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Valgrind User Manual |  |
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Valgrind User Manual | |
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Table of Contents
To use this tool, you must specify
--tool=massif on the Valgrind
command line.
Massif is a heap profiler. It measures how much heap memory your program uses. This includes both the useful space, and the extra bytes allocated for book-keeping purposes and alignment purposes. It can also measure the size of your program's stack(s), although it does not do so by default.
Heap profiling can help you reduce the amount of memory your program uses. On modern machines with virtual memory, this provides the following benefits:
It can speed up your program -- a smaller program will interact better with your machine's caches and avoid paging.
If your program uses lots of memory, it will reduce the chance that it exhausts your machine's swap space.
Also, there are certain space leaks that aren't detected by traditional leak-checkers, such as Memcheck's. That's because the memory isn't ever actually lost -- a pointer remains to it -- but it's not in use. Programs that have leaks like this can unnecessarily increase the amount of memory they are using over time. Massif can help identify these leaks.
Importantly, Massif tells you not only how much heap memory your program is using, it also gives very detailed information that indicates which parts of your program are responsible for allocating the heap memory.
First off, as for the other Valgrind tools, you should compile with
debugging info (the -g flag). It shouldn't
matter much what optimisation level you compile your program with, as this
is unlikely to affect the heap memory usage.
Then, to gather heap profiling information about the program
prog, type:
% valgrind --tool=massif prog
The program will execute (slowly). Upon completion, no summary
statistics are printed to Valgrind's commentary; all of Massif's profiling
data is written to a file. By default, this file is called
massif.out.<pid>, where
<pid> is the process ID.
To see the information gathered by Massif in an easy-to-read form, use
the ms_print script. If the output file's name is
massif.out.12345, type:
% ms_print massif.out.12345
ms_print will produce (a) a graph showing the memory consumption over the program's execution, and (b) detailed information about the responsible allocation sites at various points in the program, including the point of peak memory allocation. The use of a separate script for presenting the results is deliberate: it separates the data gathering from its presentation, and means that new methods of presenting the data can be added in the future.
An example will make things clear. Consider the following C program (annotated with line numbers) which allocates a number of different blocks on the heap.
1 #include <stdlib.h>
2
3 void g(void)
4 {
5 malloc(4000);
6 }
7
8 void f(void)
9 {
10 malloc(2000);
11 g();
12 }
13
14 int main(void)
15 {
16 int i;
17 int* a[10];
18
19 for (i = 0; i < 10; i++) {
20 a[i] = malloc(1000);
21 }
22
23 f();
24
25 g();
26
27 for (i = 0; i < 10; i++) {
28 free(a[i]);
29 }
30
31 return 0;
32 }
After running this program under Massif, the first part of ms_print's output contains a preamble which just states how the program, Massif and ms_print were each invoked:
-------------------------------------------------------------------------------- Command: example Massif arguments: (none) ms_print arguments: massif.out.12797 --------------------------------------------------------------------------------
The next part is the graph that shows how memory consumption occurred as the program executed:
KB
19.63^ #
| #
| #
| #
| #
| #
| #
| #
| #
| #
| #
| #
| #
| #
| :#
| :#
| :#
| :#
| :#
| :#
0 +----------------------------------------------------------------------->ki
0 121.8
Number of snapshots: 25
Detailed snapshots: [9, 14 (peak), 24]
Why is most of the graph empty, with only a couple of bars at the very
end? By default, Massif uses "instructions executed" as the unit of time.
For very short-run programs such as the example, most of the executed
instructions involve the loading and dynamic linking of the program. The
execution of main (and thus the heap
allocations) only occur at the very end. For a short-running program like
this, we can use the --time-unit=B option
to specify that we want the time unit to instead be the number of bytes
allocated/deallocated on the heap and stack(s).
