Perl 7 Feature Request: sealed subs for typed lexicals
Joe Schaefer
The Problem
Perl 5’s OO runtime method lookup has 50% more performance overhead than a direct, named subroutine invocation.
The initial solution: Doug MacEachern’s method lookup optimizations
Doug was the creator of the mod_perl project back in the mid-90s, so obviously writing high performance Perl was his forté. One of his many contributions to p5p was to cut the performance penalty of OO method lookup overhead in half, by using a method + @ISA heirarchy cache to make the runtime object method lookup for mod_perl objects like Apache2::RequestRec as streamlined as possible. But it only gets us half-way there.
This isn’t a trifling issue with calls to C struct get-set accessor methods — the common situation with many mod_perl APIs. Perl’s runtime method-call lookup penalty on httpd’s struct request_rec *, that mod_perl exposes via the Apache2::RequestRec module, is on the same order of magnitude of the full execution of the call. For mod_perl backed sites making millions of XS method calls a second, this is an awful waste of precious CPU cycles.
What Doug was looking for was a way to tell Perl 5 to perform the method lookup at compile time, the way it does with named subroutine calls. Every time Doug tried, he hit roadblocks of either a social or technical nature. Perhaps it’s time to make another pass at this idea with the advent of Perl 7.
Benchmark script
#!/usr/bin/env -S perl -Ilib
use Test::More tests => 2;
use POSIX 'dup2';
dup2 fileno(STDERR), fileno(STDOUT);
use strict;
use warnings;
use Benchmark ':all';
our ($x, $z);
$x = bless {}, "Foo";
$z = Foo->can("foo");
sub method {$x->foo}
sub class {Foo->foo}
sub anon {$z->($x)}
sub bar { 1 }
sub reentrant;
BEGIN {
package Foo;
use base 'sealed';
use sealed 'deparse';
sub foo { shift }
my $n;
sub _foo :Sealed { my main $x = shift; $n++ ? $x->bar : $x->reentrant }
}
sub func {Foo::foo($x)}
BEGIN {our @ISA=qw/Foo/}
use sealed 'deparse'; # invokes Lexical::Types->import into this namespace
# assigned to 'main' by UNIVERSAL::TYPESCALAR
my main $y;
sub sealed :Sealed {
$y->foo();
}
sub also_sealed :Sealed {
my main $a = shift;
if ($a) {
my Benchmark $bench;
my $inner = $a;
return sub :Sealed {
my Foo $b = $a;
$inner->foo($b->foo($inner->bar, $inner, $bench->cmpthese));
$a = $inner;
$a->foo;
$b->bar;
};
}
$a->bar();
}
sub reentrant :Sealed { my main $b = shift; local our @Q=1; my $c = $b->_foo }
ok($y->reentrant()==1);
my %tests = (
func => \&func,
method => \&method,
sealed => \&sealed,
class => \&class,
anon => \&anon,
);
cmpthese 20_000_000, \%tests;
ok(1);
Benchmark results
1..2
sealed: compiling Foo->bar lookup.
B::Deparse: coderef2text() aborted: Cant locate object method "PV" via package "B::NULL" at /usr/share/perl/5.36/B/Deparse.pm line 4925.
sealed: compiling main->foo lookup.
sub sealed :sealed {
use warnings;
use strict;
$y->foo:compiled;
}
sealed: compiling Benchmark->cmpthese lookup.
sealed: compiling Foo->foo lookup.
sealed: compiling main->foo lookup.
sealed: compiling Foo->bar lookup.
sub __ANON__ :sealed {
use warnings;
use strict;
my Foo $b = $a;
$inner->foo($b->foo:compiled($inner->bar, $inner, $bench->cmpthese:compiled));
$a = $inner;
$a->foo:compiled;
$b->bar:compiled;
}
sealed: compiling main->bar lookup.
