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

[Sergey Ostanevich <sergos@tarantool.org>]

Hi!

Thanks for the patch, LGTM.

I wonder if it can be done in a more safe way in luaT_newthread() so
that if the refernece is not initialized then initialize it, rather 
assert in debug and - I suppose - fail with creation in release? I see
an overhead for condition here and making even bigger indirect 
machinery for the wrappers itself won't bring a lot either.

Regards,
Sergos


On 20 Jul 14:28, 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@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
> 
> diff --git a/src/lua/utils.c b/src/lua/utils.c
> index 0b05d7257..23ccbc3c9 100644
> --- a/src/lua/utils.c
> +++ b/src/lua/utils.c
> @@ -43,6 +43,8 @@ int luaL_nil_ref = LUA_REFNIL;
>  int luaL_map_metatable_ref = LUA_REFNIL;
>  int luaL_array_metatable_ref = LUA_REFNIL;
>  
> +static int luaT_newthread_ref = LUA_NOREF;
> +
>  static uint32_t CTID_STRUCT_IBUF;
>  static uint32_t CTID_STRUCT_IBUF_PTR;
>  static uint32_t CTID_CHAR_PTR;
> @@ -1224,6 +1226,33 @@ void luaL_iterator_delete(struct luaL_iterator *it)
>  
>  /* }}} */
>  
> +/**
> + * @brief A wrapper for <lua_newthread> to be called via luaT_call
> + * in luaT_newthread. Whether new Lua coroutine is created it is
> + * returned on the top of the guest stack.
> + * @param L is a Lua state
> + * @sa <lua_newthread>
> + */
> +static int
> +luaT_newthread_wrapper(lua_State *L)
> +{
> +	(void)lua_newthread(L);
> +	return 1;
> +}
> +
> +lua_State *
> +luaT_newthread(lua_State *L)
> +{
> +	assert(luaT_newthread_ref != LUA_NOREF);
> +	lua_rawgeti(L, LUA_REGISTRYINDEX, luaT_newthread_ref);
> +	assert(lua_isfunction(L, -1));
> +	if (luaT_call(L, 0, 1) != 0)
> +		return NULL;
> +	lua_State *L1 = lua_tothread(L, -1);
> +	assert(L1 != NULL);
> +	return L1;
> +}
> +
>  int
>  tarantool_lua_utils_init(struct lua_State *L)
>  {
> @@ -1274,5 +1303,8 @@ tarantool_lua_utils_init(struct lua_State *L)
>  	(void) rc;
>  	CTID_UUID = luaL_ctypeid(L, "struct tt_uuid");
>  	assert(CTID_UUID != 0);
> +
> +	lua_pushcfunction(L, luaT_newthread_wrapper);
> +	luaT_newthread_ref = luaL_ref(L, LUA_REGISTRYINDEX);
>  	return 0;
>  }
> diff --git a/src/lua/utils.h b/src/lua/utils.h
> index b10754e4a..404eafe9b 100644
> --- a/src/lua/utils.h
> +++ b/src/lua/utils.h
> @@ -597,34 +597,16 @@ luaL_checkfinite(struct lua_State *L, struct luaL_serializer *cfg,
>  }
>  
>  /**
> - * @brief A wrapper for lua_newthread() to pass it into luaT_cpcall
> - * @param L is a Lua State
> - * @sa lua_newthread()
> - */
> -static inline int
> -luaT_newthread_wrapper(lua_State *L)
> -{
> -	*(lua_State **)lua_touserdata(L, 1) = lua_newthread(L);
> -	return 0;
> -}
> -
> -/**
> - * @brief Safe wrapper for lua_newthread()
> - * @param L is a Lua State
> - * @sa lua_newthread()
> - */
> -static inline lua_State *
> -luaT_newthread(lua_State *L)
> -{
> -	lua_State *L1 = NULL;
> -	if (luaT_cpcall(L, luaT_newthread_wrapper, &L1) != 0) {
> -		return NULL;
> -	}
> -	assert(L1 != NULL);
> -	setthreadV(L, L->top, L1);
> -	incr_top(L);
> -	return L1;
> -}
> + * @brief Creates a new Lua coroutine in a protected frame. If
> + * <lua_newthread> call underneath succeeds, the created Lua state
> + * is on the top of the guest stack and a pointer to this state is
> + * returned. Otherwise LUA_ERRMEM error is handled and the result
> + * is NULL.
> + * @param L is a Lua state
> + * @sa <lua_newthread>
> + */
> +lua_State *
> +luaT_newthread(lua_State *L);
>  
>  /**
>   * Check if a value on @a L stack by index @a idx is an ibuf
> -- 
> 2.25.0
>