Serious performance problems (Gerrit/Danny please comment)

[Gerrit P. Haase]

Danny Smith wrote:

> 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)

As I thought before, the only difference is buried in w32-shared-ptr.c:

$ g++ -o cygspd-mingw cygspd-mingw.o

$ time ./cygspd-mingw cygspd.dat

real    0m51.071s
user    0m50.108s
sys     0m0.046s

cgf, your box must be really fast if this lasts half the time than it
lasts for me and I have already a really fast box;)

$ g++ -o cygspd-mingw cygspd-mingw.o w32-shared-ptr.o

$ time ./cygspd-mingw cygspd.dat

real    0m8.049s
user    0m7.015s
sys     0m0.093s


Is this call in w32-shared-ptr.c really needed here?

/* recreate atom after fork */
pthread_atfork (NULL,NULL,__w32_sharedptr_fixup_after_fork);


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

Sigh!
Gerrit
-- 
=^..^=

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