If we re-run the program under Massif with this option, and then re-run ms_print, we get this more useful graph:
KB
19.63^ #
| # .
| # : .
| # : : .
| : # : : : .
| : # : : : : .
| : # : : : : : .
| : # : : : : : : .
| : : # : : : : : : : .
| : : # : : : : : : : : .
| : : : # : : : : : : : : : ,
| @ : : : # : : : : : : : : : @
| : @ : : : # : : : : : : : : : @
| : : @ : : : # : : : : : : : : : @
| : : : @ : : : # : : : : : : : : : @
| : : : : @ : : : # : : : : : : : : : @
| : : : : : @ : : : # : : : : : : : : : @
| : : : : : : @ : : : # : : : : : : : : : @
| : : : : : : : @ : : : # : : : : : : : : : @
| : : : : : : : : @ : : : # : : : : : : : : : @
0 +----------------------------------------------------------------------->KB
0 29.48
Number of snapshots: 25
Detailed snapshots: [9, 14 (peak), 24]
Each vertical bar represents a snapshot, i.e. a measurement of the
memory usage at a certain point in time. The text at the bottom show that
25 snapshots were taken for this program, which is one per heap
allocation/deallocation, plus a couple of extras. Massif starts by taking
snapshots for every heap allocation/deallocation, but as a program runs for
longer, it takes snapshots less frequently. It also discards older
snapshots as the program goes on; when it reaches the maximum number of
snapshots (100 by default, although changeable with the
--max-snapshots option) half of them are
deleted. This means that a reasonable number of snapshots are always
maintained.
Most snapshots are normal, and only basic information is recorded for them. Normal snapshots are represented in the graph by bars consisting of ':' and '.' characters.
Some snapshots are detailed. Information about
where allocations happened are recorded for these snapshots, as we will see
shortly. Detailed snapshots are represented in the graph by bars consisting
of '@' and ',' characters. The text at the bottom show that 3 detailed
snapshots were taken for this program (snapshots 9, 14 and 24). By default,
every 10th snapshot is detailed, although this can be changed via the
--detailed-freq option.
Finally, there is at most one peak snapshot. The peak snapshot is a detailed snapshot, and records the point where memory consumption was greatest. The peak snapshot is represented in the graph by a bar consisting of '#' and ',' characters. The text at the bottom shows that snapshot 14 was the peak. Note that for tiny programs that never deallocate heap memory, Massif will not record a peak snapshot.
Some more details about the peak: the peak is determined by looking
at every allocation, i.e. it is not just the peak among
the regular snapshots. However, recording the true peak is expensive, and
so by default Massif records a peak whose size is within 1% of the size of
the true peak. See the description of the
--peak-inaccuracy option below for more
details.
The following graph is from an execution of Konqueror, the KDE web browser. It shows what graphs for larger programs look like.
MB
3.952^ #.
| ,@#:
| , .. :@@#:
| @ :: :@@#:.
| ,@ :: :@@#::
| , .@@@ :: :@@#:.
| .@ :@@@ :: :@@#::
| : :@ :@@@ :: :@@#::
| ,: :@ :@@@ :: :@@#::
| ,:@: :@ :@@@ :: :@@#::.
| @@:@: :@ :@@@ :: :@@#:::
| ,,: .:: . , .::@@:@: :@ :@@@ :: :@@#:::
| .:@@: .: ::: ::: @ :::@@:@: :@ :@@@ :: :@@#:::
| ,: ::@@: ::: ::::::: @ :::@@:@: :@ :@@@ :: :@@#:::
| @: ::@@: ::: ::::::: @ :::@@:@: :@ :@@@ :: :@@#::.
| @: ::@@: ::: ::::::: @ :::@@:@: :@ :@@@ :: :@@#:::
| , @: ::@@:: ::: ::::::: @ :::@@:@: :@ :@@@ :: :@@#:::
| ::@ @: ::@@:: ::: ::::::: @ :::@@:@: :@ :@@@ :: :@@#:::
| , :::::@ @: ::@@:: ::: ::::::: @ :::@@:@: :@ :@@@ :: :@@#:::
| ..@ :::::@ @: ::@@:: ::: ::::::: @ :::@@:@: :@ :@@@ :: :@@#:::
0 +----------------------------------------------------------------------->Mi
0 626.4
Number of snapshots: 63
Detailed snapshots: [3, 4, 10, 11, 15, 16, 29, 33, 34, 36, 39, 41,
42, 43, 44, 49, 50, 51, 53, 55, 56, 57 (peak)]
Note that the larger size units are KB, MB, GB, etc. As is typical for memory measurements, these are based on a multiplier of 1024, rather than the standard SI multiplier of 1000. Strictly speaking, they should be written KiB, MiB, GiB, etc.
