[ruby-core:85179] [Ruby trunk Feature#14370] Directly mark instruction operands and avoid mark_ary usage on rb_iseq_constant_body

Issue #14370 has been updated by ko1 (Koichi Sasada).


>　Approximately 60% of the instruction sequences have 0 markable objects. Those 60% account for 35% of the total iseq_encoded that needs to be walked:

Interesting.

Just an idea. Now we allocate Array with extra area and the size of extra area called "capa" (`RArray::as::heap::aux::capa`). I tried to show the size and capa with the following patch:

```diff
Index: iseq.c
===================================================================
--- iseq.c	(リビジョン 62078)
+++ iseq.c	(作業コピー)
@@ -418,6 +418,10 @@
     if (ruby_vm_event_enabled_flags & ISEQ_TRACE_EVENTS) {
 	rb_iseq_trace_set(iseq, ruby_vm_event_enabled_flags & ISEQ_TRACE_EVENTS);
     }
+
+    fprintf(stderr, "size:%d, capa:%d\n",
+            (int)RARRAY_LEN(iseq->body->mark_ary),
+            (int)(FL_TEST(iseq->body->mark_ary, RARRAY_EMBED_FLAG) ? 0 : RARRAY(iseq->body->mark_ary)->as.heap.aux.capa));
     return Qtrue;
 }
 
```

and test-all shows:

```
size:3, capa:0
size:11, capa:20
size:4, capa:20
size:4, capa:20
size:6, capa:20
size:3, capa:0
size:5, capa:20
size:4, capa:20
size:5, capa:20
size:8, capa:20
size:4, capa:20
size:5, capa:20
size:8, capa:20
size:3, capa:0
size:5, capa:20
size:3, capa:0
size:5, capa:20
size:6, capa:20
size:3, capa:0
size:5, capa:20
size:6, capa:20
size:5, capa:20
size:7, capa:20
size:5, capa:20
size:7, capa:20
size:7, capa:20
size:5, capa:20
size:3, capa:0
size:5, capa:20
size:5, capa:20
size:23, capa:37
size:3, capa:0
size:6, capa:20
size:3, capa:0
size:5, capa:20
size:4, capa:20
size:4, capa:20
size:4, capa:20
size:4, capa:20
size:4, capa:20
...
```

(sorry, no formal statistics data)

It seems many extra data we are holding. If we shrink such extra space (capa) just after compiling, it can reduce memory.

Of course, Aaron's approach helps more (because we don't need to keep `mark_ary` any more for referencing objects). But it is simple and no computation cost on marking phase.

Thoughts?


----------------------------------------
Feature #14370: Directly mark instruction operands and avoid mark_ary usage on rb_iseq_constant_body
https://bugs.ruby-lang.org/issues/14370#change-69924

* Author: tenderlovemaking (Aaron Patterson)
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* Target version: 
----------------------------------------
Hi,

I've attached a patch that changes rb_iseq_mark to directly mark instruction operands rather than adding them to a mark array.  I observed a ~3% memory reduction by directly marking operands, and I didn't observe any difference in GC time.  To test memory usage, I used a basic Rails application, logged all malloc / free calls to a file, then wrote a script that would sum the live memory at each sample (each sample being a call to malloc).  I graphed these totals so that I could see the memory usage as malloc calls were made:

![memory usage graph](https://user-images.githubusercontent.com/3124/35020270-1b0ded20-fae0-11e7-9cbd-1d028a6c9484.png)

The red line is trunk, the blue line is trunk + the patch I've attached.  Since the X axis is sample number (not time), the blue line is not as long as the red line because the blue line calls `malloc` fewer times.  The Y axis in the graph is the total number of "live" bytes that have been allocated (all allocations minus their corresponding frees).  You can see from the graph that memory savings start adding up as more code gets loaded.

I was concerned that this patch might impact GC time, but `make gcbench-rdoc` didn't seem to show any significant difference in GC time between trunk and this patch.  If it turns out there is a performance impact, I think I could improve the time while still keeping memory usage low by generating a bitmap during iseq compilation.

There is a bit more information where I've been working, but I think I've summarized everything here.

  https://github.com/github/ruby/pull/39


---Files--------------------------------
iseq_mark.diff (6.28 KB)
iseq_mark.diff (6.28 KB)
iseq_mark.diff (7.26 KB)
benchmark_methods.diff (1.23 KB)
bench.rb (3.01 KB)


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