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Date: Fri, 26 May 2017 20:59:50 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: towlower performance
On Fri, May 26, 2017 at 05:39:31PM -0700, Andre McCurdy wrote:
> On Thu, May 25, 2017 at 11:01 AM, maksis .
> <maksis@...enaline-network.com> wrote:
> > Hi,
> >
> > After switching my C++ app from a glibc-based build to a static build using
> > musl, I've noticed significant differences in performance with certain
> > loading operations, which seems to be related to using std::map with a
> > case-insensitive UTF-8 sorting algorithm. For example, parsing a XML file
> > with a flat listing of ~350k directory items and mapping them by name takes
> > about 3 seconds with glibc vs 13 seconds with musl. After modifying the sort
> > algorithm by removing all calls to towlower, the loading time with musl
> > drops to 3.5 seconds.
> >
> > I put together a C++ benchmark with the same sorting algorithm, which seems
> > to produce similar results (GCC 6.3, 64 bit, -O3):
> >
> > https://gist.github.com/maksis/92ad04f525d69043283350675d04f160
> >
> > glibc: ~2 seconds (Arch Linux)
> >
> > musl: ~7 seconds (Alpine Linux 3.6.0)
> >
> > What might be causing the difference?
>
> I'm not sure if it maps directly to your results, but when building a
> gcc based musl toolchain, libstdc++ gets configured to use the
> "generic" OS specific include dir, which seems to contain some less
> than optimal ctype code. e.g. ctype_inline.h comes with the following
> warning:
>
> // The following definitions are portable, but insanely slow. If one
> // cares at all about performance, then specialized ctype
> // functionality should be added for the native os in question: see
> // the config/os/bits/ctype_*.h files.
>
> https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=libstdc%2B%2B-v3/config/os/generic/ctype_inline.h
That's a bit of an overstatement. While it's possible for some
operations to be slightly to moderately faster with a
pokes-at-libc-internals implementation of c++ locale/ctype
functionality, the portable code is not "insanely" slow. For wchar
functionality I don't think it's even slower at all; as far as I know
glibc only provides function-bypass tables for the 8bit char is*
functions, and for the past couple decades they haven't even fully
bypassed function calls since a function call is used to obtain the
thread-local locale tables.
In any case the portable-interface-based implementation (generic) is
the correct thing to do. musl does not and won't provide backdoors to
bypass that.
Thankfully, though, I think the issue maksis reported is more a matter
of the towlower/towupper implementation being oriented towards size
(we have all the Unicode case mappings plus code to perform them in
about 1k!) rather than performance. A simple fix that would make this
case fast (maybe faster than glibc even) is short-circuiting the
common ascii case, but a more multilingual-friendly solution might be
using some sort of O(1) first-stage table to find the relevant parts
of the mapping tables instead of using linear searches and multiple
special-case branches up front. I suspect we can make the code very
fast while keeping the size under 3k and probably less.
Rich
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