NAME¶
PDL::DiskCache -- Non-memory-resident array object
SYNOPSIS¶
NON-OO:
use PDL::DiskCache;
tie @a,'PDL::DiskCache', \@files, \%options;
imag $a[3];
OO:
use PDL::DiskCache;
$a = diskcache(\@files,\%options);
imag $a->[3];
or
use PDL::DiskCache;
$a = new PDL::DiskCache(\@files,\%options);
imag $a->[4];
- \@files
- an array ref containing a list of file names
- \%options
- a hash ref containing options for the PDL::DiskCache object
(see "TIEARRAY" below for details)
DESCRIPTION¶
A PDL::DiskCache object is a perl "tied array" that is useful for
operations where you have to look at a large collection of PDLs one or a few
at a time (such as tracking features through an image sequence). You can write
prototype code that uses a perl list of a few PDLs, then scale up to to
millions of PDLs simply by handing the prototype code a DiskCache tied array
instead of a native perl array. The individual PDLs are stored on disk and a
few of them are swapped into memory on a FIFO basis. You can set whether the
data are read-only or writeable.
By default, PDL::DiskCache uses FITS files to represent the PDLs, but you can
use any sort of file at all -- the read/write routines are the only place
where it examines the underlying data, and you can specify the routines to use
at construction time (or, of course, subclass PDL::DiskCache).
Items are swapped out on a FIFO basis, so if you have 10 slots and an expression
with 10 items in it then you're OK (but you probably want more slots than
that); but if you use more items in an expression than there are slots,
thrashing will occur!
The hash ref interface is kept for historical reasons; you can access the
sync() and
purge() method calls directly from the returned array
ref.
Shortcomings & caveats¶
There's no file locking, so you could really hose yourself by having two of
these things going at once on the same files.
Since this is a tied array, things like Dumper traverse it transparently. That
is sort-of good but also sort-of dangerous. You wouldn't want to
PDL::Dumper::sdump() a large PDL::DiskCache, for example -- that would
defeat the purpose of using a PDL::DiskCache in the first place.
Author, license, no warranty¶
Copyright 2001, Craig DeForest
This code may be distributed under the same terms as Perl itself (license
available at <
http://www.perl.org>). Copying, reverse engineering,
distribution, and modification are explicitly allowed so long as this notice
is preserved intact and modified versions are clearly marked as such.
If you modify the code and it's useful, please send a copy of the modified
version to cdeforest@solar.stanford.edu.
This package comes with NO WARRANTY.
FUNCTIONS¶
diskcache¶
Object constructor.
$a = diskcache(\@f,\%options);
Options
- •
- See the TIEARRAY options,below.
TIEARRAY¶
Tied-array constructor; invoked by perl during object construction.
TIEARRAY(class,\@f,\%options)
Options
- ro (default 0)
- If set, treat the files as read-only (modifications to the
tied array will only persist until the changed elements are swapped
out)
- rw (default 1)
- If set, allow reading and writing to the files. Because
there's currently no way to determine reliably whether a PDL has been
modified, rw files are always written to disk when they're swapped out --
this causes a slight performance hit.
- mem (default 20)
- Number of files to be cached in memory at once.
- read (default \&rfits)
- A function ref pointing to code that will read list objects
from disk. The function must have the same syntax as rfits: $object =
rfits(filename).
- write (default \&wfits)
- A function ref pointing to code that will write list
objects to disk. The function must have the same syntax as wfits:
func(object,filename).
- bless (default 0)
- If set to a nonzero value, then the array ref gets blessed
into the DiskCache class for for easier access to the "purge"
and "sync" methods. This means that you can say
"$a->sync" instead of the more complex "(%{tied
@$a})->sync", but "ref $a" will return
"PDL::DiskCache" instead of "ARRAY", which could break
some code.
- verbose (default 0)
- Get chatty.
purge¶
Remove an item from the oldest slot in the cache, writing to disk as necessary.
You also send in how many slots to purge (default 1; sending in -1 purges
everything.)
For most uses, a nice MODIFIED flag in the data structure could save some hassle
here. But PDLs can get modified out from under us with slicing and .= -- so
for now we always assume everything is tainted and must be written to disk.
sync¶
In a rw cache, flush all items out to disk but retain them in the cache. This is
useful primarily for cache protection and could be slow. Because we have no
way of knowing what's modified and what's not in the cache, all elements are
always flushed from an rw cache. For ro caches, this is a not-too-slow (but
safe) no-op.
DESTROY¶
This is the perl hook for object destruction. It just makes a call to
"sync", to flush the cache out to disk. Destructor calls from perl
don't happen at a guaranteed time, so be sure to call "sync" if you
need to ensure that the files get flushed out, e.g. to use 'em somewhere
else.