[Tarantool-discussions] [RFC luajit v2] rfc: describe a LuaJIT memory profiler
Sergey Kaplun
skaplun at tarantool.org
Fri Dec 25 16:14:43 MSK 2020
Hi!
Thanks for the review!
On 22.12.20, Sergey Ostanevich wrote:
> Hi!
>
> Thanks for the patch!
>
> See some comments below, after being applied is LGTM.
Please, see new verison here [1].
>
> Sergos
>
> > On 16 Dec 2020, at 22:09, Sergey Kaplun <skaplun at tarantool.org> wrote:
> >
> > Part of #5442
> > ---
> >
> > Issues: https://github.com/tarantool/tarantool/issues/5442
> > Branch: https://github.com/tarantool/tarantool/tree/skaplun/gh-5442-luajit-memory-profiler
> >
> > 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.
> >
> > doc/rfc/5442-luajit-memory-profiler.md | 306 +++++++++++++++++++++++++
> > 1 file changed, 306 insertions(+)
> > create mode 100644 doc/rfc/5442-luajit-memory-profiler.md
> >
> > diff --git a/doc/rfc/5442-luajit-memory-profiler.md b/doc/rfc/5442-luajit-memory-profiler.md
> > new file mode 100644
> > index 000000000..720105009
> > --- /dev/null
> > +++ b/doc/rfc/5442-luajit-memory-profiler.md
> > @@ -0,0 +1,306 @@
> > +# 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.
>
> This is something that is very questionable. Do you have any data on performance
> of Lua code against the GC code in real life? Is there any about Tarantool
> applications?
>
> I’m not questioning the mem profiler itself, rather trying to weigh the GC problem.
I can give benchmarks for vshard [2] for now. There are approximately
20% of time spent in `lj_gc_steps()`, IINM.
>
> > +Memory usage of Lua application should be profiled to find out various
> ^^^^^^^ locate
Fixed.
>
> > +memory-unoptimized code blocks. If the application has memory leaks they can be
> > +found with the profiler.
> ^ also.
Fixed.
> > +
> > +## Detailed design
> > +
> > +The whole toolchain of memory profiling will be divided by several parts:
> ^^^ into
Thank you! Fixed.
> > +1) Prerequisites.
> > +2) Recording information about memory usage and saving it.
> > +3) Reading saved data and display it in human-readable format.
> > +
> > +### Prerequisites
> > +
> > +This section describes additional changes in LuaJIT required to feature
> ^^ for the
Fixed.
> > +implementation. This version of LuaJIT memory profiler does not support
> > +reporting allocations from traces. But trace code semantics should be totally
> > +the same as for Lua interpreter. So profiling with `jit.off()` should be
> > +enough.
> > +
> > +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 special function's type aka Fast Function. It
> ^ a
Fixed.
> > +is used for LuaJIT builtins.
> > +
> > +Fast function allocation events always belong to the previous frame with
> > +considering of tail call optimizations (TCO).
> > +
> > +Assume we have the following Lua chunk named <test.lua>:
> > +
> > +```
> > +1 jit.off()
> > +2 misc.memprof.start("memprof_new.bin")
> > +3 local function append(str, rep)
> > +4 return string.rep(str, rep)
> > +5 end
> > +6
> > +7 local t = {}
> > +8 for _ = 1, 1e5 do
> > +9 table.insert(t,
> > +10 append('q', _)
> > +11 )
> > +12 end
> > +13 misc.memprof.stop()
> > +```
> > +
> > +Profilers output is like the follows:
> > +```
> > +ALLOCATIONS
> > + at test.lua:0, line 10: 100007 5004638934 0
> > + at test.lua:0, line 5: 1 40 0
> > + at test.lua:0, line 7: 1 72 0
> > + at test.lua:0, line 9: 1 48 0
> > +
> > +REALLOCATIONS
> > + at test.lua:0, line 9: 16 4194496 2097376
> > + Overrides:
> > + @test.lua:0, line 9
> > +
> > + at test.lua:0, line 10: 12 262080 131040
> > + Overrides:
> > + @test.lua:0, line 10
> > +
> > +
> > +DEALLOCATIONS
> > +INTERNAL: 21 0 2463
> > + at test.lua:0, line 10: 8 0 1044480
> > + Overrides:
> > + @test.lua:0, line 10
> > +```
> > +
> > +In Lua functions for profile events, we had to determine the line number of the
> > +function definition and corresponding `GCproto` address. For C functions only
> > +address will be enough. If Fast function is called from Lua function we had to
> > +report the Lua function for more meaningful output. Otherwise report the C
> > +function.
> > +
> > +So we need to know in what type of function CALL/RETURN virtual machine (VM)
> ^^^^^^^ a ^ the
> > +is. LuaJIT has already determined C function execution VM state but neither
> ^ currently in.
