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

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Date:	Fri, 29 Jul 2011 09:26:19 -0400
From:	Andrew Lutomirski <luto@....edu>
To:	Dave Chinner <david@...morbit.com>
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 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?

[1] http://sourceware.org/bugzilla/show_bug.cgi?id=12813

--Andy
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