[Tarantool-patches] [PATCH v2] rfc: luajit metrics

[Sergey Kaplun <skaplun@tarantool.org>]

Part of #5187
---

This patch adds RFC to LuaJIT metrics interfaces. Nevertheless name
`misc` for builtin library is not good and should be discussed, because
tons of user modules can use that name for their own libraies.

Branch: https://github.com/tarantool/tarantool/tree/skaplun/5187-luajit-metrics
Issue: https://github.com/tarantool/tarantool/issues/5187

Changes in v2:
- Fixed typos
- Made comments more verbose
- Avoided flushing any of metrics after each call of luaM_metrics()

 doc/rfc/5187-luajit-metrics.md | 126 +++++++++++++++++++++++++++++++++
 1 file changed, 126 insertions(+)
 create mode 100644 doc/rfc/5187-luajit-metrics.md

diff --git a/doc/rfc/5187-luajit-metrics.md b/doc/rfc/5187-luajit-metrics.md
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+# LuaJIT metrics
+
+* **Status**: In progress
+* **Start date**: 17-07-2020
+* **Authors**: Sergey Kaplun @Buristan skaplun@tarantool.org,
+               Igor Munkin @igormunkin imun@tarantool.org,
+               Sergey Ostanevich @sergos sergos@tarantool.org
+* **Issues**: [#5187](https://github.com/tarantool/tarantool/issues/5187)
+
+## Summary
+
+LuaJIT metrics provide extra information about the Lua state. They consists of
+GC metrics (overall amount of objects and memory usage), JIT stats (both
+related to the compiled traces and the engine itself), string hash hits/misses.
+
+## Background and motivation
+
+One can be curious about their application performance. We are going to provide
+various metrics about the several platform subsystems behaviour. GC pressure
+produced by user code can weight down all application performance. Irrelevant
+traces compiled by the JIT engine can just burn CPU time with no benefits as a
+result. String hash collisions can lead to DoS caused by a single request. All
+these metrics should be well monitored by users wanting to improve the
+performance of their application.
+
+## Detailed design
+
+For C API we introduce additional extension header <lmisclib.h> that provides
+interfaces for new LuaJIT C API extensions. The first interface in this header
+will be the following:
+
+```
+/* API for obtaining various metrics from the platform. */
+
+LUAM_API struct luam_Metrics *luaM_metrics(lua_State *L,
+					   struct luam_Metrics *dst);
+```
+
+This function fills the structure pointed by `dst` with the corresponding
+metrics related to Lua state anchored to the given coroutine `L`. The result of
+the function is a pointer to the filled structure (the same `dst` points to).
+
+The `struct luam_Metrics` has the following definition:
+
+```
+struct luam_Metrics {
+	/*
+	 * Strings amount founded in string hash
+	 * instead of allocation of new one.
+	 */
+	size_t strhash_hit;
+	/* Strings amount allocated and put into string hash. */
+	size_t strhash_miss;
+
+	size_t strnum;   /* Amount of allocated string objects. */
+	size_t tabnum;   /* Amount of allocated table objects. */
+	size_t udatanum; /* Amount of allocated udata objects. */
+	size_t cdatanum; /* Amount of allocated cdata objects. */
+
+	/* Memory currently allocated. */
+	size_t gc_total;
+	/* Total amount of freed memory. */
+	size_t gc_freed;
+	/* Total amount of allocated memory. */
+	size_t gc_allocated;
+
+	/* Count of incremental GC steps per state. */
+	size_t gc_steps_pause;
+	size_t gc_steps_propagate;
+	size_t gc_steps_atomic;
+	size_t gc_steps_sweepstring;
+	size_t gc_steps_sweep;
+	size_t gc_steps_finalize;
+
+	/*
+	 * Overall number of snap restores (and number of stopped
+	 * trace executions) for given jit_State.
+	 */
+	size_t jit_snap_restore;
+	/* Overall number of abort traces for given jit_State. */
+	size_t jit_trace_abort;
+	/* Total size of all allocated machine code areas. */
+	size_t jit_mcode_size;
+	/* Amount of JIT traces. */
+	unsigned int jit_trace_num;
+};
+```
+
+All metrics are collected throughout the platform uptime. But some of them
+(namely `strhash_hit`, `strhash_miss`, `gc_freed`, `gc_allocated`,
+`gc_steps_pause`, `gc_steps_propagate`, `gc_steps_atomic`,
+`gc_steps_sweepstring`, `gc_steps_sweep`, `gc_steps_finalize`,
+`jit_snap_restore` and `jit_trace_abort`) increase monotonic and can overflow.
+They make sense only with comparing with their value from a previous
+`luaM_metrics()` call.
+
+There is also a complement introduced for Lua space -- `misc.getmetrics()`.
+This function is just a wrapper for `luaM_metrics()` returning a Lua table with
+the similar metrics. Its usage is quite simple:
+```
+$ ./src/tarantool
+Tarantool 2.5.0-267-gbf047ad44
+type 'help' for interactive help
+tarantool> misc.getmetrics()
+---
+- tabnum: 1812
+  gc_total: 1369927
+  strnum: 5767
+  jit_trace_num: 0
+  cdatanum: 89
+  jit_mcode_size: 0
+  udatanum: 17
+  jit_snap_restore: 0
+  gc_freed: 2239391
+  strhash_hit: 53759
+  gc_steps_finalize: 0
+  gc_allocated: 3609318
+  gc_steps_atomic: 6
+  gc_steps_sweep: 296
+  gc_steps_sweepstring: 17920
+  jit_trace_abort: 0
+  strhash_miss: 6874
+  gc_steps_propagate: 10106
+  gc_steps_pause: 7
+...
+```
-- 
2.24.1