File i/o performance problems

[mihalicyn]

Let's see following code:

-- file have size 34406 bytes
f = io.open("/root/dev/libs_port.patch", 'r')

function fsop()
    f:seek('set', 0)
    s = f:read('all')
end

starttime = os.clock()
for i=1,18000
do
    fsop()
end
endtime = (os.clock() - starttime)

print(s:len())
f:close()

print("os.clock() diff " .. endtime)

Results

9000 fsop ~650 timediff kern
        ~0.18 timediff user
4500 fsop ~320 kernel
        ~0.1 user
18000 fsop ~1280 kernel
            ~0.37 user

As we can see scalability factor is two and times ratio is approximately two.

After that I made some experiments...
/usr/src/external/mit/lua/dist/src/liolib.c
original code

static void read_all (lua_State *L, FILE *f) {
  size_t nr;
  luaL_Buffer b;
  luaL_buffinit(L, &b);
  do {  /* read file in chunks of LUAL_BUFFERSIZE bytes */
    char *p = luaL_prepbuffer(&b);
    nr = fread(p, sizeof(char), LUAL_BUFFERSIZE, f);
    luaL_addsize(&b, nr);
  } while (nr == LUAL_BUFFERSIZE);
  luaL_pushresult(&b);  /* close buffer */
}

change to:

static void read_all (lua_State *L, FILE *f) {
  size_t nr;
  luaL_Buffer b;
  luaL_buffinit(L, &b);
  int fsize = 34406;
  do {  /* read file in chunks of LUAL_BUFFERSIZE bytes */
    char *p = luaL_prepbuffer(&b);
    //nr = fread(p, sizeof(char), LUAL_BUFFERSIZE, f);
    fsize -= LUAL_BUFFERSIZE;
    nr = LUAL_BUFFERSIZE;
    if (fsize < 0)
        nr = fsize + LUAL_BUFFERSIZE;        

    luaL_addsize(&b, nr);
  } while (nr == LUAL_BUFFERSIZE);
  luaL_pushresult(&b);  /* close buffer */
}

and times changes to ~1000 for 18k, ~500 for 9k...
Therefore, performance problems caused by memory allocator not by dofilereadv used in fread implementation.

I'm using code from that repo with NetBSD 7.1 generic kernel.