sub also_sealed :sealed {
use warnings;
use strict;
my main $a = shift();
if ($a) {
my Benchmark $bench;
my $inner = $a;
return sub {
my Foo $b = $a;
$inner->foo($b->foo:compiled($inner->bar, $inner, $bench->cmpthese:compiled));
$a = $inner;
$a->foo:compiled;
$b->bar:compiled;
}
;
}
$a->bar:compiled;
}
sealed: compiling main->_foo lookup.
sub reentrant :sealed {
use warnings;
use strict;
my main $b = shift();
(local our(@Q)) = 1;
my $c = $b->_foo:compiled;
}
ok 1
Rate class method anon func sealed
class 15503876/s -- -9% -19% -20% -36%
method 17094017/s 10% -- -10% -12% -29%
anon 19047619/s 23% 11% -- -2% -21%
func 19417476/s 25% 14% 2% -- -19%
sealed 24096386/s 55% 41% 27% 24% --
ok 2
Proposed Perl 7 solution: :sealed subroutines for typed lexicals
Sample code:
use v7.0;
use Apache2::RequestRec;
sub handler :sealed {
my Apache2::RequestRec $r = shift;
$r->content_type("text/html"); #compile time method lookup
}
Production-Quality, Robust Perl v5.28+ Prototype: sealed.pm v6.0.1 (on CPAN)
Compile instructions for perl 5.30+ are available in the sealed.pm pod should you want to run mod_perl2 w/ ithreads and httpd-2.4 w/ event mpm, and not segfault at any scale. Tested on Solaris 11.4 and Ubuntu 22.04 on amd64.
For fun, try this monkey patch to ModPerl::RegistryCooker:
<VirtualHost *:443>
PerlModule ModPerl::RegistryCookerSealed
PerlResponseHandler ModPerl::Registry
AddHandler perl-script .pl
Options +ExecCGI
</VirtualHost>
It enables the effects of sub handler :Sealed {} on all of your ModPerl::Registry scripts, something like this one.
~/src/cms% h2load -n 100000 -c 1000 -m 100 -t 10 http://localhost/perl-script/enquiry.pl\?lang=.es
starting benchmark...
spawning thread #0: 100 total client(s). 10000 total requests
spawning thread #1: 100 total client(s). 10000 total requests
spawning thread #2: 100 total client(s). 10000 total requests
spawning thread #3: 100 total client(s). 10000 total requests
spawning thread #4: 100 total client(s). 10000 total requests
spawning thread #5: 100 total client(s). 10000 total requests
spawning thread #6: 100 total client(s). 10000 total requests
spawning thread #7: 100 total client(s). 10000 total requests
spawning thread #8: 100 total client(s). 10000 total requests
spawning thread #9: 100 total client(s). 10000 total requests
Application protocol: h2c
progress: 10% done
progress: 20% done
progress: 30% done
progress: 40% done
progress: 50% done
progress: 60% done
progress: 70% done
progress: 80% done
progress: 90% done
progress: 100% done
finished in 13.07s, 7652.14 req/s, 11.83MB/s
requests: 100000 total, 100000 started, 100000 done, 100000 succeeded, 0 failed, 0 errored, 0 timeout
status codes: 100000 2xx, 0 3xx, 0 4xx, 0 5xx
traffic: 154.61MB (162119955) total, 566.39KB (579980) headers (space savings 95.47%), 152.30MB (159700000) data
min max mean sd +/- sd
time for request: 5.74ms 12.77s 6.39s 3.61s 58.14%
time for connect: 304us 293.01ms 70.17ms 76.83ms 74.80%
time to 1st byte: 7.86ms 7.87s 3.33s 1.82s 50.40%
req/s : 7.71 248.17 19.60 28.07 92.70%
See https://github.com/SunStarSys/sealed/blob/master/lib/sealed.pm. Look for t/bench.pl in the parent directory.
This will allow Perl 5 to do the sample code’s content_type method-lookup at compile time, without causing any back-compat issues or aggrieved CPAN coders, since this feature would target application developers. Not inheritable OO-module authors.
This Perlish idea is gratuitously stolen from Dylan. Read this for the CPython effort from over a decade ago.