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Nevertheless, switching the JIT compilation off before the profiling is not "a must". It is rather a recommendation, and in some cases, for example in a production environment, you may need to keep JIT compilation on to see the full picture of all the memory allocations. In this case, the majority of the ``INTERNAL`` events are most probably caused by traces.
As for investigating the Lua code with the help of profiling reports, it is always code-dependent and there can't be hundred per cent definite recommendations in this regard. Nevertheless, you can see some of the things in the :ref:`Profiling report analysis example <profiler_analysis>` later.
Also, below is the :ref:`FAQ <profiler_faq>` section with the questions that most probably can arise while using profiler.
FAQ
In this section, some profiler-related points are discussed in a Q&A format.
**Question (Q)**: Is the profiler suitable for C allocations or allocations inside C code?
**Answer (A)**: The profiler reports only allocation events caused by the Lua allocator. All Lua-related allocations, like table or string creation are reported. But the profiler doesn't report allocations made by ``malloc()`` or other non-Lua allocators. You can use ``valgrind`` to debug them.
**Q**: Why are there so many ``INTERNAL`` allocations in my profiling report? What does it mean?
**A**: ``INTERNAL`` means that these allocations/reallocations/deallocations are related to the internal LuaJIT structures or are made on traces. Currently, the profiler doesn't verbosely report allocations of objects that are made during trace execution. Try :ref:`adding jit.off() <profiler_usage_internal_jitoff>` before the profiler start.
**Q**: Why are there some reallocations/deallocations without an ``Overrides`` section?
**A**: These objects can be created before the profiler starts. Adding ``collectgarbage()`` before the profiler's start enables collecting all previously allocated objects that are dead when the profiler starts.
**Q**: Why are some objects not collected during profiling? Is it a memory leak?
**A**: LuaJIT uses incremental Garbage Collector (GC). A GC cycle may not be finished at the moment the profiler stops. Add ``collectgarbage()`` before stopping the profiler to collect all the dead objects for sure.
**Q**: Can I profile not just a current chunk but the entire running application? Can I start the profiler when the application is already running?
**A**: Yes. Here is an example of code that can be inserted in the Tarantool console for a running instance.
local fiber = require "fiber"
local log = require "log"

fiber.create(function()
fiber.name("memprof")

collectgarbage() -- Collect all objects already dead
log.warn("start of profile")

local st, err = misc.memprof.start(FILENAME)
if not st then
log.error("failed to start profiler: %s", err)
end

fiber.sleep(TIME)

collectgarbage()
st, err = misc.memprof.stop()

if not st then
log.error("profiler on stop error: %s", err)
end

log.warn("end of profile")
end)
where
``FILENAME``—the name of the binary file where profiling events are written
``TIME``—duration of profiling, in seconds.
Also, you can directly call ``misc.memprof.start()`` and ``misc.memprof.stop()`` from a console.
Profiling report analysis example
In the example below, the following Lua code named ``format_concat.lua`` is investigated with the help of the memory profiler reports.
-- Prevent allocations on new traces.
jit.off()

local function concat(a)
local nstr = a.."a"
return nstr
end

local function format(a)
local nstr = string.format("%sa", a)
return nstr
end

collectgarbage()

local binfile = "/tmp/memprof_"..(arg[0]):match("([^/]*).lua")..".bin"

local st, err = misc.memprof.start(binfile)
assert(st, err)

-- Payload.
for i = 1, 10000 do
local f = format(i)
local c = concat(i)
end
collectgarbage()

local st, err = misc.memprof.stop()
assert(st, err)

os.exit()
When you run this code :ref:`under Tarantool <profiler_usage_generate>` and then :ref:`parse <profiler_usage_parse_command>` the binary memory profile in /tmp/memprof_format_concat.bin, you will get the following profiling report:
ALLOCATIONS
@format_concat.lua:10: 19996 events +624284 bytes -0 bytes
INTERNAL: 1 events +65536 bytes -0 bytes

REALLOCATIONS

DEALLOCATIONS
INTERNAL: 19996 events +0 bytes -558778 bytes
Overrides:
@format_concat.lua:10

@format_concat.lua:10: 2 events +0 bytes -98304 bytes
Overrides:
@format_concat.lua:10

HEAP SUMMARY:
INTERNAL holds 65536 bytes: 1 allocs, 0 frees
Reasonable questions regarding the report can be:
Why are there no allocations related to the ``concat()`` function?
Why is the number of allocations not a round number?
Why are there about 20K allocations instead of 10K?
First of all, LuaJIT doesn't create a new string if the string with the same payload exists (see details on `lua-users.org/wiki <http://lua-users.org/wiki/ImmutableObjects>`_). This is called `string interning <https://en.wikipedia.org/wiki/String_interning>`_. So, when a string is created via the ``format()`` function, there is no need to create the same string via the ``concat()`` function, and LuaJIT just uses the previous one.
That is also the reason why the number of allocations is not a round number as could be expected from the cycle operator ``for i = 1, 10000...``: Tarantool creates some strings for internal needs and built‑in modules, so some strings already exist.

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testing_project / Documentation: reference/tooling/luajit_memprof — English

local fiber = require "fiber"
local log = require "log"

fiber.create(function()
fiber.name("memprof")

collectgarbage() -- Collect all objects already dead
log.warn("start of profile")

local st, err = misc.memprof.start(FILENAME)
if not st then
log.error("failed to start profiler: %s", err)
end

fiber.sleep(TIME)

collectgarbage()
st, err = misc.memprof.stop()

if not st then
log.error("profiler on stop error: %s", err)
end

log.warn("end of profile")
end)
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String age
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