[Bug 1797335] Re: strstr() on ubuntu18.04 8 times slower than on ubuntu16

[Steve Langasek]

I notice that your cpu flags list avx2; this suggests the possibility
that the code path being used is one that's specific to avx2, not just
sse2.  I have attempted to reproduce this on CPUs both with and without
avx2 and I cannot.  I get no statistically-significant difference in
runtime, on either the avx2 system or the non-avx2 system, with glibc
from Ubuntu 16.04 vs. glibc from Ubuntu 18.04.

This could be a CPU bug or a bug in vagrant rather than in glibc;
there's currently not enough information to tell.  I think we would need
a trace that shows which implementation is being selected in glibc in
this environment first and foremost, to confirm whether it's the correct
one.

** Changed in: glibc (Ubuntu)
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/1797335

Title:
  strstr() on ubuntu18.04 8 times slower than on ubuntu16

Status in glibc package in Ubuntu:
  Incomplete

Bug description:
  strstr() from glibc is up to 8 times slower than it was on ubuntu16.
  Looks like the sse2 optimized code is not used. To reproduce compile
  attached file (strstr.c) :

  $ gcc -o strstr strstr.c

  and then run on the 2 platforms :

  $ vagrant init ubuntu/bionic64
  $ vagrant up
  $ vagrant ssh
  Welcome to Ubuntu 18.04.1 LTS (GNU/Linux 4.15.0-36-generic x86_64)

   * Documentation:  https://help.ubuntu.com
   * Management:     https://landscape.canonical.com
   * Support:        https://ubuntu.com/advantage

    System information as of Thu Oct 11 09:02:46 UTC 2018

    System load:  0.0              Processes:             97
    Usage of /:   9.8% of 9.63GB   Users logged in:       0
    Memory usage: 12%              IP address for enp0s3: 10.0.2.15
    Swap usage:   0%

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  0 packages can be updated.
  0 updates are security updates.

  Last login: Thu Oct 11 09:02:17 2018 from 10.0.2.2
  vagrant at ubuntu-bionic:~$ cd /vagrant
  vagrant at ubuntu-bionic:/vagrant$ time ./strstr aa
  matches = 3000000

  real	0m4.877s
  user	0m4.852s
  sys	0m0.012s
  vagrant at ubuntu-bionic:/vagrant$

  Same machine :

  $ vagrant init ubuntu/xenial64
  $ vagrant up
  $ vagrant ssh
  Welcome to Ubuntu 16.04.5 LTS (GNU/Linux 4.4.0-135-generic x86_64)

   * Documentation:  https://help.ubuntu.com
   * Management:     https://landscape.canonical.com
   * Support:        https://ubuntu.com/advantage

    Get cloud support with Ubuntu Advantage Cloud Guest:
      http://www.ubuntu.com/business/services/cloud

  0 packages can be updated.
  0 updates are security updates.

  New release '18.04.1 LTS' available.
  Run 'do-release-upgrade' to upgrade to it.

  Last login: Thu Oct 11 08:22:26 2018 from 10.0.2.2
  vagrant at ubuntu-xenial:~$ cd /vagrant
  vagrant at ubuntu-xenial:/vagrant$ time ./strstr aa
  matches = 3000000

  real	0m0.525s
  user	0m0.508s
  sys	0m0.000s
  vagrant at ubuntu-xenial:/vagrant$
  vagrant at ubuntu-xenial:/vagrant$ time ./strstr aa
  matches = 3000000

  real	0m0.525s
  user	0m0.508s
  sys	0m0.000s
  vagrant at ubuntu-xenial:/vagrant$

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