Question to Cache::Memcached

[Joachim Bauernberger]

Hi,

On Tuesday 08 June 2004 20:06, Brad Fitzpatrick wrote:
> Whoa, good bug!
>
> I guess I assumed Perl ints would wrap around, not get magically upcast to
> some float/bigint type, or whatever's going on.
>
> Can you change it to this:
>
>  $hash =3D ($hash*33 + ord($_)) % (2 ** 31);
>
> That should work, right?  *scratchs head*

Why not do something like this in _hashfunc():

$hash .=3D int(ord($_));

and just make use of "use integer" within get_sock()

=2E.. this would make _hashfunc slimmer and maybe faster (with large keys y=
ou=20
end up doing more $hash*33) and you only need "use integer" in get_sock() .=
=2E.

Regards,
~/joachim

>
> On Tue, 8 Jun 2004, Joachim Bauernberger wrote:
> > Hi,
> >
> > consider the snipped below which extracts the _hashfuc from Memcached.p=
m.
> >
> > Feeding a key to set or add which looks like this:
> > "020/dating/profilcache/profilAnsicht.xmlindex=3Dpage1userid=3D1812379"
> >
> > will result in an internal key like:
> > 2.34523032118619e+15
> >
> > doing a % modulo on this value will always result in 0 therefore the
> > array index is 0 and the second server is ignored.
> >
> > Best regards,
> > ~/joachim
> >
> > #########################################
> > #!/usr/bin/perl
> > use strict;
> >
> > my $res =3D  _hashfunc($ARGV[0])."\n";
> >
> > print "$res\n";
> > print "res % 2: ". $res % 2 ."\n";
> >
> > sub _hashfunc {
> >     my $hash =3D 0;
> >     foreach (split //, shift) {
> >             $hash =3D $hash*33 + ord($_);
> >     }
> > return $hash;
> > }
> > ########################################
> >
> > Not sure if something like the following would work better in this case
> > (or break something else):
> >
> > foreach (split //, shift) {
> >  $hash +=3D sprintf("%d",ord($_));
> > }
> >
> > Thanks & Best regards,
> > ~/joachim
> >
> > Regards,
> > ~/joachim
> >
> > On Monday 07 June 2004 22:56, Joachim Bauernberger wrote:
> > > Hi Brad,
> > >
> > > On Monday 07 June 2004 21:09, Brad Fitzpatrick wrote:
> > > > > How does the module decide which backend to give it too (if no
> > > > > weight value is specified)?
> > > >
> > > > It runs the key through a hashing function, then mods that number
> > > > with the number of buckets (number of servers * each's weight) and
> > > > uses the result as the bucket number.
> > > >
> > > > So if you have two servers:
> > > >
> > > > 10.0.0.1 weight 1   ->  1 bucket  (bucket 0)
> > > > 10.0.0.2 weight 3   ->  3 buckets (bucket 1, 2, 3)
> > > >
> > > > Total:                  4 buckets
> > > >
> > > > So now we have key "foobar".  HashValue("foobar") =3D 2384234324234=
34.
> > > >
> > > > Taking the hash value, mod 4 (4 buckets)...
> > > >
> > > >  238423432423434 % 4 =3D 2 (bucket 2)
> > > >
> > > > So it uses 10.0.0.2
> > > >
> > > > > Is there a restriction on how the keys may look?
> > > >
> > > > No spaces or newlines.
> > > >
> > > > > My keys are similar to this (without quotes):
> > > > >
> > > > > "010/dating/profilansicht/profilAnsicht.xmlindex=3Dpage1userid=3D=
217079
> > > > >7"
> > > >
> > > > That looks fine.
> > > >
> > > > So the mystery is:  what hash values is it calculating for your key=
s?
> > > > It'd be way too weird if they were all 0.
> > >
> > > I just added some debugging to Memcached.pm:
> > >
> > > first in get_sock() just after asigning a value to $hv
> > >
> > > print STDERR "[JMBDEBUG] hv =3D $hv
> > > bucketcount=3D".$self->{'bucketcount'}."\n";
> > >
> > > this results in the following output:
> > >
> > > [JMBDEBUG] hv =3D 8.25640843592502e+101 bucketcount=3D2
> > >
> > >
> > > then in get_sock() just after $host gets calculated:
> > > print STDERR "[JMBDEBUG] host=3D".$host." ".$self->{'buckets'}[0]." ".
> > > $self->{'buckets'}[1]."\n";
> > >
> > > which results in:
> > > [JMBDEBUG] host=3D127.0.0.1:11211 127.0.0.1:11211 127.0.0.1:11212
> > >
> > > Somehow I always get 0 from this:
> > >  $hv % $self->{'bucketcount'}
> > >
> > > Thanks again,
> > > ~/joachim
> >
> > --
> > Phone: +49 (0) 89 490 267 726
> > Fax: +49 (0) 89 490 267 701
> > Mobile: +49 (0) 179 674 3611
> > mailto: joachim.bauernberger@friendscout24.de =A0 =A0 =A0 =A0
> > Web: http://www.friendscout24.de

=2D-=20
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