who can explain the -jv output in detail?

buf.lua

-- Copyright (C) Jinhua Luo

local select = select
local type = type
local tostring = tostring
local ipairs = ipairs
local tinsert = table.insert
local tremove = table.remove

local tpool = {n = 0}

local function append(t, ...)
	local n = select("#", ...)
	if n > 0 then
		local v = ...
		local typ = type(v)
		if typ == "table" then
			t.size = t.size + v.size
		else
			if typ ~= "string" then v = tostring(v) end
			t.size = t.size + #v
		end
		tinsert(t, v)
		if n > 1 then
			return append(t, select(2, ...))
		end
	end
end

local function clear(t, idx)
	idx = idx or #t
	if idx > 0 then
		local v = t[idx]
		if type(v) == "table" then
			v:free()
		end
		t[idx] = nil
		if idx > 1 then
			return clear(t, idx - 1)
		end
	end
end

local function free(t)
	t:clear()
	t.size = 0
	tinsert(tpool, t)
	tpool.n = tpool.n + 1
end

local mt = {
	__index = {
		free = free,
		append = append,
		clear = clear,
	}
}

local function alloc()
	local t
	if tpool.n > 0 then
		t = tremove(tpool)
		tpool.n = tpool.n - 1
	else
		t = setmetatable({size = 0}, mt)
	end
	return t
end

local total = 0

for i = 1, 10000000 do
	local t = alloc()
	t:append("hello")
	t:append("world")
	t:append("foo")
	local t2 = alloc()
	t2:append(1,2,3)
	t:append(t2)
	total = total + #t
	t:free()
	-- local t = {}
	-- tinsert(t, "hello")
	-- tinsert(t, "world")
	-- tinsert(t, "foo")
	-- local t2 = {}
	-- tinsert(t2, 1)
	-- tinsert(t2, 2)
	-- tinsert(t2, 3)
	-- tinsert(t, t2)
	-- total = total + #t
end

print(total)

return {
	alloc = alloc
}

output.txt

root@debian:~/ljio2# time luajit -jv lib/ljio/http/buf.lua 
[TRACE   1 buf.lua:30 return]
[TRACE --- (1/3) buf.lua:39 -- down-recursion, restarting at buf.lua:42]
[TRACE --- buf.lua:42 -- inner loop in root trace at buf.lua:73]
[TRACE --- (1/0) buf.lua:31 -- down-recursion, restarting at buf.lua:42]
[TRACE --- buf.lua:42 -- inner loop in root trace at buf.lua:73]
[TRACE --- (1/0) buf.lua:31 -- loop unroll limit reached at buf.lua:25]
[TRACE --- (1/0) buf.lua:31 -- loop unroll limit reached at buf.lua:25]
[TRACE --- (1/0) buf.lua:31 -- loop unroll limit reached at buf.lua:25]
[TRACE   2 (1/0) buf.lua:31 -- fallback to interpreter]
[TRACE --- (1/3) buf.lua:39 -- down-recursion, restarting at buf.lua:42]
[TRACE --- buf.lua:42 -- inner loop in root trace at buf.lua:73]
[TRACE --- (1/3) buf.lua:39 -- down-recursion, restarting at buf.lua:42]
[TRACE --- buf.lua:42 -- inner loop in root trace at buf.lua:73]
[TRACE --- (1/3) buf.lua:39 -- down-recursion, restarting at buf.lua:42]
[TRACE --- buf.lua:42 -- inner loop in root trace at buf.lua:73]
[TRACE   3 (1/3) buf.lua:39 -- fallback to interpreter]
[TRACE   4 buf.lua:44 return]
[TRACE   5 buf.lua:59 return]
[TRACE   6 buf.lua:72 loop]
40000000

real	0m2.373s
user	0m2.336s
sys	0m0.028s

root@debian:~/ljio2# luajit -v
LuaJIT 2.1.0-alpha -- Copyright (C) 2005-2015 Mike Pall. http://luajit.org/