Slow shared memory issue

bash has asked for the wisdom of the Perl Monks concerning the following question:

Hello,
Need advice why shared memory is so slow. My assumption was that shared memory is like usual memory. But this simple test show contrary.
I guess that my test is bad...

#!/usr/bin/evn perl
use strict;
use warnings;

use Benchmark qw(:all :hireswallclock);
use IPC::SysV qw(IPC_CREAT);
use Data::Dumper::Concise;

my $data;
my $size = 65536;

my $id = shmget('12345', $size, IPC_CREAT | 0666);

my $mem = join('', map { ('a' .. 'z')rand(26) } (1..65536));
open(MEMORY, "<", \$mem);

shmwrite( $id, $mem, 0, $size );

cmpthese( -1, {
    'shmread'     => sub { shmread( $id, $data, 0, $size ) || die "shmread: $!" },
    'memory read' => sub { read( MEMORY, $data, $size, 0 ) // die "read: $!" },
});

$ perl bin/test/bench/shmread.pl 
                  Rate     shmread memory read
shmread        36202/s          --       -100%
memory read 10155128/s      27951%          --

Comment on Slow shared memory issue

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Re: Slow shared memory issue by Anonymous Monk on Mar 27, 2016 at 17:00 UTC
You have to check the return value of `sysread`, like so `'memory read' => sub { sysread( MEMORY, $data, 0, $size ) // die $!; }` [download] you'll see 'Bad file descriptor'. `sysread` cannot read from a filehandle opened on a string; it needs a real file descriptor. Also, you probably meant `sysread( MEMORY, $data, $size, 0 )` (the third argument is the number of bytes to read and the fourth is offset - thats different from `shmread`). `shmread` will probably still be slower, since it reads or writes the System V shared memory segment... by attaching to it, copying in/out, and detaching from it.	[reply] [d/l] [select]
Re^2: Slow shared memory issue by bash (Scribe) on Mar 27, 2016 at 19:00 UTC
shmat (attach) -> memread (read) works 8x times faster So now I have only one question. Why IPC::Shareable do not uses benefits of shmat -> memread flow ? It uses shmread which is obviously slow... https://metacpan.org/source/MSOUTH/IPC-Shareable-0.61/lib/IPC/Shareable/SharedMem.pm#L97	[reply]
Re^2: Slow shared memory issue by bash (Scribe) on Mar 27, 2016 at 18:35 UTC
You're right... missed errors checks. Thank you for the answer about attaching to shared segment. It's quite expensive operation. It clarify performance issue. Don't understand why there is no description - http://perldoc.perl.org/functions/shmread.html	[reply]