>
> > +is. LuaJIT has already determined C function execution VM state but neither
> Currently VM state identifies C function execution only, so Fast and Lua
> functions states will be added.
Reformulated in the new version.
>
> > +Fast functions nor Lua function. So corresponding VM states will be added.
> > +
> > +To determine currently allocating coroutine (that may not be equal to currently
> > +executed) new field will be added to `global_State` structure named `mem_L`
> ^ a
Fixed.
>
> > +kept coroutine address. This field sets at each reallocation to corresponding
> ^^^ to keep the ^^^ is set
Fixed.
>
> > +`L` with which it was called.
> > +
> > +There is the static function (`lj_debug_getframeline`) returned line number for
> ^^^ a ^^^ that returns
Fixed.
> > +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 <ljp_symtab.h>
> > +** 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: [FTUUSSEE]
> > +** * EE : 2 bits for representing allocation event type (AEVENT_*)
> > +** * SS : 2 bits for representing allocation source type (ASOURCE_*)
> > +** * UU : 2 unused bits
> > +** * T : Reserved. 0 for regular events, 1 for the events marked with
> > +** the timestamp mark. It is assumed that the time distance between
>
> No representation of a timestamp so far? If so - maybe remove excessive description
> and put ‘reserved for timestamp’.
I've dropped it according your and Igors comments.
>
> > +** two marked events is approximately the same and is equal
> > +** to 1 second. Always zero for now.
> > +** * 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) dumps at start of profiling to avoid determine and write line
> > +number of Lua code and corresponding chunk of code each time, when memory event
> > +happens. 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 <ljp_symtab.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 default allocation function is replaced by the
> > +new allocation function as additional wrapper to write inspected profiling
> > +events. When profiler stops old allocation function is substituted back.
> > +
> > +Starting profiler from Lua is quite simple:
> > +```lua
> > +local started, err = 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`.
> > +Final callback calls `fclose()` at the end of profiling. 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 = 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
>
> Why do you need to separate an ‘error’ from a ’system error’?
I've used the same formulation as in Lua Reference Manual [3].
It's errno, in fact, but IINM it is system dependent.
>
> > +`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 `luaM_memprof_*`. 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 from memory profiler and render it in 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 example above.
> > +
> > +`INTERNAL` means that this 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 usage:
> Either ‘usage’ or ‘example of use’
Fixed, thank you!
>
>
> > +```bash
> > +/usr/bin/time -f"array3d %U" ./luajit $BENCH_DIR/array3d.lua 300 >/dev/null
> > +```
> > +
> > +Benchmark results before and after the patch (less is better):
>
> Comparative to ‘BEFORE’ is more reafable, dispersion should also help with
> reading of results.
Done in the new version.
>
> > +
> > +```
> > + | BEFORE | AFTER,memprof off | AFTER,memprof on
> > +---------------+--------+-------------------+-----------------
> > +array3d | 0.22 | 0.20 | 0.21
> > +binary-trees | 3.32 | 3.33 | 3.94
> > +chameneos | 2.92 | 3.18 | 3.12
> > +coroutine-ring | 0.99 | 1.00 | 0.99
> > +euler14-bit | 1.04 | 1.05 | 1.03
> > +fannkuch | 6.77 | 6.69 | 6.64
> > +fasta | 8.27 | 8.30 | 8.25
> > +life | 0.48 | 0.48 | 1.03
> > +mandelbrot | 2.69 | 2.70 | 2.75
> > +mandelbrot-bit | 1.99 | 2.00 | 2.08
> > +md5 | 1.57 | 1.61 | 1.56
> > +nbody | 1.35 | 1.38 | 1.33
> > +nsieve | 2.11 | 2.19 | 2.09
> > +nsieve-bit | 1.50 | 1.55 | 1.47
> > +nsieve-bit-fp | 4.40 | 4.63 | 4.44
> > +partialsums | 0.54 | 0.58 | 0.55
> > +pidigits-nogmp | 3.48 | 3.50 | 3.47
> > +ray | 1.63 | 1.68 | 1.64
> > +recursive-ack | 0.19 | 0.22 | 0.20
> > +recursive-fib | 1.62 | 1.71 | 1.63
> > +scimark-fft | 5.78 | 5.94 | 5.69
> > +scimark-lu | 3.26 | 3.57 | 3.59
> > +scimark-sor | 2.34 | 2.35 | 2.33
> > +scimark-sparse | 5.03 | 4.92 | 4.91
> > +series | 0.94 | 0.96 | 0.95
> > +spectral-norm | 0.96 | 0.96 | 0.95
> > +```
> > --
> > 2.28.0
> >
>
[1]: https://lists.tarantool.org/pipermail/tarantool-discussions/2020-December/000147.html
[2]: https://github.com/tarantool/vshard/issues/224#issuecomment-637632349
[3]: https://www.lua.org/manual/5.1/manual.html#5.7
--
Best regards,
Sergey Kaplun
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