[Tarantool-discussions] [RFC luajit v3] rfc: describe a LuaJIT memory profiler
Sergey Kaplun
skaplun at tarantool.org
Fri Dec 25 14:34:31 MSK 2020
Part of #5442
---
RFC on branch: https://github.com/tarantool/tarantool/blob/skaplun/gh-5442-luajit-memory-profiler/doc/rfc/5442-luajit-memory-profiler.md
Changes in v3:
* More comments in example.
* More verbose benchmark information.
* Grammar and spelling fixes.
Changes in v2:
* Removed C API, Tarantool integration and description of additional
features -- they will be added in another RFC if necessary.
* Removed checking profile is running from the public API.
* Added benchmarks and more meaningful example.
* Grammar fixes.
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+# LuaJIT memory profiler
+
+* **Status**: In progress
+* **Start date**: 24-10-2020
+* **Authors**: Sergey Kaplun @Buristan skaplun at tarantool.org,
+ Igor Munkin @igormunkin imun at tarantool.org,
+ Sergey Ostanevich @sergos sergos at tarantool.org
+* **Issues**: [#5442](https://github.com/tarantool/tarantool/issues/5442)
+
+## Summary
+
+LuaJIT memory profiler is a toolchain for analysis of memory usage by user's
+application.
+
+## Background and motivation
+
+Garbage collector (GC) is a curse of performance for most of Lua applications.
+Memory usage of Lua application should be profiled to locate various
+memory-unoptimized code blocks. If the application has memory leaks they can be
+found with the profiler also.
+
+## Detailed design
+
+The whole toolchain of memory profiling will be divided into several parts:
+1) [Prerequisites](#prerequisites).
+2) [Recording information about memory usage and saving it](#information-recording).
+3) [Reading saved data and display it in human-readable format](#reading-and-displaying-saved-data).
+
+### Prerequisites
+
+This section describes additional changes in LuaJIT required for the feature
+implementation. This version of LuaJIT memory profiler does not support verbose
+reporting allocations from traces. All allocation from traces are reported as
+internal. But trace code semantics should be totally the same as for the Lua
+interpreter (excluding sink optimizations). Also all deallocations reported as
+internal too.
+
+There are two different representations of functions in LuaJIT: the function's
+prototype (`GCproto`) and the function object so called closure (`GCfunc`).
+The closures are represented as `GCfuncL` and `GCfuncC` for Lua and C closures
+correspondingly. Also LuaJIT has a special function's type aka Fast Function.
+It is used for LuaJIT builtins.
+
+Tail call optimization does not create a new call frame, so all allocations
+inside the function called via `CALLT`/`CALLMT` are attributed to its caller.
+
+Usually developers are not interested in information about allocations inside
+builtins. So if fast function was called from a Lua function all
+allocations are attributed to this Lua function. Otherwise attribute this event
+to a C function.
+
+Assume we have the following Lua chunk named <test.lua>:
+
+```
+1 jit.off()
+2 misc.memprof.start("memprof_new.bin")
+3 -- Lua does not create a new frame to call string.rep and all allocations are
+4 -- attributed not to `append()` function but to the parent scope.
+5 local function append(str, rep)
+6 return string.rep(str, rep)
+7 end
+8
+9 local t = {}
+10 for _ = 1, 1e5 do
+11 -- table.insert is a builtin and all corresponding allocations
+12 -- are reported in the scope of main chunk
+13 table.insert(t,
+14 append('q', _)
+15 )
+16 end
+17 misc.memprof.stop()
+```
+
+If one run the chunk above the profiler reports approximately the following
+(see legend [here](#reading-and-displaying-saved-data)):
+```
+ALLOCATIONS
+ at test.lua:0, line 14: 1002 531818 0
+ at test.lua:0, line 13: 1 24 0
+ at test.lua:0, line 9: 1 32 0
+ at test.lua:0, line 7: 1 20 0
+
+REALLOCATIONS
+ at test.lua:0, line 13: 9 16424 8248
+ Overrides:
+ @test.lua:0, line 13
+
+ at test.lua:0, line 14: 5 1984 992
+ Overrides:
+ @test.lua:0, line 14
+
+
+DEALLOCATIONS
+INTERNAL: 20 0 1481
+ at test.lua:0, line 14: 3 0 7168
+ Overrides:
+ @test.lua:0, line 14
+```
+
+So we need to know a type of function being executed by the virtual machine
+(VM). Currently VM state identifies C function execution only, so Fast and Lua
+functions states will be added.
