NAME
IPC::ScoreBoard - IPC similar to the apache scoreboard
SYNOPSIS
use IPC::ScoreBoard;
# create an anonymous scoreboard
my $sb=SB::anon $nslots, $slotsize, $extra;
# create a file base board
my $sb=SB::named $filename, $nslots, $slotsize, $extra;
# open a file based board
my ($sb, $nslots, $slotsize, $extra)=SB::open $filename;
# set/set a value
SB::set $sb, $slotidx, $elidx, $integer_value;
$value=SB::get $sb, $slotidx, $elidx;
@values=SB::get_all $sb, $slotidx;
# increment/decrement
SB::incr $sb, $slotidx, $elidx, $integer_value;
SB::decr $sb, $slotidx, $elidx, $integer_value;
# sum functions
$sum=SB::sum $sb, $elidx;
@sums=SB::sum_all $sb;
# access extra space
SB::set_extra $sb, $elidx, $integer_value;
$value=SB::get_extra $sb, $elidx;
@values=SB::get_all_extra $sb;
SB::incr_extra $sb, $elidx, $integer_value;
SB::decr_extra $sb, $elidx, $integer_value;
# fetch parameters
$nslots=SB::nslots $sb;
$slotsize=SB::slotsize $sb;
$nextra=SB::nextra $sb;
# does the compiler provide atomic increment/decrement operations?
if( SB::have_atomics ) {
# increment and decrement operations are atomic
}
INSTALLATION
perl Makefile.PL
make
make test
make install
DEPENDENCIES
* perl 5.8.8
* File::Map 0.21
DESCRIPTION
A scoreboard is a set of integer numbers residing in shared memory. It
is organized as 2-dimensional array where a line in one of the
dimensions is called a slot. So, in other words the scoreboard is a set
of slots and each slot is a set of integer numbers.
The idea is that in a system of processes or threads of execution each
process *owns* a slot. A process can change the values in its own slot
at will but must adhere to read-only access to other slots.
There is one extra slot at the end of the scoreboard that is allowed to
be used by every process. However this module does not provide any kind
of locking to synchronize access.
The extra slot can differ in size from the other normal slots.
A scoreboard can be anonymous or it can have a name in the file system
and hence be accessed by unrelated processes.
What is that good for?
Suppose a system of processes that handle certain requests. Now, you
want to implement a monitor that shows the overall number of requests
handled so far by the system as a whole.
One way to do that is to use a shared variable that is incremented each
time a process has finished a request. But access to this variable has
to be synchronized by some type of locking. Otherwise 2 or even more
processes can read the shared variable at the same time. Then each of
them increments its own value and writes it back. In the end only the
value written by the last process hits the memory. All other increments
are lost.
A lock free way could be to have each process increment its own
variable. Then the monitor would have to sum up all the variables of the
processes. But the system is now lock-free.
USAGE
A scoreboard object is a reference to a scalar. Its methods can be
invoked in the usual object style, "$sb->get(42,19)", or as subroutines,
"IPC::ScoreBoard::get $sb, 42, 19". The latter variant is a bit faster
but involves a lot of typing.
To mitigate that all functions are exported to the "SB::" namespace if
the module is included via "use". If it is included via "require"
nothing is exported. Neither is it if the parameter ":noshortcuts" is
passed to "use":
use IPC::ScoreBoard; # generates shortcuts SB::get & co.
use IPC::ScoreBoard (); # no shortcuts
use IPC::ScoreBoard ':noshortcuts'; # no shortcuts
require IPC::ScoreBoard; # no shortcuts
All data access functions throw an exception if access outside the
boundaries of the slot or scoreboard is tried.
The following section shows only the shortcut usage. Remember all
functions can also be called as
$object_or_class->functionname(@param);
or as
IPC::ScoreBoard::functionname $scoreboard, @param;
Scoreboard creation
SB::anon $nslots, $slotsize, $nextra
creates an anonymous scoreboard with space for $nslots slots and
$slotsize "IV" values per slot. The extra slot contains $nextra "IV"
values.
"anon" returns the scoreboard object.
In case of an error an exception is thrown.
Example:
my $sb=IPC::ScoreBoard->anon($nslots, $slotsize, $nextra);
my $sb=IPC::ScoreBoard::anon $nslots, $slotsize, $nextra;
my $sb=SB::anon $nslots, $slotsize, $nextra;
SB::named $filename, $nslots, $slotsize, $nextra
similar to "anon" but creates a named scoreboard with the name
$filename.
