Re: towlower performance

[Andre McCurdy <armccurdy@gmail.com>]

On Thu, May 25, 2017 at 11:01 AM, maksis .
<maksis@adrenaline-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