1Test::MemoryGrowth(3) User Contributed Perl DocumentationTest::MemoryGrowth(3)
2
6 "Test::MemoryGrowth" - assert that code does not cause growth in memory
7 usage
8
10 use Test::More;
11 use Test::MemoryGrowth;
12 use Some::Class;
14 no_growth {
16 my $obj = Some::Class->new;
17 } 'Constructing Some::Class does not grow memory';
18 my $obj = Some::Class->new;
20 no_growth {
21 $obj->do_thing;
22 } 'Some::Class->do_thing does not grow memory';
23 #### This test will fail ####
26 my @list;
27 no_growth {
28 push @list, "Hello world";
29 } 'pushing to an array does not grow memory';
30 done_testing;
32
34 This module provides a function to check that a given block of code
35 does not result in the process consuming extra memory once it has
36 finished. Despite the name of this module it does not, in the strictest
37 sense of the word, test for a memory leak: that term is specifically
38 applied to cases where memory has been allocated but all record of it
39 has been lost, so it cannot possibly be reclaimed. While the method
40 employed by this module can detect such bugs, it can also detect cases
41 where memory is still referenced and reachable, but the usage has grown
42 more than would be expected or necessary.
43 The block of code will be run a large number of times (by default
45 10,000), and the difference in memory usage by the process before and
46 after is compared. If the memory usage has now increased by more than
47 one byte per call, then the test fails.
48 In order to give the code a chance to load initial resources it needs,
50 it will be run a few times first (by default 10); giving it a chance to
51 load files, AUTOLOADs, caches, or any other information that it
52 requires. Any extra memory usage here will not count against it.
53 This simple method is not a guaranteed indicator of the absence of
55 memory resource bugs from a piece of code; it has the possibility to
56 fail in both a false-negative and a false-positive way.
57 False Negative
59 It is possible that a piece of code causes memory usage growth that
60 this module does not detect. Because it only detects memory growth
61 of at least one byte per call, it cannot detect cases of linear
62 memory growth at lower rates than this. Most memory usage growth
63 comes either from Perl-level or C-level bugs where memory objects
64 are created at every call and not reclaimed again. (These are
65 either genuine memory leaks, or needless allocations of objects
66 that are stored somewhere and never reclaimed). It is unlikely such
67 a bug would result in a growth rate smaller than one byte per call.
68 A second failure case comes from the fact that memory usage is
70 taken from the Operating System's measure of the process's Virtual
71 Memory size, so as to be able to detect memory usage growth in C
72 libraries or XS-level wrapping code, as well as Perl functions.
73 Because Perl does not agressively return unused memory to the
74 Operating System, it is possible that a piece of code could use un-
75 allocated but un-reclaimed memory to grow into; resulting in an
76 increase in its requirements despite not requesting extra memory
77 from the Operating System.
78 False Positive
80 It is possible that the test will claim that a function grows in
81 memory, when the behaviour is in fact perfectly normal for the code
82 in question. For example, the code could simply be some function
83 whose behaviour is required to store extra state; for example,
84 adding a new item into a list. In this case it is in fact expected
85 that the memory usage of the process will increase.
86 By careful use of this test module, false indications can be minimised.
88 By splitting tests across many test scripts, each one can be started in
89 a new process state, where most of the memory assigned from the
90 Operating System is in use by Perl, so anything extra that the code
91 requires will have to request more. This should reduce the false
92 negative indications.
93 By keeping in mind that the module simply measures the change in
95 allocated memory size, false positives can be minimised, by not
96 attempting to assert that certain pieces of code do not grow in memory,
97 when in fact it would be expected that they do.
98 Devel::MAT Integration
100 If Devel::MAT is installed, this test module will use it to dump the
101 state of the memory after a failure. It will create a .pmat file named
102 the same as the unit test, but with the trailing .t suffix replaced
103 with -TEST.pmat where "TEST" is the number of the test that failed (in
104 case there was more than one). It will then run the code under test one
105 more time, before writing another file whose name is suffixed with
106 -TEST-after.pmat. This pair of files may be useful for differential
107 analysis.
108
110 no_growth
111 no_growth { CODE } %opts, $name
112 Assert that the code block does not consume extra memory.
114 Takes the following named arguments:
116 calls => INT
118 The number of times to call the code during growth testing.
119 burn_in => INT
121 The number of times to call the code initially, before watching
122 for memory usage.
123
125 ยท Don't be Linux Specific
126 Currently, this module uses a very Linux-specific method of
128 determining process memory usage (namely, by inspecting
129 /proc/self/status). This should really be fixed to some OS-
130 neutral abstraction. Currently I am unaware of a simple
131 portable mechanism to query this. Patches very much welcome. :)
132
134 Paul Evans <leonerd@leonerd.org.uk>
135perl v5.30.1 2020-01-30 Test::MemoryGrowth(3)