[Tarantool-patches] [RFC v3] 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 libraries.

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()
Changes in v3:
- Added colors count metrics description
- Added description about how metrics are collected
- Added benchmarks

 doc/rfc/5187-luajit-metrics.md | 218 +++++++++++++++++++++++++++++++++
 1 file changed, 218 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 consist 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
+
+The additional header <lmisclib.h> is introduced to extend the existing LuaJIT
+C API with new interfaces. The first function provided via this header is the
+following:
+
+```
+/* API for obtaining various platform metrics. */
+
+LUAMISC_API void luaM_metrics(lua_State *L, struct luam_Metrics *metrics);
+```
+
+This function fills the structure pointed to by `metrics` with the corresponding
+metrics related to Lua state anchored to the given coroutine `L`.
+
+The `struct luam_Metrics` has the following definition:
+
+```
+struct luam_Metrics {
+  /* Strings amount found 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;
+
+  /* Amount of allocated string objects. */
+  size_t gc_strnum;
+  /* Amount of allocated table objects. */
+  size_t gc_tabnum;
+  /* Amount of allocated udata objects. */
+  size_t gc_udatanum;
+  /* Amount of allocated cdata objects. */
+  size_t gc_cdatanum;
+
+  /* Amount of white objects. */
+  size_t gc_whitenum;
+  /* Amount of gray objects. */
+  size_t gc_graynum;
+  /* Amount of black objects. */
+  size_t gc_blacknum;
+
+  /* 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 (amount of guard assertions
+  ** leading to stopping trace executions and trace exits,
+  ** that are not stitching with other traces).
+  */
+  size_t jit_snap_restore;
+  /* Overall number of abort traces. */
+  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;
+};
+```
+
+Couple of words about how metrics are collected:
+- `strhash_*` -- whenever existing string is returned after attemption to
+  create new string there is incremented `strhash_hit` counter, if new string
+  created then `strhash_miss` is incremented instead.
+- `gc_*num`, `jit_trace_num` -- corresponding counter incremented whenever new
+  object is allocated. When object become garbage collected its counter is
+  decremented.
+- `gc_whitenum`, `gc_graynum`, `gc_blacknum` -- in so far as all objects are
+  created  as the current white, `gc_whitenum` is incremented at any object
+  creation. Whenever  color of object changes counter for old color is
+  decremented and counter for  new color is incremented instead.
+  *NB*: after full cycle of Garbage Collector there are only white objects.
+- `gc_total`, `gc_allocated`, `gc_freed` -- any time when allocation function
+  is called `gc_allocated` and/or `gc_freed` is increased and `gc_total`
+  increase when memory is allocated or reallocated, decrease when memory is
+  freed.
+- `gc_steps_*` -- corresponding counter increments whenever Garbage Collector
+  starts to execute 1 step of garbage collection.
+- `jit_snap_restore` -- whenever JIT machine exits from the trace and restores
+  interpreter state `jit_snap_restore` counter is incremented.
+- `jit_trace_abort` -- whenever JIT compiler can't record the trace in case NYI
+  BC this counter is incremented.
+- `jit_mcode_size` -- whenever new MCode area is allocated `jit_mcode_size` is
+  increased at corresponding size in bytes. Sets to 0 when all mcode area is
+  freed.
+
+All metrics are collected throughout the platform uptime. These metrics
+increase monotonically and can overflow:
+  - `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`
+  - `jit_trace_abort`
+
+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. All returned values are presented as numbers with cast to
+double, so there is a corresponding precision loss. Function usage is quite
+simple:
+```
+$ ./src/tarantool
+Tarantool 2.5.0-267-gbf047ad44
+type 'help' for interactive help
+tarantool> misc.getmetrics()
+---
+- gc_graynum: 4443
+  strhash_hit: 53965
+  gc_steps_atomic: 6
+  strhash_miss: 6879
+  gc_steps_sweepstring: 17920
+  gc_strnum: 5759
+  gc_tabnum: 1813
+  gc_cdatanum: 89
+  jit_snap_restore: 0
+  gc_total: 1370836
+  gc_udatanum: 17
+  gc_steps_finalize: 0
+  gc_allocated: 3616689
+  jit_trace_num: 0
+  gc_whitenum: 3460
+  jit_mcode_size: 0
+  gc_steps_sweep: 297
+  jit_trace_abort: 0
+  gc_freed: 2245853
+  gc_steps_pause: 7
+  gc_steps_propagate: 10171
+  gc_blacknum: 3979
+...
+```
+
+## Benchmarks
+
+Benchmarks was taken from repo:
+[LuaJIT-test-cleanup](https://github.com/LuaJIT/LuaJIT-test-cleanup).
+
+Example of usage:
+```
+/usr/bin/time -f"array3d %U" ./luajit $BENCH_DIR/array3d.lua  300 >/dev/null
+```
+Taking into account the measurement error ~ 2%, it can be said that there is no
+difference in the performance.
+
+Benchmark results after and before patch (less is better):
+```
+   Benchmark   | AFTER (s) | BEFORE (s)
+---------------+-----------+-----------
+array3d        |   0.21    |   0.20
+binary-trees   |   3.34    |   3.24
+chameneos      |   2.95    |   2.99
+coroutine-ring |   1.02    |   1.02
+euler14-bit    |   1.04    |   1.05
+fannkuch       |   6.99    |   6.81
+fasta          |   8.28    |   8.28
+life           |   0.48    |   0.46
+mandelbrot     |   2.66    |   2.68
+mandelbrot-bit |   2.01    |   1.97
+md5            |   1.59    |   1.54
+nbody          |   1.36    |   1.56
+nsieve         |   2.11    |   2.06
+nsieve-bit     |   1.54    |   1.50
+nsieve-bit-fp  |   4.51    |   4.60
+partialsums    |   0.58    |   0.55
+pidigits-nogmp |   3.48    |   3.46
+ray            |   1.62    |   1.63
+recursive-ack  |   0.19    |   0.20
+recursive-fib  |   1.64    |   1.67
+scimark-fft    |   5.84    |   5.86
+scimark-lu     |   3.33    |   3.64
+scimark-sor    |   2.34    |   2.34
+scimark-sparse |   4.99    |   4.93
+series         |   0.95    |   0.94
+spectral-norm  |   0.95    |   0.97
+```
-- 
2.28.0