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

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Date:	Mon, 1 Aug 2011 08:29:47 -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 Sun, Jul 31, 2011 at 7:01 AM, Dave Chinner <david@...morbit.com> wrote:
> 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.

Are you sure?  I stuck a trace event in do_emulate_vsyscall and it's
not getting hit at all in fs_mark, at least on my system.  I'll send
out the patch tomorrow.

--Andy
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