[Tarantool-patches] [PATCH] lua/utils: improve luaT_newthread performance

[Igor Munkin]

Kirill,

Please proceed with the patch.

On 20.07.20, Igor Munkin wrote:
> <luaT_newthread> created a new GCfunc object for the helper invoked in a
> protected <lua_cpcall> frame (i.e. <luaT_newthread_wrapper>) on each
> call. The change introduces a static reference to a GCfunc object for
> <luaT_newthread_wrapper> to be initialized on Tarantool startup to
> reduce Lua GC memory usage.
> 
> Furthermore, since <lua_cpcall> yields nothing on guest stack, the newly
> created Lua coroutine need to be pushed back to prevent its sweep. So
> to reduce guest stack manipulations <lua_cpcall> is replaced with
> <lua_pcall> and the resulting Lua thread is obtained via guest stack.
> 
> Signed-off-by: Igor Munkin <imun at tarantool.org>
> ---
> Recently we discussed with Timur his struggling with linking his binary
> Lua module against Tarantool. The reason is LuaJIT internals usage for
> manipulations with the guest stack that are not provided by the binary.
> I glanced the current <luaT_newthread> implementation and found out two
> another problems related to the platform overall performance (as it is
> proved with the corresponding benchmarks).
> 
> The first problem is the similar one <box_process_lua> had prior to the
> patch[1]: <lua_cpcall> creates an auxiliary GCfunc object for the
> function to be called in protected frame. However this function is the
> same throughout the platform uptime. It can be created on Taranool
> startup and I see no reason to clobber GC that way.
> 
> Another problem I found are excess manipulations with the guest stack:
> one need the newly born coroutine on it to prevent it being collected by
> GC, but <lua_cpcall> purges everything left on the stack in scope of the
> invoked function. As a result the obtained Lua coroutine is "pushed"
> back to the guest stack via LuaJIT internal interfaces. It's a bit
> ridiculous, since one can just use public API to get the same results:
> Lua coroutine on the guest stack and the corresponding pointer to it.
> 
> I tested the platform performance with the same benchmark[2] I made for
> the <box_process_lua> patch and here are the numbers I obtained after
> the 15 runs:
> * Vanilla bleeding master (mean):
> | ===== 2.5.0-267-gbf047ad44 =====
> | call by ref GC: 921877 Kb
> | call by ref time: 1.340172 sec
> | call GC: 476563.991667 Kb
> | eval GC: 655274.935547 Kb
> * Patched bleeding master (mean):
> | ===== 2.5.0-268-gec0eb12f4 =====
> | call by ref GC: 859377 Kb
> | call by ref time: 1.215410 sec
> | call GC: 445313 Kb
> | eval GC: 624024 Kb
> * Relative measurements (before -> after):
> | call by ref GC: -6% (-62500 Kb)
> | call by ref time: -9% (-0.124762 sec)
> | call GC: -6% (-31250 Kb)
> | eval GC: -4% (-31250 Kb)
> 
> There is one hot path I left unverified -- Lua-born fibers creation, but
> I guess the relative numbers are quite similar to the ones I mentioned
> above. However, if one wonders these results, feel free to ask me.
> 
> Branch: https://github.com/tarantool/tarantool/tree/imun/experimental-luaT-newthread
> 
> @ChangeLog:
> * Improved safe Lua coroutine creation for the case of fiber
>   initialization. Prepared GCfunc object is used instead of temporary
>   one, resulting in 3-6% garbage collection reduction. Also excess guest
>   stack manipulations are removed.
> 
>  src/lua/utils.c | 32 ++++++++++++++++++++++++++++++++
>  src/lua/utils.h | 38 ++++++++++----------------------------
>  2 files changed, 42 insertions(+), 28 deletions(-)
> 
> [1]: https://lists.tarantool.org/pipermail/tarantool-patches/2020-June/017834.html
> [2]: https://gist.github.com/igormunkin/c941074fa9fdf0f7a4f068498fb5e24c
> 

<snipped>

> 

-- 
Best regards,
IM