Logalloc is a replace-malloc library for Firefox (see memory/build/replace_malloc.h) that dumps a log of memory allocations to a given file descriptor or file name. That log can then be replayed against Firefox's default memory allocator independently or through another replace-malloc library, allowing the testing of other allocators under the exact same workload. To get an allocation log the following environment variables need to be set when starting Firefox: - on Linux: LD_PRELOAD=/path/to/liblogalloc.so - on Mac OSX: DYLD_INSERT_LIBRARIES=/path/to/liblogalloc.dylib - on Windows: MOZ_REPLACE_MALLOC_LIB=/path/to/logalloc.dll - on Android: MOZ_REPLACE_MALLOC_LIB=/path/to/liblogalloc.so (see https://wiki.mozilla.org/Mobile/Fennec/Android#Arguments_and_Environment_Variables for how to pass environment variables to Firefox for Android) - on all platforms: MALLOC_LOG=/path/to/log-file or MALLOC_LOG=number When MALLOC_LOG is a number below 10000, it is considered as a file descriptor number that is fed to Firefox when it is started. Otherwise, it is considered as a file name. As those allocation logs can grow large quite quickly, it can be useful to pipe the output to a compression tool. MALLOC_LOG=1 would send to Firefox's stdout, MALLOC_LOG=2 would send to its stderr. Since in both cases that could be mixed with other output from Firefox, it is usually better to use another file descriptor by shell redirections, such as: MALLOC_LOG=3 firefox 3>&1 1>&2 | gzip -c > log.gz (3>&1 copies the `| gzip` pipe file descriptor to file descriptor #3, 1>&2 then copies stderr to stdout. This leads to: fd1 and fd2 sending to stderr of the parent process (the shell), and fd3 sending to gzip.) Each line of the allocations log is formatted as follows: <pid> <tid> <function>([<args>])[=<result>] where <args> is a comma separated list of values. The number of <args> and the presence of <result> depend on the <function>. Example log: 18545 18545 malloc(32)=0x7f90495120e0 18545 18545 calloc(1,148)=0x7f9049537480 18545 18545 realloc(0x7f90495120e0,64)=0x7f9049536680 18545 18545 posix_memalign(256,240)=0x7f9049583300 18545 18545 jemalloc_stats() 18545 18545 free(0x7f9049536680) This log can be replayed with the logalloc-replay tool in memory/replace/logalloc/replay. However, as the goal of that tool is to reproduce the recorded memory allocations, it needs to avoid as much as possible doing its own allocations for bookkeeping. Reading the logs as they are would require data structures and memory allocations. As a consequence, the logs need to be preprocessed beforehand. The logalloc_munge.py script is responsible for that preprocessing. It simply takes a raw log on its stdin, and outputs the preprocessed log on its stdout. It replaces pointer addresses with indexes the logalloc-replay tool can use in a large (almost) linear array of allocation tracking slots (prefixed with '#'). It also replaces the pids with numbers starting from 1 (such as the first seen pid number is 1, the second is 2, etc.). The above example log would become the following, once preprocessed: 1 1 malloc(32)=#1 1 1 calloc(1,148)=#2 1 1 realloc(#1,64)=#1 1 1 posix_memalign(256,240)=#3 1 1 jemalloc_stats() 1 1 free(#1) The logalloc-replay tool then takes the preprocessed log on its stdin and replays the allocations printed there, but will only replay those with the same process id as the first line (which normally is 1). As the log files are simple text files, though, it is easy to separate out the different processes log with e.g. grep, and feed the separate processes logs to logalloc-replay. The logalloc-replay program won't output anything unless jemalloc_stats records appears in the log. You can expect those to be recorded when going to about:memory in Firefox, but they can also be added after preprocessing. Here is an example of what one can do: gunzip -c log.gz | python logalloc_munge.py | \ awk '$1 == "2" { print $0 } !(NR % 10000) { print "2 1 jemalloc_stats()" }' | \ ./logalloc-replay The above command replays the allocations of process #2, with some stats output every 10000 records. The logalloc-replay tool itself being hooked with replace-malloc, it is possible to set LD_PRELOAD/DYLD_INSERT_LIBRARIES/MOZ_REPLACE_MALLOC_LIB and replay a log through a different allocator. For example: LD_PRELOAD=libreplace_jemalloc.so logalloc-replay < log Will replay the log against jemalloc4 (which is, as of writing, what libreplace_jemalloc.so contains).