Returning to our example, the graph is followed by the detailed information for each snapshot. The first nine snapshots are normal, so only a small amount of information is recorded for each one:
-------------------------------------------------------------------------------- n time(B) total(B) useful-heap(B) extra-heap(B) stacks(B) -------------------------------------------------------------------------------- 0 0 0 0 0 0 1 1,008 1,008 1,000 8 0 2 2,016 2,016 2,000 16 0 3 3,024 3,024 3,000 24 0 4 4,032 4,032 4,000 32 0 5 5,040 5,040 5,000 40 0 6 6,048 6,048 6,000 48 0 7 7,056 7,056 7,000 56 0 8 8,064 8,064 8,000 64 0
Each normal snapshot records several things.
Its number.
The time it was taken. In this case, the time unit is
bytes, due to the use of
--time-unit=B.
The total memory consumption at that point.
The number of useful heap bytes allocated at that point. This reflects the number of bytes asked for by the program.
The number of extra heap bytes allocated at that point. This reflects the number of bytes allocated in excess of what the program asked for. There are two sources of extra heap bytes.
First, every heap block has administrative bytes associated with it.
The exact number of administrative bytes depends on the details of the
allocator. By default Massif assumes 8 bytes per block, as can be seen
from the example, but this number can be changed via the
--heap-admin option.
Second, allocators often round up the number of bytes asked for to a
larger number. By default, if N bytes are asked for, Massif rounds N up
to the nearest multiple of 8 that is equal to or greater than N. This is
typical behaviour for allocators, and is required to ensure that elements
within the block are suitably aligned. The rounding size can be changed
with the --alignment option, although it
cannot be less than 8, and must be a power of two.
The size of the stack(s). By default, stack profiling is off as it slows Massif down greatly. Therefore, the stack column is zero in the example.
The next snapshot is detailed. As well as the basic counts, it gives an allocation tree which indicates exactly which pieces of code were responsible for allocating heap memory:
9 9,072 9,072 9,000 72 0 99.21% (9,000B) (heap allocation functions) malloc/new/new[], --alloc-fns, etc. ->99.21% (9,000B) 0x804841A: main (example.c:20)
The allocation tree can be read from the top down. The first line
indicates all heap allocation functions such as malloc
and C++ new. All heap allocations go through these
functions, and so all 9,000 useful bytes (which is 99.21% of all allocated
bytes) go through them. But how were malloc and new
called? At this point, every allocation so far has been due to line 21
inside main, hence the second line in the tree. The
-> indicates that main (line 20) called
malloc.
Let's see what the subsequent output shows happened next:
-------------------------------------------------------------------------------- n time(B) total(B) useful-heap(B) extra-heap(B) stacks(B) -------------------------------------------------------------------------------- 10 10,080 10,080 10,000 80 0 11 12,088 12,088 12,000 88 0 12 16,096 16,096 16,000 96 0 13 20,104 20,104 20,000 104 0 14 20,104 20,104 20,000 104 0 99.48% (20,000B) (heap allocation functions) malloc/new/new[], --alloc-fns, etc. ->49.74% (10,000B) 0x804841A: main (example.c:20) | ->39.79% (8,000B) 0x80483C2: g (example.c:5) | ->19.90% (4,000B) 0x80483E2: f (example.c:11) | | ->19.90% (4,000B) 0x8048431: main (example.c:23) | | | ->19.90% (4,000B) 0x8048436: main (example.c:25) | ->09.95% (2,000B) 0x80483DA: f (example.c:10) ->09.95% (2,000B) 0x8048431: main (example.c:23)
The first four snapshots are similar to the previous ones. But then the global allocation peak is reached, and a detailed snapshot is taken. Its allocation tree shows that 20,000B of useful heap memory has been allocated, and the lines and arrows indicate that this is from three different code locations: line 20, which is responsible for 10,000B (49.74%); line 5, which is responsible for 8,000B (39.79%); and line 10, which is responsible for 2,000B (9.95%).
