to make koreader on Android more stable
and with these opt params:
```
require("jit.opt").start("sizemcode=64","maxmcode=64", "hotloop=10000")
```
The strategy here is that we only use precious mcode memory (jitting)
on deep loops like the several blitting methods in blitbuffer.lua and
the pixel-copying methods in mupdf.lua. So that a small amount of mcode
memory (64KB) allocated when koreader is launched in the android.lua
is enough for the program and it won't need to jit other parts of lua
code and thus won't allocate mcode memory any more which by our
observation will be harder and harder as we run koreader.
This workaround makes a reservation of mmap slots that are hopefully
near the address of `mcode_alloc_at`. Then it unmaps those slots and
lets luajit allocate a large slot for mcode.
On kindle, kobo and pocketbook the data directory is the current
running directory but on Android the app is installed in system
defined location and users may have no access to that location.
The same circumstances should be true for the upcoming Koreader for
Ubuntu touch, so the data directory (in which tessdata, dictionaries,
global settings, persistant defaults and probably history data are
stored) could be stored in another place.
Anyone who encounters random freeze of koreader for Android
is encouraged to enable this by adding a file at
"/sdcard/koreader/patch.lua" with the content of:
```
require("jit.opt").start("sizemcode=256","maxmcode=256")
for i=1,100 do end -- Force allocation of one large segment
```
The sizemcode and maxmcode could be adjusted from 196 to 512 to
make koreader best fit on your Android device.
