1DiskCache(3) User Contributed Perl Documentation DiskCache(3)
2
6 PDL::DiskCache -- Non-memory-resident array object
7
9 NON-OO:
10 use PDL::DiskCache;
12 tie @a,'PDL::DiskCache', \@files, \%options;
13 imag $a[3];
14 OO:
16 use PDL::DiskCache;
18 $a = diskcache(\@files,\%options);
19 imag $a->[3];
20 or
22 use PDL::DiskCache;
24 $a = new PDL::DiskCache(\@files,\%options);
25 imag $a->[4];
26 \@files
28 an array ref containing a list of file names
29 \%options
31 a hash ref containing options for the PDL::DiskCache object (see
32 "TIEARRAY" below for details)
33
35 A PDL::DiskCache object is a perl "tied array" that is useful for
36 operations where you have to look at a large collection of PDLs one or
37 a few at a time (such as tracking features through an image sequence).
38 You can write prototype code that uses a perl list of a few PDLs, then
39 scale up to to millions of PDLs simply by handing the prototype code a
40 DiskCache tied array instead of a native perl array. The individual
41 PDLs are stored on disk and a few of them are swapped into memory on a
42 FIFO basis. You can set whether the data are read-only or writeable.
43 By default, PDL::DiskCache uses FITS files to represent the PDLs, but
45 you can use any sort of file at all -- the read/write routines are the
46 only place where it examines the underlying data, and you can specify
47 the routines to use at construction time (or, of course, subclass
48 PDL::DiskCache).
49 Items are swapped out on a FIFO basis, so if you have 10 slots and an
51 expression with 10 items in it then you're OK (but you probably want
52 more slots than that); but if you use more items in an expression than
53 there are slots, thrashing will occur!
54 The hash ref interface is kept for historical reasons; you can access
56 the sync() and purge() method calls directly from the returned array
57 ref.
58
60 There's no file locking, so you could really hose yourself by having
61 two of these things going at once on the same files.
62 Since this is a tied array, things like Dumper traverse it
64 transparently. That is sort-of good but also sort-of dangerous. You
65 wouldn't want to PDL::Dumper::sdump() a large PDL::DiskCache, for
66 example -- that would defeat the purpose of using a PDL::DiskCache in
67 the first place.
68
70 Copyright 2001, Craig DeForest
71 This code may be distributed under the same terms as Perl itself
73 (license available at http://www.perl.org). Copying, reverse
74 engineering, distribution, and modification are explicitly allowed so
75 long as this notice is preserved intact and modified versions are
76 clearly marked as such.
77 If you modify the code and it's useful, please send a copy of the
79 modified version to cdeforest@solar.stanford.edu.
80 This package comes with NO WARRANTY.
82
84 diskcache
85 Object constructor.
86 Synopsis
88 $a = diskcache(\@f,\%options);
89 Options
91 see the TIEARRAY options,below.
92 TIEARRAY
94 Tied-array constructor; invoked by perl during object construction.
95 Synopsis
97 TIEARRAY(class,\@f,\%options)
98 Options
100 ro (default 0): If set, treat the files as read-only (modifications
101 to the tied array will only persist until the changed elements are
102 swapped out)
103 rw (default 1): If set, allow reading and writing to the files.
105 Because there's currently no way to determine reliably whether a PDL
106 has been modified, rw files are always written to disk when they're
107 swapped out -- this causes a slight performance hit.
108 mem (default 20): Number of files to be cached in memory at once.
110 read (default \&rfits): A function ref pointing to code that will
112 read list objects from disk. The function must have the same syntax
113 as rfits: $object = rfits(filename).
114 write (default \&wfits): A function ref pointing to code that will
116 write list objects to disk. The function must have the same syntax
117 as wfits: func(object,filename).
118 bless (default 0): If set to a nonzero value, then the array ref
120 gets blessed into the DiskCache class for for easier access to the
121 "purge" and "sync" methods. This means that you can say "$a-"sync>
122 instead of the more complex "(%{tied @$a})-"sync>, but "ref $a" will
123 return "PDL::DiskCache" instead of "ARRAY", which could break some
124 code.
125 verbose (default 0): Get chatty.
127 purge
129 Remove an item from the oldest slot in the cache, writing to disk as
130 necessary. You also send in how many slots to purge (default 1;
131 sending in -1 purges everything.)
132 For most uses, a nice MODIFIED flag in the data structure could save
134 some hassle here. But PDLs can get modified out from under us with
135 slicing and .= -- so for now we always assume everything is tainted and
136 must be written to disk.
137 sync
139 In a rw cache, flush all items out to disk but retain them in the
140 cache. This is useful primarily for cache protection and could be
141 slow. Because we have no way of knowing what's modified and what's not
142 in the cache, all elements are always flushed from an rw cache. For ro
143 caches, this is a not-too-slow (but safe) no-op.
144 DESTROY
146 This is the perl hook for object destruction. It just makes a call to
147 "sync", to flush the cache out to disk. Destructor calls from perl
148 don't happen at a guaranteed time, so be sure to call "sync" if you
149 need to ensure that the files get flushed out, e.g. to use 'em
150 somewhere else.
151perl v5.12.3 2009-10-17 DiskCache(3)