[Tarantool-patches] [PATCH 2.X 5/7] module api: luaT_temp_luastate & luaT_release_temp_luastate

[Vladislav Shpilevoy]

On 29.09.2020 23:03, Alexander Turenko wrote:
> On Tue, Sep 29, 2020 at 06:10:02PM +0300, Igor Munkin wrote:
>> Vlad,
>>
>> I guess there should be a question to be asked in scope of the original
>> series: what performance enhancement can we obtain with such hacky
>> optimization? Do we have any benchmarks? If the performance impact is
>> negligible I believe these interfaces should be left internal.
> 
> Why hacky? I'm a module developer and I ask Tarantool: Hey, I know you
> have a cache of Lua states. Give me one? Can I yield while the state is
> in use? I can, nice. Can I share it between fibers? No, okay.

It is not really a cache of lua states. Maybe of a single lua state, and
working only in Lua fibers (in C fibers you will create/delete the state
constantly), but not of states.

A proper Lua state cache/pool can be implemented easily without fiber at
all. Just have a pool with these states, where you either take a free one,
or allocate a new when the pool is empty. And when you don't need a state,
put it back into the pool. Then you will have at most the same number of
states in this pool, as the number of fibers, using this pool. Does not
seem to be a complex thing, will fit in 50-70 lines on C I think, top.
Neither seem to be too heavy (I didn't check), since all these states will
have empty stack, cleared before putting them back to the pool.

> That's all. Quite simple.

It is not that simple. Whatever you export now, we will need to support
potentially forever. So exporting not vital things increases support costs
for us for a very long time by saving a few time to you and Timur so you
don't need to change merger code and can move it as is. I would better
one time made merger less depending on Tarantool internals, then do long
support of the new pile of internal methods exposed in a hurry.