+
+To determine currently allocating coroutine (that may not be equal to currently
+executed one) a new field called `mem_L` is added to `global_State` structure
+to keep the coroutine address. This field is set at each reallocation to
+corresponding `L` with which it was called.
+
+There is a static function (`lj_debug_getframeline`) that returns line number
+for current `BCPos` in `lj_debug.c` already. It will be added to the debug
+module API to be used in memory profiler.
+
+### Information recording
+
+Each allocate/reallocate/free is considered as a type of event that are
+reported. Event stream has the following format:
+
+```c
+/*
+** Event stream format:
+**
+** stream := symtab memprof
+** symtab := see symtab description
+** memprof := prologue event* epilogue
+** prologue := 'l' 'j' 'm' version reserved
+** version := <BYTE>
+** reserved := <BYTE> <BYTE> <BYTE>
+** event := event-alloc | event-realloc | event-free
+** event-alloc := event-header loc? naddr nsize
+** event-realloc := event-header loc? oaddr osize naddr nsize
+** event-free := event-header loc? oaddr osize
+** event-header := <BYTE>
+** loc := loc-lua | loc-c
+** loc-lua := sym-addr line-no
+** loc-c := sym-addr
+** sym-addr := <ULEB128>
+** line-no := <ULEB128>
+** oaddr := <ULEB128>
+** naddr := <ULEB128>
+** osize := <ULEB128>
+** nsize := <ULEB128>
+** epilogue := event-header
+**
+** <BYTE> : A single byte (no surprises here)
+** <ULEB128>: Unsigned integer represented in ULEB128 encoding
+**
+** (Order of bits below is hi -> lo)
+**
+** version: [VVVVVVVV]
+** * VVVVVVVV: Byte interpreted as a plain integer version number
+**
+** event-header: [FUUUSSEE]
+** * EE : 2 bits for representing allocation event type (AEVENT_*)
+** * SS : 2 bits for representing allocation source type (ASOURCE_*)
+** * UUU : 3 unused bits
+** * F : 0 for regular events, 1 for epilogue's *F*inal header
+** (if F is set to 1, all other bits are currently ignored)
+*/
+```
+
+It is enough to know the address of LUA/C function to determine it. Symbolic
+table (symtab) is dumped at the start of profiling to avoid dumping function
+location on each memory event for saving both CPU usage and binary profile
+size.
+
+Each line contains the address, Lua chunk definition as the filename and line
+number of the function's declaration. This table of symbols has the following
+format described at <lj_memprof.h>:
+
+```c
+/*
+** symtab format:
+**
+** symtab := prologue sym*
+** prologue := 'l' 'j' 's' version reserved
+** version := <BYTE>
+** reserved := <BYTE> <BYTE> <BYTE>
+** sym := sym-lua | sym-final
+** sym-lua := sym-header sym-addr sym-chunk sym-line
+** sym-header := <BYTE>
+** sym-addr := <ULEB128>
+** sym-chunk := string
+** sym-line := <ULEB128>
+** sym-final := sym-header
+** string := string-len string-payload
+** string-len := <ULEB128>
+** string-payload := <BYTE> {string-len}
+**
+** <BYTE> : A single byte (no surprises here)
+** <ULEB128>: Unsigned integer represented in ULEB128 encoding
+**
+** (Order of bits below is hi -> lo)
+**
+** version: [VVVVVVVV]
+** * VVVVVVVV: Byte interpreted as a plain numeric version number
+**
+** sym-header: [FUUUUUTT]
+** * TT : 2 bits for representing symbol type
+** * UUUUU : 5 unused bits
+** * F : 1 bit marking the end of the symtab (final symbol)
+*/
+```
+
+So when memory profiling starts the current allocation function is replaced by
+the new allocation function additionally wrapped to write the profiling events.
+When profiler stops the previous allocation function is restored.
+
+Starting profiler from Lua is quite simple:
+```lua
+local started, err, errno = misc.memprof.start(fname)
+```
+where `fname` is name of the file where profile events are written. Writer for
+this function perform `fwrite()` for each call retrying in case of `EINTR`.
+When the profiling is stopped the `fclose()` is called. If it is impossible to
+open a file for writing or profiler fails to start, returns `nil` on failure
+(plus an error message as a second result and a system-dependent error code as
+a third result). Otherwise returns some true value.
+
+Stopping profiler from Lua is simple too:
+```lua
+local stopped, err, errno = misc.memprof.stop()
+```
+
+If there is any error occurred at profiling stopping (an error when file
+descriptor was closed) `memprof.stop()` returns `nil` (plus an error message as
+a second result and a system-dependent error code as a third result). Returns
+`true` otherwise.
