Constant memory growth

[sebt3]

Based on the valgrind massif tool, I'm getting this :
->13.06% (3,590,851B) 0x65BB270: luaM_realloc_ (lmem.c:84)
| ->08.34% (2,292,707B) 0x65B9797: luaC_newobj (lgc.c:215)
| | ->04.31% (1,183,539B) 0x65BF6C3: luaS_newlstr (lstring.c:103)
| | | ->04.20% (1,155,555B) 0x65B2540: lua_pushlstring (lapi.c:505)
| | | | ->03.75% (1,030,540B) 0x65C4E6A: luaL_pushresult (lauxlib.c:473)
| | | | | ->03.75% (1,030,540B) 0x65C8C63: read_line (liolib.c:386)
| | | | | ->03.75% (1,030,540B) 0x65C8CD1: g_read (liolib.c:430)
| | | | | ->03.75% (1,030,540B) 0x65C93D3: io_readline (liolib.c:493)
| | | | | ->03.75% (1,030,540B) 0x3C94F4: watcheD::LuaCollector::collect() (luacollectors.cpp:71)
| | ->02.72% (746,944B) 0x65BF7B0: luaS_newlstr (lstring.c:103)
| | | ->01.10% (302,687B) 0x65B2540: lua_pushlstring (lapi.c:505)
... Some time later
->13.11% (3,645,476B) 0x65BB270: luaM_realloc_ (lmem.c:84)
| ->08.42% (2,342,628B) 0x65B9797: luaC_newobj (lgc.c:215)
| | ->04.39% (1,221,577B) 0x65BF6C3: luaS_newlstr (lstring.c:103)
| | | ->04.29% (1,192,648B) 0x65B2540: lua_pushlstring (lapi.c:505)
| | | | ->03.84% (1,067,633B) 0x65C4E6A: luaL_pushresult (lauxlib.c:473)
| | | | | ->03.84% (1,067,633B) 0x65C8C63: read_line (liolib.c:386)
| | | | | ->03.84% (1,067,633B) 0x65C8CD1: g_read (liolib.c:430)
| | | | | ->03.84% (1,067,633B) 0x65C93D3: io_readline (liolib.c:493)
| | | | | ->03.84% (1,067,633B) 0x3C94F4: watcheD::LuaCollector::collect() (luacollectors.cpp:71)
| | ->02.70% (751,563B) 0x65BF7B0: luaS_newlstr (lstring.c:103)
| | | ->01.10% (306,908B) 0x65B2540: lua_pushlstring (lapi.c:505)

All "LuaCollector::collect()" do is calling the same lua function again and a again at regular interval. The lua function should not allocate new memory...
It seams like all new string are attached to a lua table and never freed.

What am I missing? What can I do for this ?