Re: [lmi] Compiling takes longer with gcc-4.9.2

[Greg Chicares]

On 2016-01-17 18:36, Vadim Zeitlin wrote:
> On Sun, 17 Jan 2016 17:51:45 +0000 Greg Chicares <address@hidden> wrote:
> 
> GC> On 2015-12-22 00:28, Greg Chicares wrote:
> GC> > My local tree contains makefile changes to use MinGW-w64 gcc-4.9.2,
> GC> > with '-std=c++11', some new warning flags, and various other
> GC> > adjustments.
> GC> 
> GC> I'm almost ready to commit those changes. First, however, I tried them
> GC> out on different hardware, and found a PCH surprise. Timings quoted
> GC> from the 2015-12-22 original used old hardware; new timings reported
> GC> today use new hardware unless explicitly indicated otherwise.
> GC> 
> GC>   old: 2 x E5520,      3 Gbps sata ii,  wd black WD2003FZEX, debian-7.7
> GC>   new: 2 x #5-2530 v3, 6 Gpbs sata iii, samsung 850 pro 1TB, debian-7.9
>                ^^^^^^^
>  This is probably E5-2630.

Yes. I used to be able to touch-type numbers. Letters, too.

> GC> I expect the new hardware to perform uniformly better than the old.
> 
>  I would obviously expect it as well... Something seems to be really wrong
> here, especially with
> 
> GC> > make --jobs=8 -f install_wx.make > ../log 2>&1
> GC> > 2662.23s user 1594.59s system 475% cpu 14:55.38 total
> GC> 
> GC> make --jobs=8 -f install_wx.make > ../log 2>&1
> GC> 2727.15s user 6098.59s system 214% cpu 1:08:39.33 total
> 
> this, of course. It takes twice less of the CPU time yet 4 times the wall
> time? I just can't fathom this.

I was using
  --host=i686-pc-mingw32
where this
  --host=i686-w64-mingw32
is wanted. I'll have to retest now. I wonder why that even worked, or
seemed to.

> GC> The bottleneck might even be the preprocessor.
> 
>  So the IO layer then? It looks plausible but this would just mean that KVM
> doesn't do its job well.

'virtio' drivers are provided for most popular OS's. But there is none
for msw-xp; or, properly speaking, IIRC, one exists, but it does nothing,
and its author recommends against using it.

> I know you don't want to use it but I'm really
> curious to know how faster would the compilation inside the same VM be
> inside VMware.

Is installing it as simple as 'apt-get [whatever]'? Would installing it
pull in lots of changes I don't want? Is it provided by debian, in the
"free" repository--or does it require "nonfree", or, worse, an external
repository? I could consider this now that I have two machines that can
run VMs--it's not a catastrophe if I destabilize the backup.

> GC> New hardware makes a small difference for gcc-4.9.2, though it seemed
> GC> to make no difference for gcc-3.4.5 .
> 
>  I'd like to see your numbers for cross-compiling. Normally they should be
> significantly better than mine (and so an order of magnitude better than
> what you have now), if this is not the case neither I'd seriously start
> suspecting some hardware problem in the new machine.

Cross-compiling from GNU/Linux: all in due time. First I need to get
the i686-pc-cygwin + i686-w64-mingw32 combination working, so that it
can be tested in the office.