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Re: Optimizing with Caching vs. Parallelizing (MCE::Map)

by marioroy (Parson)
on Apr 06, 2020 at 05:55 UTC ( #11115117=note: print w/replies, xml ) Need Help??


in reply to Optimizing with Caching vs. Parallelizing (MCE::Map)

Hi 1nickt,

One may simulate extra CPU time to better understand the benefit of caching. I converted Laurent_R's example to consume multiple CPU cores. Laurent's example incurs extra overhead due to calling a subroutine inside the loop and returning a full list. I changed the number of iterations from 1 million to 100 thousand now that simulating extra CPU cycles.

Nick's example

use strict; use warnings; use feature 'say'; use MCE::Util; use MCE::Map max_workers => MCE::Util::get_ncpu(); use Time::HiRes 'usleep'; my @output = mce_map_s { my $input = $_; my $n = $input; my @result = $input; while ( $n != 1 ) { $n = $n % 2 ? 3 * $n + 1 : $n / 2; usleep 20; # simulate extra computation push @result, $n; } return [ $input, scalar @result ]; } 1, 100000; MCE::Map->finish; @output = sort { $b->[1] <=> $a->[1] } @output; say sprintf('%s : length %s', $_->[0], $_->[1]) for @output[0..19];

Laurent's example - no caching

use strict; use warnings; use feature qw/say/; use MCE::Util; use MCE::Map max_workers => MCE::Util::get_ncpu(); use Time::HiRes qw/usleep/; sub collatz_seq { my $n = shift; my @result; while ($n != 1) { my $new_n = $n % 2 ? 3 * $n + 1 : $n / 2; usleep 20; # simulate extra computation push @result, $new_n; $n = $new_n; } return @result; } my @long_seqs = mce_map_s { my $num = $_; my @seq = ($num, collatz_seq $num); return [ $num, scalar @seq ]; } 1, 100000; MCE::Map->finish; @long_seqs = sort { $b->[1] <=> $a->[1] } @long_seqs; say "$_->[0] : length $_->[1]" for @long_seqs[0..19];

Laurent's example - with caching

use strict; use warnings; use feature qw/say/; use constant MAX => 300000; use MCE::Util; use MCE::Map max_workers => MCE::Util::get_ncpu(); use Time::HiRes qw/usleep/; my %cache; sub collatz_seq { my $input = shift; my $n = $input; my @result; while ($n != 1) { if (exists $cache{$n}) { push @result, @{ $cache{$n} }; last; } my $new_n = $n % 2 ? 3 * $n + 1 : $n / 2; usleep 20; # simulate extra computation push @result, $new_n; $cache{$n} = [ $new_n, @{ $cache{$new_n} } ] if $n < MAX && exists $cache{$new_n}; $n = $new_n; } $cache{$input} = \@result if $n < MAX; return @result; } my @long_seqs = mce_map_s { my $num = $_; my @seq = ($num, collatz_seq $num); return [ $num, scalar @seq ]; } 1, 100000; MCE::Map->finish; @long_seqs = sort { $b->[1] <=> $a->[1] } @long_seqs; say "$_->[0] : length $_->[1]" for @long_seqs[0..19];

Program output

77031 : length 351 52527 : length 340 78791 : length 338 60975 : length 335 87087 : length 333 88059 : length 333 91463 : length 333 63387 : length 330 95081 : length 328 99067 : length 328 99721 : length 328 71310 : length 325 71311 : length 325 74791 : length 325 74793 : length 325 35655 : length 324 53483 : length 322 56095 : length 322 80225 : length 320 81159 : length 320

Time to run with 12 cores using a Linux VM

Notice the extra 4 seconds user time (likely function overhead inside loop), but not felt in real time (wall clock time).

Nick's example: 1m32.958s real, 0m56.561s user - no caching Laurent's example: 1m33.454s real, 1m00.491s user - no caching Laurent's example: 0m18.761s real, 0m20.990s user - caching

Caching is helpful here (5x faster) because I added sleep to simulate more CPU cycles. However, caching is not helpful when the overhead involved is greater than the computation itself as demonstrated by 1nickt's comparison above.

Regards, Mario

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