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 $x = diskcache(\@files,\%options);
19 imag $x->[3];
20 or
22 use PDL::DiskCache;
24 $x = new PDL::DiskCache(\@files,\%options);
25 imag $x->[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 $x = diskcache(\@f,\%options);
88 Options
90 • See the TIEARRAY options, below.
92 TIEARRAY
94 Tied-array constructor; invoked by perl during object construction.
95 TIEARRAY(class,\@f,\%options)
97 Options
99 ro (default 0)
101 If set, treat the files as read-only (modifications to the tied
102 array will only persist until the changed elements are swapped out)
103 rw (default 1)
105 If set, allow reading and writing to the files. Because there's
106 currently no way to determine reliably whether a PDL has been
107 modified, rw files are always written to disk when they're swapped
108 out -- this causes a slight performance hit.
109 mem (default 20)
111 Number of files to be cached in memory at once.
112 read (default \&rfits)
114 A function ref pointing to code that will read list objects from
115 disk. The function must have the same syntax as rfits: $object =
116 rfits(filename).
117 write (default \&wfits)
119 A function ref pointing to code that will write list objects to
120 disk. The function must have the same syntax as wfits:
121 func(object,filename).
122 bless (default 0)
124 If set to a nonzero value, then the array ref gets blessed into the
125 DiskCache class for for easier access to the "purge" and "sync"
126 methods. This means that you can say "$x->sync" instead of the more
127 complex "(%{tied @$x})->sync", but "ref $x" will return
128 "PDL::DiskCache" instead of "ARRAY", which could break some code.
129 verbose (default 0)
131 Get chatty.
132 purge
134 Remove an item from the oldest slot in the cache, writing to disk as
135 necessary. You also send in how many slots to purge (default 1;
136 sending in -1 purges everything.)
137 For most uses, a nice MODIFIED flag in the data structure could save
139 some hassle here. But PDLs can get modified out from under us with
140 slicing and .= -- so for now we always assume everything is tainted and
141 must be written to disk.
142 sync
144 In a rw cache, flush items out to disk but retain them in the cache.
145 Accepts a single scalar argument, which is the index number of a single
147 item that should be written to disk. Passing (-1), or no argument,
148 writes all items to disk, similar to purge(-1).
149 For ro caches, this is a not-too-slow (but safe) no-op.
151 DESTROY
153 This is the perl hook for object destruction. It just makes a call to
154 "sync", to flush the cache out to disk. Destructor calls from perl
155 don't happen at a guaranteed time, so be sure to call "sync" if you
156 need to ensure that the files get flushed out, e.g. to use 'em
157 somewhere else.
158perl v5.36.0 2022-07-22 DiskCache(3)