linux-kernel - Re: [3.0-rc0 Regression]: legacy vsyscall emulation increases user CPU time by 20%

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Date:	Sun, 31 Jul 2011 21:01:19 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Andrew Lutomirski <luto@....edu>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu
Subject: Re: [3.0-rc0 Regression]: legacy vsyscall emulation increases user
 CPU time by 20%

On Fri, Jul 29, 2011 at 09:26:19AM -0400, Andrew Lutomirski wrote:
> On Fri, Jul 29, 2011 at 8:17 AM, Andrew Lutomirski <luto@....edu> wrote:
> > On Fri, Jul 29, 2011 at 3:24 AM, Dave Chinner <david@...morbit.com> wrote:
> >> On Thu, Jul 28, 2011 at 11:30:49PM -0400, Andrew Lutomirski wrote:
> >>> >
> >>> > Assuming this is the problem, can this be fixed without requiring
> >>> > the whole world having to wait for the current glibc dev tree to
> >>> > filter down into distro repositories?
> >>>
> >>> How old is your glibc?  gettimeofday has used the vdso since:
> >>
> >> It's 2.11 on the test machine, whatever that translates to. I
> >> haven't really changed the base userspace for about 12 months
> >> because if I do I invalidate all my historical benchmark results
> >> that I use for comparisons.
> >
> > 2.11 is from 2009 and appears to contain that commit.  Does your
> > workload call time() very frequently?  That's the largest slowdown.
> > With the old code, time() took 4-5 ns and with the new code time() is
> > about as slow as gettimeofday().  I suggested having a config option
> > to allow time() to stay fast until glibc 2.14 became widespread, but a
> > few other people disagreed.
> 
> *sigh*
> 
> fs_mark: fs_mark.o lib_timing.o
>         ${CC} -static -o fs_mark fs_mark.o lib_timing.o
> 
> Even brand-new glibc still issues vsyscalls when statically linked,
> and Ulrich has said [1] that he doesn't care that much about
> performance of statically linked code.
> 
> How bad would it be to just remove the -static from the makefile?

Results in 270s +-5s user CPU time, so user CPU time is still ~10%
up on 3.0 numbers.  IOWs, a non-static link roughly halves the
regression but doesn't get rid of it.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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