Re: Yielding across C boundaries
- From: Mike Pall <mike-1301@xxxxxxxxxx>
- To: luajit@xxxxxxxxxxxxx
- Date: Thu, 3 Jan 2013 00:17:25 +0100
Daurnimator wrote:
> Can we expect to eventually see lua_yieldk, lua_callk, and lua_pcallk
> added to luajit?
Not without switching to the Lua 5.2 ABI (i.e. unlikely anytime
soon).
> > You're probably using LuaJIT for its performance benefits. But
> > callbacks from C into the VM are slow -- really slow compared to
> > everything else. If possible, rewrite the intervening C code in
> > Lua (maybe with the help of the FFI) and/or switch to a pull-style
> > API (iterator instead of callbacks).
>
> Btw, why is this? It came up in an irc channel a couple of days ago:
> I said that iteration would be favourable; but was unable to cite why...
Entering the VM needs a lot of state setup and leaving it isn't
free either. Constantly entering and leaving the VM via a callback
from C *to* Lua has a high overhead. For short callbacks, the
switching overhead between C and Lua may completely dominate the
total CPU time.
Calling an iterator written in C via the FFI *from* a Lua program
is much cheaper -- this compiles down to a simple call instruction.
Staying within Lua code and calling a Lua function from Lua (both
callback-style or iterator-style) is even cheaper: the compiler
can inline the callee, which helps many other optimizations.
--Mike
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