bug#60807: closed ([PATCH 1/2] mtime: use Time::HiRes::stat when available for subsecond resolution)

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Your message dated Fri, 3 Feb 2023 22:13:37 -0800
with message-id 
<CA+8g5KH7qx3Ze57Y6_bymNiMEo_8SKQVdbnswg3kn0h2Q0z+HA@mail.gmail.com>
and subject line Re: [bug#60807] [PATCH 1/2] mtime: use Time::HiRes::stat when 
available for subsecond resolution
has caused the debbugs.gnu.org bug report #60807,
regarding [PATCH 1/2] mtime: use Time::HiRes::stat when available for subsecond 
resolution
to be marked as done.

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-- 
60807: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=60807
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> --- Begin Message ---
>
>
>
> Subject: 
>
> [PATCH 1/2] mtime: use Time::HiRes::stat when available for subsecond resolution
>
>
>
>
> Date: 
>
> Sat, 14 Jan 2023 03:02:14 -0500
>
>
>
>
> Perl's builtin stat function returns timestamps that have 1 second
> resolution.  This can lead automake needlessly regenerating files
> because it compares timestamps as "older than or equal to" rather
> than only "older than".  This is perfectly reasonable as we have
> no way of knowing what file is older if they have the same value
> which means we must be pessimistic & assume an upate is required.
>
> However, given modern systems that are quite fast and can easily
> generate many files in less than an second, we end up doing a lot
> of extra work.
>
> Until Perl gets around to figuring out how to support subsecond
> timestamp resolution, optionally import the Time::HiRes module and
> use its stat function instead.  If it's available, great, if not,
> then we're no worse off than we are today.
>
> Performance-wise, at least by using the testsuite, there doesn't
> seem to be any measurable difference.
>
> * lib/Automake/FileUtils.pm: Use Time::HiRes to lookup mtimes on
> files if available.
> ---
>  lib/Automake/FileUtils.pm | 21 +++++++++++++++++----
>  1 file changed, 17 insertions(+), 4 deletions(-)
>
> diff --git a/lib/Automake/FileUtils.pm b/lib/Automake/FileUtils.pm
> index 848ff22d1761..78e0942e9f53 100644
> --- a/lib/Automake/FileUtils.pm
> +++ b/lib/Automake/FileUtils.pm
> @@ -42,6 +42,11 @@ use Exporter;
>  use File::stat;
>  use IO::File;
>  
> +# Perl's builtin stat does not provide sub-second resolution.  Use Time::HiRes
> +# if it's available instead.  Hopefully one day perl will update.
> +# https://github.com/Perl/perl5/issues/17900
> +my $have_time_hires = eval { require Time::HiRes; };
> +
>  use Automake::Channels;
>  use Automake::ChannelDefs;
>  
> @@ -115,10 +120,18 @@ sub mtime ($)
>    return 0
>      if $file eq '-' || ! -f $file;
>  
> -  my $stat = stat ($file)
> -    or fatal "cannot stat $file: $!";
> -
> -  return $stat->mtime;
> +  if ($have_time_hires)
> +    {
> +      my @stat = Time::HiRes::stat ($file)
> +       or fatal "cannot stat $file: $!";
> +      return $stat[9];
> +    }
> +  else
> +    {
> +      my $stat = stat ($file)
> +       or fatal "cannot stat $file: $!";
> +      return $stat->mtime;
> +    }
>  }
>  
>  
> -- 
> 2.39.0
>
>
>
>
> --- End Message ---
> --- Begin Message ---
>
>
>
> Subject: 
>
> Re: [bug#60807] [PATCH 1/2] mtime: use Time::HiRes::stat when available for subsecond resolution
>
>
>
>
> Date: 
>
> Fri, 3 Feb 2023 22:13:37 -0800
>
>
>
>
> On Sun, Jan 15, 2023 at 12:27 AM Mike Frysinger <vapier@gentoo.org> wrote:
> > On 14 Jan 2023 21:27, Jacob Bachmeyer wrote:
> > > Mike Frysinger wrote:
> > > > --- a/lib/Automake/FileUtils.pm
> > > > +++ b/lib/Automake/FileUtils.pm
> > > > @@ -42,6 +42,11 @@ use Exporter;
> > > >  use File::stat;
> > > >  use IO::File;
> > > >
> > > > +# Perl's builtin stat does not provide sub-second resolution.  Use 
> > > > Time::HiRes
> > > > +# if it's available instead.  Hopefully one day perl will update.
> > > > +# https://github.com/Perl/perl5/issues/17900
> > > > +my $have_time_hires = eval { require Time::HiRes; };
> > > > +
> > > >  use Automake::Channels;
> > > >  use Automake::ChannelDefs;
> > > >
> > > > @@ -115,10 +120,18 @@ sub mtime ($)
> > > >    return 0
> > > >      if $file eq '-' || ! -f $file;
> > > >
> > > > -  my $stat = stat ($file)
> > > > -    or fatal "cannot stat $file: $!";
> > > > -
> > > > -  return $stat->mtime;
> > > > +  if ($have_time_hires)
> > > > +    {
> > > > +      my @stat = Time::HiRes::stat ($file)
> > > > +   or fatal "cannot stat $file: $!";
> > > > +      return $stat[9];
> > > > +    }
> > > > +  else
> > > > +    {
> > > > +      my $stat = stat ($file)
> > > > +   or fatal "cannot stat $file: $!";
> > > > +      return $stat->mtime;
> > > > +    }
> > > >  }
> > >
> > > If you change that variable to a constant, you can eliminate the runtime
> > > overhead entirely, since Perl optimizes if(1) and if(0) and folds
> > > constants at compile time.
> > >
> > > Something like:
> > >
> > >     use constant HAVE_Time_HiRes => eval { require Time::HiRes; };
> > >
> > > Then:
> > >
> > >     if (HAVE_Time_HiRes)
> > >        ...
> > >
> > > If you do this, Perl will inline the block actually used and elide the
> > > branch at runtime.  This is generally useful for any test that can only
> > > go one way in a specific run of the program.
> >
> > thanks, i'll integrate that idea.  i'm by no means a perl programmer.
>
> Thanks, Mike.
> It looks like this has been resolved slightly differently by a recent
> change from Paul Eggert, so I'm closing this.
>
>
> --- End Message ---