Re: Serious performance problems (Gerrit/Danny please comment)

[Danny Smith <dannysmith at clear dot net dot nz>]

Re: Serious performance problems (Gerrit/Danny please comment)
From: Christopher Faylor <cgf-no-personal-reply-please at cygwin dot com>
To: cygwin at cygwin dot com
Date: Sat, 28 May 2005 14:57:20 -0400
Subject: Re: Serious performance problems (Gerrit/Danny please comment)
References: <http://sources.redhat.com/ml/cygwin/2005-05/msg01305.html>
<http://sources.redhat.com/ml/cygwin/2005-05/msg01319.html>
Reply-to: cygwin at cygwin dot com

--------------------------------------------------------------------------------

> On Sat, May 28, 2005 at 01:24:31PM +0200, Vaclav Haisman wrote:
> >Somebody mentioned that malloc implementation could be the problem. Dunno. I
> >has also crossed my mind that another difference between FreeBSD and Cygwin
is
> >implementation of C++ exceptions. Maybe the SJLJ implementation that Cygwin
> >AFAIK uses has too big overhead.
>
> To test this theory, I just tried replacing Cygwin's "Unwind" functions
> with those from mingw and saw a noticeable speed up in the execution of
> this program.  I did this by extracting the contents of mingw's libgcc
> to a directory and then including unwind-c.o and unwind-sjlj.o on the
> command line when linking the test case.  I had to modify the test case
> by adding these two lines to the bottom:
>
>   int __mingwthr_key_dtor;
>   int _CRT_MT;
>
> to avoid undefined symbol errors so this is obviously not intended as a
> complete solution.
>
> On doing this, the program went from taking 25 seconds to execute to
> taking 7 seconds to execute.  That's still 4x slower than mingw but it
> is, nonetheless, a noticeable difference.
>
> Gerrit and Danny do you know what the difference between the mingw and
> cygwin implementations of these functions might be?
>

I too suspect sjlj exceptions to be the problem. This has already been
reported on GCC lists: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14563

The sjlj overhead affects both cygwun and mingw, but cygwin has the
additional overhead of posix threads to ensure thread-safe allocation
and destruction of structures used by exception handling, while mingw
uses win32api directly
(In CRT_MT == 0 case, it doesn't even bother cleaning up)

Enabling Dwarf2 exceptions helps a lot, since it eliminates the need for
the code for these object in f function prologue.


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