+
+If you want to build LuaJIT without memory profiler, you should build it with
+`-DLUAJIT_DISABLE_MEMPROF`. If it is disabled `misc.memprof.start()` and
+`misc.memprof.stop()` always return `false`.
+
+Memory profiler is expected to be thread safe, so it has a corresponding
+lock/unlock at internal mutex whenever you call corresponding memprof
+functions. If you want to build LuaJIT without thread safety use
+`-DLUAJIT_DISABLE_THREAD_SAFE`.
+
+### Reading and displaying saved data
+
+Binary data can be read by `lj-parse-memprof` utility. It parses the binary
+format provided by memory profiler and render it on human-readable format.
+
+The usage is very simple:
+```
+$ ./luajit-parse-memprof --help
+luajit-parse-memprof - parser of the memory usage profile collected
+ with LuaJIT's memprof.
+
+SYNOPSIS
+
+luajit-parse-memprof [options] memprof.bin
+
+Supported options are:
+
+ --help Show this help and exit
+```
+
+Plain text of profiled info has the following format:
+```
+@<filename>:<function_line>, line <line where event was detected>: <number of events> <allocated> <freed>
+```
+See the example [above](#prerequisites).
+
+`INTERNAL` means that these allocations are caused by internal LuaJIT
+structures. Note that events are sorted from the most often to the least.
+
+`Overrides` means what allocation this reallocation overrides.
+
+## Benchmarks
+
+Benchmarks were taken from repo:
+[LuaJIT-test-cleanup](https://github.com/LuaJIT/LuaJIT-test-cleanup).
+
+Example of measuring:
+```bash
+/usr/bin/time -f"array3d %U" ./luajit $BENCH_DIR/array3d.lua 300 >/dev/null
+```
+
+This table shows performance deviation in relation to REFerence value (before
+commit) with stopped and running profiler. The table shows the average value
+for 11 runs. The first field of the column indicates the change in the average
+time in seconds (less is better). The second field is the standard deviation
+for the found difference.
+
+```
+ Name | REF | AFTER, memprof off | AFTER, memprof on
+----------------+------+--------------------+------------------
+array3d | 0.21 | +0.00 (0.01) | +0.00 (0.01)
+binary-trees | 3.25 | -0.01 (0.06) | +0.53 (0.10)
+chameneos | 2.97 | +0.14 (0.04) | +0.13 (0.06)
+coroutine-ring | 1.00 | +0.01 (0.04) | +0.01 (0.04)
+euler14-bit | 1.03 | +0.01 (0.02) | +0.00 (0.02)
+fannkuch | 6.81 | -0.21 (0.06) | -0.20 (0.06)
+fasta | 8.20 | -0.07 (0.05) | -0.08 (0.03)
+life | 0.46 | +0.00 (0.01) | +0.35 (0.01)
+mandelbrot | 2.65 | +0.00 (0.01) | +0.01 (0.01)
+mandelbrot-bit | 1.97 | +0.00 (0.01) | +0.01 (0.02)
+md5 | 1.58 | -0.01 (0.04) | -0.04 (0.04)
+nbody | 1.34 | +0.00 (0.01) | -0.02 (0.01)
+nsieve | 2.07 | -0.03 (0.03) | -0.01 (0.04)
+nsieve-bit | 1.50 | -0.02 (0.04) | +0.00 (0.04)
+nsieve-bit-fp | 4.44 | -0.03 (0.07) | -0.01 (0.07)
+partialsums | 0.54 | +0.00 (0.01) | +0.00 (0.01)
+pidigits-nogmp | 3.47 | -0.01 (0.02) | -0.10 (0.02)
+ray | 1.62 | -0.02 (0.03) | +0.00 (0.02)
+recursive-ack | 0.20 | +0.00 (0.01) | +0.00 (0.01)
+recursive-fib | 1.63 | +0.00 (0.01) | +0.01 (0.02)
+scimark-fft | 5.72 | +0.06 (0.09) | -0.01 (0.10)
+scimark-lu | 3.47 | +0.02 (0.27) | -0.03 (0.26)
+scimark-sor | 2.34 | +0.00 (0.01) | -0.01 (0.01)
+scimark-sparse | 4.95 | -0.02 (0.04) | -0.02 (0.04)
+series | 0.95 | +0.00 (0.02) | +0.00 (0.01)
+spectral-norm | 0.96 | +0.00 (0.02) | -0.01 (0.02)
+```
--
2.28.0
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