Date: Sat, 19 Apr 2008 08:55:19 -0700 From: Steve Kargl <sgk@troutmask.apl.washington.edu> To: Jeremie Le Hen <jeremie@le-hen.org> Cc: freebsd-arch@freebsd.org Subject: Re: Integration of ProPolice in FreeBSD Message-ID: <20080419155519.GA55562@troutmask.apl.washington.edu> In-Reply-To: <20080419074638.GH4840@obiwan.tataz.chchile.org> References: <20080418132749.GB4840@obiwan.tataz.chchile.org> <200804181945.59189.max@love2party.net> <20080418204738.GE4840@obiwan.tataz.chchile.org> <20080419001555.GA50009@troutmask.apl.washington.edu> <20080419074638.GH4840@obiwan.tataz.chchile.org>
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On Sat, Apr 19, 2008 at 09:46:38AM +0200, Jeremie Le Hen wrote:
> Hi Steve,
>
> On Fri, Apr 18, 2008 at 05:15:55PM -0700, Steve Kargl wrote:
> > On Fri, Apr 18, 2008 at 10:47:38PM +0200, Jeremie Le Hen wrote:
> > >
> > > Certainly. I would like to hear opinion from other committers if SSP
> > > should be enabled by default.
> >
> > I'm not a committer, but I'll ask a question anyway.
> >
> > Can you quantify the performance impact, in particular for
> > numerically intensive codes with heavy use of libm?
>
> I don't run such application, so I can't answer. Sorry. If you are
> willing to give a try, I would be pleased to help you to run your tests,
> or even run them on my side.
>
> BTW for the sake of my curiosity, is there a technical reason for
> ProPolice to be heavier for libm?
>
Most numerical applications, that I'm familiar with, tend
to contain nested loops that make calls to functions in
libm. Simple example in one of my codes is a 3 deep loop
that computes what is known as the thermal dose.
for (k = 0; k < kmax; k++)
for (j = 0; j < jmax; j++)
for (i = 0; i < imax; i++)
td += exp(a * b[k][j][i])
Now, put the above loops inside a time loop with n time steps.
exp() will be called kmax*jmax*imax*n times where this product
can be quite large (order of 5e11). Any overhead caused by PP
will increase the simulation time. A 1% increase in time is
probably tolerable, but a 10% increase would be detrimental to
simulations that takes days to complete (yes, I have a few that
run that long).
I'll see if I can get you some numbers this weekend.
--
Steve
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