeed. (Although thinking really really long term, I think a Poppler
> that is using std::vector, std::string and friends everywhere, could
> loose a lot of distributed fat in the form of a lot of memory
> indirections and benefit from the highly optimized standard library. But
> it just is not a single patch thing to reap any of these benefits.)
> 
> > Which bring us to the code side. First i know it sucks but your spacing is
> > broken, please check the whole patch
> > 
> > -	nameToGID->removeInt(macGlyphNames[j]);
> > -	nameToGID->add(new GooString(macGlyphNames[j]), i);
> > +          nameToGID[macGlyphNames[j]] = i;
> > 
> > i could fix it, but it's better (for me) if you do :D
> 
> Definitely for me to fix. The main problem is that the spacing in those
> files was already broken and I am unsure whether I should fix the
> surrounding spacing even if I am not touching it otherwise. Due to that,
> there sometimes is no correct way in the current patch. If you do not
> say otherwise, I will try to make at least the touched blocks of code
> consistent.

Are you sure the spacing is broken? I'd say it's just xpdf weird spacing rules 
of using 2 soft spaces and then hard tabs at 8.

> 
> > Now onto the code,
> > 
> >   const auto emplaceRangeMap = [&](const char* encodingName, GBool
> >   unicodeOut,> 
> > UnicodeMapRange* ranges, int len) {
> > 
> >     residentUnicodeMaps.emplace(
> >     
> >       std::piecewise_construct,
> >       std::forward_as_tuple(encodingName),
> >       std::forward_as_tuple(encodingName, unicodeOut, ranges, len)
> >     
> >     );
> >   
> >   };
> > 
> > It's the first time in my life i see std::piecewise_construct and
> > std::forward_as_tuple, yes that probably makes me a bad C++ developer, but
> > given there's lots of us around, it doesn't make me happy now i don't
> > understand what the code does.
> 
> The problem is that most internal Poppler types lack proper construction
> and assignment operators, hence I need to work harder to construct those
> UnicodeMap instances in-place inside the map (GooHash just stored a
> pointer to it, so there was no problem.)
> 
> The whole piecewise_construct and forward_as_tuple dance just ensures,
> that those parameters to emplace are properly grouped before being
> unpacked to become the parameters of std::string and UnicodeMap
> construction again. If UnicodeMap was movable, this would probably look like
> 
> residentUnicodeMaps.emplace(
>   encodingName,
>   UnicodeMap{encodingName, unicodeOut, ranges, len}
> );
> 
> If you like, I can try to make Unicode a move-only type and simplify the
> mentioned helper functions?

you can give it a try, not sure how easy that's going to be.

> 
> > Then there's the benefit/risk ratio. The code using GooHash is mostly
> > rocksolid and we haven't probably touched any line in it for a long time
> > and we have probably neither written new code that uses GooHash.
> > 
> > In that regard we risk breaking working code.
> > 
> > The benefit is not more speed nor less memory usage as your measurements
> > show.
> > 
> > Mostly the benefit is "removing code from maintainership", which i agree
> > is a good thing, but as mentioned before it's some code "we mostly
> > ignore", so it's not that much of a good thing.
> 
> I very much agree with the risk assessment.
> 
> But I also think the code will ossify (or maybe already is?) due to
> those custom data structures and the less than idiomatic C++ usage.
> Hence I think, Poppler would not just loose code, but the remaining code
> should become easier to maintain. (Of course, the piecewise_construct
> fiasco shows that this has intermediate costs. But again, I think this
> is just an incentive to provide types with the usual C++ semantics which
> should make all code using those types better.)

Yeah, i guess you're right.

Cheers,
  Albert

> 
> Best regards, Adam.
> 
> > So i'm hesitant of what to do. It really sounds like a nice thing to not
> > have custom structures, but on the other hand it's not like they get much
> > in the way of coding.
> > 
> > I'd really appreciate other people's comments on this.
> > 
> > Cheers,
> > 
> >   Albert
> >> 
> >> I'll also attach the detailed comparison, but the gist seems to be that
> >> if there are significant changes, the run time is reduced but the memory
> >> usage is increased in the majority of cases. But this does not seem to
> >> show up in the cumulative results.
> >> 
> >> Best regards, Adam.
> >> 
> >> P.S.: One could try to improve the memory usage by tuning the load
> >> factor or calling shrink_to_fit where appropriate. Would you like me to
> >> try to do this?
> >> 
> >> P.P.S.: One obvious area for improvement would be better
> >> interoperability between GooString and std::string, i.e. not converting
> >> them as C strings so that the length information does not need to be
> >> recomputed. I will try to prepare this as a separate patch on top of
> >> this one or should I include this here?
> >> 
> >> Best regards, Adam.
> >> 
> >>> Concerning code size, a release build of libpoppler.so goes from
> >>> 
> >>>    text    data     bss     dec     hex filename
> >>> 
> >>> 2464034  288852     360 2753246  2a02de libpoppler.so.73.0.0
> >>> 
> >>> for the current master to
> >>> 
> >>>    text    data     bss     dec     hex filename
> >>> 
> >>> 2482129  288756     360 2771245  2a492d libpoppler.so.73.0.0
> >>> 
> >>> with the patch applied, i.e. a 0.65% increase in binary size.
> >>> 
> >>> 
> >>> Please note that in my original message, I was referring only to source
> >>> code size, i.e.
> >>> 
> >>> git diff --stat master...remove-goo-hash
> >>> 
> >>>  18 files changed, 168 insertions(+), 803 deletions(-)
> >>>  
> >>>> Cheers,
> >>>> 
> >>>>   Albert
> >>> 
> >>> Best regards, Adam.
> >>> 
> >>>>> Best regards, Adam.
> >>>> 
> >>>> _______________________________________________
> >>>> poppler mailing list
> >>>> poppler at lists.freedesktop.org
> >>>> https://lists.freedesktop.org/mailman/listinfo/poppler
> >>> 
> >>> _______________________________________________
> >>> poppler mailing list
> >>> poppler at lists.freedesktop.org
> >>> https://lists.freedesktop.org/mailman/listinfo/poppler
> > 
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