We can then drill down further in the allocation tree. For example, of the 8,000B asked for by line 5, half of it was due to a call from line 11, and half was due to a call from line 25.
In short, Massif collates the stack trace of every single allocation point in the program into a single tree, which gives a complete picture of how and why all heap memory was allocated.
Note that the tree entries correspond not to functions, but to
individual code locations. For example, if function A
calls malloc, and function B calls
A twice, once on line 10 and once on line 11, then
the two calls will result in two distinct stack traces in the tree. In
contrast, if B calls A repeatedly
from line 15 (e.g. due to a loop), then each of those calls will be
represented by the same stack trace in the tree.
Note also that tree entry with children in the example satisfies an invariant: the entry's size is equal to the sum of its children's sizes. For example, the first entry has size 20,000B, and its children have sizes 10,000B, 8,000B, and 2,000B. In general, this invariant almost always holds. However, in rare circumstances stack traces can be malformed, in which case a stack trace can be a sub-trace of another stack trace. This means that some entries in the tree may not satisfy the invariant -- the entry's size will be greater than the sum of its children's sizes. Massif can sometimes detect when this happens; if it does, it issues a warning:
Warning: Malformed stack trace detected. In Massif's output,
the size of an entry's child entries may not sum up
to the entry's size as they normally do.
However, Massif does not detect and warn about every such occurrence. Fortunately, malformed stack traces are rare in practice.
Returning now to ms_print's output, the final part is similar:
-------------------------------------------------------------------------------- n time(B) total(B) useful-heap(B) extra-heap(B) stacks(B) -------------------------------------------------------------------------------- 15 21,112 19,096 19,000 96 0 16 22,120 18,088 18,000 88 0 17 23,128 17,080 17,000 80 0 18 24,136 16,072 16,000 72 0 19 25,144 15,064 15,000 64 0 20 26,152 14,056 14,000 56 0 21 27,160 13,048 13,000 48 0 22 28,168 12,040 12,000 40 0 23 29,176 11,032 11,000 32 0 24 30,184 10,024 10,000 24 0 99.76% (10,000B) (heap allocation functions) malloc/new/new[], --alloc-fns, etc. ->79.81% (8,000B) 0x80483C2: g (example.c:5) | ->39.90% (4,000B) 0x80483E2: f (example.c:11) | | ->39.90% (4,000B) 0x8048431: main (example.c:23) | | | ->39.90% (4,000B) 0x8048436: main (example.c:25) | ->19.95% (2,000B) 0x80483DA: f (example.c:10) | ->19.95% (2,000B) 0x8048431: main (example.c:23) | ->00.00% (0B) in 1+ places, all below ms_print's threshold (01.00%)
The final detailed snapshot shows how the heap looked at termination.
The 00.00% entry represents the code locations for which memory was
allocated and then freed (line 20 in this case, the memory for which was
freed on line 28). However, no code location details are given for this
entry; by default, Massif only records the details for code locations
responsible for more than 1% of useful memory bytes, and ms_print likewise
only prints the details for code locations responsible for more than 1%.
The entries that do not meet this threshold are aggregated. This avoids
filling up the output with large numbers of unimportant entries. The
thresholds can be changed with the
--threshold option that both Massif and
ms_print support.
If your program forks, the child will inherit all the profiling data that has been gathered for the parent.
If the output file format string (controlled by
--massif-out-file) does not contain %p, then
the outputs from the parent and child will be intermingled in a single output
file, which will almost certainly make it unreadable by ms_print.
Massif-specific options are:
--heap=<yes|no> [default: yes]
Specifies whether heap profiling should be done.