Example:
my $sb=IPC::ScoreBoard->named($filename, $nslots, $slotsize, $nextra);
my $sb=IPC::ScoreBoard::named $filename, $nslots, $slotsize, $nextra;
my $sb=SB::named $filename, $nslots, $slotsize, $nextra;
SB::open $filename
similar to "anon" but connects to or opens an existing named scoreboard
with the name $filename.
Besides the scoreboard object the scoreboard parameters $nslots,
$slotsize, $nextra are returned:
Example:
my ($sb, $nslots, $slotsize, $extra)=IPC::ScoreBoard->open($filename);
my ($sb, $nslots, $slotsize, $extra)=IPC::ScoreBoard::open $filename;
my ($sb, $nslots, $slotsize, $extra)=SB::open $filename;
Data manipulation
SB::set $sb, $slotidx, $elidx, $value;
sets the $elidxth (counting from 0) element in slot number $slotidx
(also counting from 0) to $value. $value is interpreted as integer.
The new value is returned.
SB::get $sb, $slotidx, $elidx;
reads the value at position $elidx in slot number $slotidx.
SB::incr $sb, $slotidx, $elidx, $amount;
SB::decr $sb, $slotidx, $elidx, $amount;
these 2 functions increment or decrement the value at position $elidx in
slot number $slotidx. $amount is optional. If ommitted 1 is used.
If supported by the compiler atomic operations are used to do that. That
means, even if multiple processes increment or decrement a certain value
in parallel nothing is lost as described in the DESCRIPTION.
The new value is returned.
SB::sum $sb, $elidx;
sums up the values at a position $elidx over all slots (except for the
extra one).
SB::get_all $sb, $slotidx;
returns a list of all values of slot number $slotidx.
SB::sum_all $sb;
returns a list of sums. The equivalent in perl could read:
@sums=map { SB::sum $sb, $_ } 0..$slotsize;
SB::set_extra $sb, $elidx, $value;
sets the value at position $elidx in the extra slot.
SB::get_extra $sb, $elidx;
reads the value at position $elidx in the extra slot.
SB::incr_extra $sb, $elidx, $amount;
SB::decr_extra $sb, $elidx, $amount;
these 2 functions increment or decrement the value at position $elidx in
the extra slot. $amount is optional. If ommitted 1 is used.
If supported by the compiler atomic operations are used to do that. That
means, even if multiple processes increment or decrement a certain value
in parallel nothing is lost as described in the DESCRIPTION.
The new value is returned.
SB::get_all_extra $sb;
returns the list of all values from the extra slot.
Auxiliary functions
SB::nslots $sb
returns the number of slots in the scoreboard
SB::slotsize $sb
returns the number of "IV"s in each slot
SB::nextra $sb
returns the number of "IV"s in the extra slot
SB::offset_of $sb, $slotidx, $elidx
converts a slotnumber and an index within the slot into a byte offset
from the beginning of the scoreboard.
If both $slotidx and $elidx are given the offset of the slot element is
returned. If $elidx is ommitted or undefined $slotidx is taken as an
element index within the extra slot:
$sb->offset_of(2, 3); # 3rd IV in 2nd slot
SB::offset_of($sb, 3); # 3rd IV of extra slot
This allows to store data other than integers.
Example:
# store "hugo" in extra[2..4]
substr($$sb, $sb->offset_of(2), 3*$ivlen, pack( "Z".(3*$ivlen), "hugo"));
# retrieve "hugo"
(unpack "x".$sb->offset_of(2)."Z*", $$sb)[0]
Notes on the example:
* Make sure the replacement string has exactly the length as given in
the 3rd "substr" parameter.
* Don't use the lvalue form of "substr". It stores a reference to $$sb
and hence if $$sb goes out of scope the scoreboard won't be
unmapped.
Don't do:
substr($$sb, $sb->offset_of(2), 3*$ivlen)=pack( "Z".(3*$ivlen), "hugo");
* Remember, the "Z" pack format stores always a "NULL" byte at the end
of the string. So, the example works because "hugo" does not contain
a "NULL" byte. If you need to store binary data a more sophisticated
format as "W/a" could be used. But watch out for the maximum length.
SB::have_atomics
returns true if "IPC::ScoreBoard" has been compiled with a compiler that
supports atomic increment/decrement operations.
EXPORT
Nothing.
SEE ALSO
* <http://www.alexonlinux.com/multithreaded-simple-data-type-access-an
d-atomic-variables>
for more information about atomic operations
* File::Map
the underlying memory mapper
AUTHOR
Torsten Förtsch, <torsten.foertsch@gmx.net>
COPYRIGHT AND LICENSE
Copyright (C) 2010 by Torsten Förtsch
This library is free software; you can redistribute it and/or modify it
under the same terms as Perl itself, either Perl version 5.10.0 or, at
your option, any later version of Perl 5 you may have available.