[Pdx-pm] Hash question
Bruce J Keeler
bruce at gridpoint.com
Sat Nov 22 15:38:18 CST 2003
On Fri, 2003-11-21 at 20:25, Randal L. Schwartz wrote:
>
> I think it's even been shown that iteration is better:
>
> my @array = $db2->DataHash();
> while (@array) {
> $SignUpInfo{shift @array} = shift @array;
> }
>
Makes sense as it doesn't have to compute hash keys for %tmp.
> Of course, I'm cheating here, knowing that the left side
> is eval'ed before the right. If you don't want that much magic:
>
> my %tmp = $db2->DataHash();
> while (my ($k, $v) = each %tmp) {
> $SignUpInfo{$k} = $v;
> }
You're saying that's cheaper than
> @SignUpInfo{keys %tmp} = values %tmp;
?
This I found hard to believe. Why would Perl pessimize it so? I
whipped up the following:
#!/usr/bin/perl
use Benchmark qw( cmpthese );
push (@array, rand, rand) for (1..100);
cmpthese ( -10, {
iterated_hash => sub {
my %dest;
my %tmp = @array;
while (my ($k, $v) = each %tmp) {
$dest{$k} = $v;
}
},
atonce => sub {
my %dest;
my %tmp = @array;
@dest{keys %tmp} = values %tmp;
},
iterated_array => sub {
my %dest;
my @tmp = @array;
while (@tmp) {
$dest{shift @tmp} = shift @tmp;
}
},
} );
Results:
Rate iterated_array iterated_hash atonce
iterated_array 1198/s -- -56% -71%
iterated_hash 2709/s 126% -- -34%
atonce 4098/s 242% 51% --
It seems that the array method is worst of all. Most interesting.
My perl is:
bruce at scrunge| /tmp % perl -V
Summary of my perl5 (revision 5.0 version 8 subversion 2) configuration:
Platform:
osname=linux, osvers=2.4.22-xfs+ti1211,
archname=i386-linux-thread-multi
uname='linux kosh 2.4.22-xfs+ti1211 #1 sat oct 25 10:11:37 est 2003
i686 gnulinux '
config_args='-Dusethreads -Duselargefiles -Dccflags=-DDEBIAN
-Dcccdlflags=-fPIC -Darchname=i386-linux -Dprefix=/usr
-Dprivlib=/usr/share/perl/5.8.2 -Darchlib=/usr/lib/perl/5.8.2
-Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5
-Dvendorarch=/usr/lib/perl5 -Dsiteprefix=/usr/local
-Dsitelib=/usr/local/share/perl/5.8.2
-Dsitearch=/usr/local/lib/perl/5.8.2 -Dman1dir=/usr/share/man/man1
-Dman3dir=/usr/share/man/man3 -Dsiteman1dir=/usr/local/man/man1
-Dsiteman3dir=/usr/local/man/man3 -Dman1ext=1 -Dman3ext=3perl
-Dpager=/usr/bin/sensible-pager -Uafs -Ud_csh -Uusesfio -Uusenm
-Duseshrplib -Dlibperl=libperl.so.5.8.2 -Dd_dosuid -des'
hint=recommended, useposix=true, d_sigaction=define
usethreads=define use5005threads=undef useithreads=define
usemultiplicity=define
useperlio=define d_sfio=undef uselargefiles=define usesocks=undef
use64bitint=undef use64bitall=undef uselongdouble=undef
usemymalloc=n, bincompat5005=undef
Compiler:
cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DTHREADS_HAVE_PIDS
-DDEBIAN -fno-strict-aliasing -I/usr/local/include -D_LARGEFILE_SOURCE
-D_FILE_OFFSET_BITS=64',
optimize='-O3',
cppflags='-D_REENTRANT -D_GNU_SOURCE -DTHREADS_HAVE_PIDS -DDEBIAN
-fno-strict-aliasing -I/usr/local/include'
ccversion='', gccversion='3.3.2 (Debian)', gccosandvers=''
intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=1234
d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=12
ivtype='long', ivsize=4, nvtype='double', nvsize=8, Off_t='off_t',
lseeksize=8
alignbytes=4, prototype=define
Linker and Libraries:
ld='cc', ldflags =' -L/usr/local/lib'
libpth=/usr/local/lib /lib /usr/lib
libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt
perllibs=-ldl -lm -lpthread -lc -lcrypt
libc=/lib/libc-2.3.2.so, so=so, useshrplib=true,
libperl=libperl.so.5.8.2
gnulibc_version='2.3.2'
Dynamic Linking:
dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-rdynamic'
cccdlflags='-fPIC', lddlflags='-shared -L/usr/local/lib'
Characteristics of this binary (from libperl):
Compile-time options: MULTIPLICITY USE_ITHREADS USE_LARGE_FILES
PERL_IMPLICIT_CONTEXT
Built under linux
Compiled at Nov 15 2003 17:52:08
@INC:
/etc/perl
/usr/local/lib/perl/5.8.2
/usr/local/share/perl/5.8.2
/usr/lib/perl5
/usr/share/perl5
/usr/lib/perl/5.8.2
/usr/share/perl/5.8.2
/usr/local/lib/site_perl
/usr/local/lib/perl/5.8.0
/usr/local/share/perl/5.8.0
.
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