src/README.mtrace

Built-in memory tracing/profiling

The basilisk preprocessor (qcc) can generate code to trace all calls to malloc(), calloc(), realloc(), strdup() and free(). This can be used to track the evolution of the amount of memory dynamically allocated by the program and can automatically detect common errors such as invalid pointers, double free’s and memory leaks. It is similar to the mtrace() functionality implemented in the GNU C library, however it will only trace calls made by code compiled with qcc (i.e. it is not a library implementation). This can be quite useful on systems where either mtrace() is not available, or where mtrace() does not play well with other libraries (MPI in particular).

To turn on memory tracing you need to set the macro MTRACE to something different from zero, for example using:

qcc -DMTRACE=1 mycode.c -o mycode -lm

or if you are using makefiles:

CFLAGS=-DMTRACE=1 make bump2D.tst

In addition to the program outputs, the tracing function will output a summary on standard error looking like:

*** MTRACE: max resident  set size:   17137664 bytes
*** MTRACE: max traced memory size:    4398337 bytes (tracing overhead 0.1%)
 max bytes                function   file
   3567536           mempool_alloc   /home/popinet/basilisk/src/grid/mempool.h:52
    796672            cache_append   /home/popinet/basilisk/src/grid/tree.h:300
    495616      cache_level_shrink   /home/popinet/basilisk/src/grid/tree.h:290
    196608      cache_level_append   /home/popinet/basilisk/src/grid/tree.h:273
    194400            new_refarray   /home/popinet/basilisk/src/grid/tree.h:89
    131072              matrix_new   /home/popinet/basilisk/src/utils.h:226
      5600          realloc_scalar   /home/popinet/basilisk/src/grid/tree.h:1234
...
*** MTRACE: To get a graph use: tail -n 2 mtrace | gnuplot -persist
*** MTRACE: No memory leaks

The first line is the maximum system memory usage (in bytes), as reported by getrusage().ru_maxrss (if this function is available on your system). The second line is the maximum amount of memory allocated by the functions traced by code compiled with qcc. A breakdown of this amount according to which function made the call (with the corresponding file and line numbers) then follows.

Trace file

In addition to the summary profiling information, a file tracing all calls is also generated. It is called mtrace by default, but this can be changed using the MTRACE environment variable. For example

MTRACE=/tmp/mtrace CFLAGS=-DMTRACE=1 make bump2D.tst

will use /tmp/trace instead. When using MPI, mtrace will be used for process 0 and mtrace-1, mtrace-2 etc… for the other processes.

There are only two types of records in the file:

  1. Index definition, something like
@ 20 mempool_new /home/popinet/basilisk/src/grid/mempool.h 31

where @ indicates that this is an index record, 20 is the index number, mempool_new is the name of the function containing the call, followed by the file and line number where the call is made. An index record always precedes the first use of this index

  1. Memory function call, something like:
+ 20 3640 2096433 384 17137664

where + is the type of memory operation, 20 is the index number (i.e. where is the call made), 3640 is the record number (starting from zero), 2096433 is the total amount of allocated memory (in bytes) after the call, 384 is the total amount of memory allocated up-to-now by this specific call (as identified by its index) and 17137664 is the ru_maxrss value (in bytes) at this point in time.

The memory operations corresponds to the functions being traced i.e.

+: memory allocation (i.e. malloc(), calloc(), strdup())

-: memory deallocation (i.e. free())

>: memory allocation by realloc()

< : memory deallocation by realloc()

The last two lines of the file contain commands which can be used by gnuplot to get a graph, for example using

gnuplot> set term x11
gnuplot> load "< tail -n2 mtrace"

which will give something like

Graph of memory trace

Graph of memory trace

By default only functions representing more than 1% of the total allocated memory are displayed. The initial amount of memory reported by ru_maxrss (here 1.7e7 bytes) is substracted from the corresponding curve in the graph.

Memory leak reporting

Note that in the previous graph, all curves (ru_maxrss excepted) should converge to zero at the end of the trace, otherwise memory leaks are present. If this the case, a message like this will be displayed:

...
/home/popinet/basilisk-octree/src/common.h:825: error: 34960 bytes leaked here
/home/popinet/basilisk/src/Makefile.defs:33: recipe for target 'bump2D.tst' failed
make: *** [bump2D.tst] Error 1

(the program exits with an error status of one). The number of bytes leaked is given together with the location of where the leaked memory was allocated.

Other memory errors

Trying to free memory twice or passing invalid pointers to free() can also be detected and will lead to the program aborting. A debugger can then be used to track the cause of the problem.

Controlling outputs

The degree of verbosity of the profiling can be controlled by increasing the value of MTRACE.

MTRACE=1: Profiling output and memory trace.

MTRACE=2: Profiling output.

MTRACE=3: Memory leak errors only.

Accessing the profiling summary

The profiling summary is contained in

struct {
  FILE * fp;                     // trace file
  size_t total, max;             // current and maximum allocated memory
  size_t overhead, maxoverhead;  // current and maximum profiling overhead
  size_t nr;                     // current number of records
  size_t startrss, maxrss;       // starting and maximum system ressource usage
  char * fname;                  // trace file name
} pmtrace;

See also