--heap-admin=<number> [default: 8]
If heap profiling is enabled, gives the number of administrative
bytes per block to use. This should be an estimate of the average,
since it may vary. For example, the allocator used by
glibc requires somewhere between 4 to
15 bytes per block, depending on various factors. It also requires
admin space for freed blocks, although Massif does not account
for this.
--stacks=<yes|no> [default: yes]
Specifies whether stack profiling should be done. This option slows Massif down greatly, and so is off by default. Note that Massif assumes that the main stack has size zero at start-up. This is not true, but measuring the actual stack size is not easy, and it reflects the size of the part of the main stack that a user program actually has control over.
--depth=<number> [default: 30]
Maximum depth of the allocation trees recorded for detailed snapshots. Increasing it will make Massif run somewhat more slowly, use more memory, and produce bigger output files.
--alloc-fn=<name>
Functions specified with this option will be treated as though
they were a heap allocation function such as
malloc. This is useful for functions that are
wrappers to malloc or new,
which can fill up the allocation trees with uninteresting information.
This option can be specified multiple times on the command line, to
name multiple functions.
Note that overloaded C++ names must be written in full. Single quotes may be necessary to prevent the shell from breaking them up. For example:
--alloc-fn='operator new(unsigned, std::nothrow_t const&)'
The full list of functions and operators that are by default considered allocation functions is as follows.
malloc calloc realloc memalign __builtin_new __builtin_vec_new operator new(unsigned) operator new(unsigned long) operator new[](unsigned) operator new[](unsigned long) operator new(unsigned, std::nothrow_t const&) operator new[](unsigned, std::nothrow_t const&) operator new(unsigned long, std::nothrow_t const&) operator new[](unsigned long, std::nothrow_t const&)
--threshold=<m.n> [default: 1.0]
The significance threshold for heap allocations, as a percentage. Allocation tree entries that account for less than this will be aggregated. Note that this should be specified in tandem with ms_print's option of the same name.
--peak-inaccuracy=<m.n> [default: 1.0]
Massif does not necessarily record the actual global memory allocation peak; by default it records a peak only when the global memory allocation size exceeds the previous peak by at least 1.0%. This is because there can be many local allocation peaks along the way, and doing a detailed snapshot for every one would be expensive and wasteful, as all but one of them will be later discarded. This inaccuracy can be changed (even to 0.0%) via this option, but Massif will run drastically slower as the number approaches zero.
--time-unit=i|ms|B [default: i]
The time unit used for the profiling. There are three possibilities: instructions executed (i), which is good for most cases; real (wallclock) time (ms, i.e. milliseconds), which is sometimes useful; and bytes allocated/deallocated on the heap and/or stack (B), which is useful for very short-run programs, and for testing purposes, because it is the most reproducible across different machines.
--detailed-freq=<n> [default: 10]
Frequency of detailed snapshots. With
--detailed-freq=1, every snapshot is
detailed.
--max-snapshots=<n> [default: 100]
The maximum number of snapshots recorded. If set to N, for all programs except very short-running ones, the final number of snapshots will be between N/2 and N.
--massif-out-file=<file> [default: massif.out.%p]
Write the profile data to file
rather than to the default output file,
massif.out.<pid>. The
%p and %q format specifiers can be
used to embed the process ID and/or the contents of an environment
variable in the name, as is the case for the core option
--log-file. See Basic Options for details.
--alignment=<n> [default: 1.0]
The minimum alignment (and thus size) of heap blocks.
ms_print's options are:
-h, --help
-v, --version
Help and version, as usual.
--threshold=<m.n> [default: 1.0]
Same as Massif's --threshold, but
applied after profiling rather than during.
--x=<m.n> [default: 72]
Width of the graph, in columns.
--y=<n> [default: 20]
Height of the graph, in rows.
Massif's file format is plain text (i.e. not binary) and deliberately easy to read for both humans and machines. Nonetheless, the exact format is not described here. This is because the format is currently very Massif-specific. We plan to make the format more general, and thus suitable for possible use with other tools. Once this has been done, the format will be documented here.