Forum: Ruby-core Introducing Incremental GC algorithm

308cbef6e86dfc49cce3b2d4cf42aedc?d=identicon&s=25 unknown (Guest)
on 2014-08-17 07:29
(Received via mailing list)
Issue #10137 has been updated by Koichi Sasada.


 I had accidentally added GC.verify_internal_consistency method for each
 test case (in after_tear_down.

 After remove it,

 trunk         : 798.756056s
 before removal: 1981.145346s
 after removal : 845.399831s

 Not so bad.


 (2014/08/17 11:55), shibata.hiroshi@gmail.com wrote:
 > Issue #10137 has been updated by Hiroshi SHIBATA.
 >
 >
 > I evaluated running time for test-all.
 >
 > ref. https://twitter.com/k_tsj/status/500523453028388864
 >
 > rincgc branch seems 2 times slower than trunk when sequential
running.
 > I tested 3 times. it's same results.
 >
 > ### trunk
 >
 > ruby -v: ruby 2.2.0dev (2014-08-17 trunk 47206) [x86_64-darwin14]
 > make test-all TESTS='-j4'  291.72s user 65.89s system 241% cpu
2:27.94 total
 >
 > ruby -v: ruby 2.2.0dev (2014-08-17 trunk 47206) [x86_64-darwin14]
 > make test-all  336.31s user 62.33s system 83% cpu 7:55.63 total
 >
 > ### rincgc branch
 >
 > ruby -v: ruby 2.2.0dev (2014-08-15 trunk 47192) [x86_64-darwin14]
 > make test-all TESTS='-j4'  359.57s user 74.27s system 213% cpu
3:23.33 total
 >
 > ruby -v: ruby 2.2.0dev (2014-08-15 trunk 47192) [x86_64-darwin14]
 > make test-all  878.35s user 65.17s system 90% cpu 17:22.43 total
 >
 >
 >
 > ----------------------------------------
 > Feature #10137: Introducing Incremental GC algorithm
 > https://bugs.ruby-lang.org/issues/10137#change-48380
 >
 > * Author: Koichi Sasada
 > * Status: Open
 > * Priority: Normal
 > * Assignee: Koichi Sasada
 > * Category: core
 > * Target version: current: 2.2.0
 > ----------------------------------------
 > # Abstract
 >
 > Introduce *incremental GC* algorithm to reduce pause time of
major/full
 > GC. This ticket includes design and implementation note and a working
 > patch.
 >
 > # Background and problem
 >
 > Ruby 2.1 uses generational GC algorithm (named RGenGC) to improve GC
 > throughput. Genrational GC algorithm separates existing objects into
 > young generation objects and old generation objects. At the most of
GC
 > timing, GC only marks young generation objects (called minor GC). If
 > there are no enough memory, then mark all of objects (called major GC
or
 > full GC). Minor GC is dramatically fast than major GC. So that total
 > throughput of application improves (10% improvement in my RDoc
benchmark,
 > [ruby-list:49896] reported that GC intensive application is 6 times
 > faster!).
 >
 > However, major GC is needed periodically and it pauses same time as
GC
 > on Ruby 2.0 and before. This problem hits response time intensive
 > application such as web application.
 >
 > # Proposal
 >
 > Introduce *Incremental GC* algorithm for major GC.
 >
 > Incremental GC algorithm is well-known GC algorithm to reduce GC
pause
 > time. Scatter GC (marking) process in several phases and run
processes
 > interleaving with Ruby's execution process. This is similar to
current
 > lazy sweep algorithm (in fact, lazy sweep is a half part of
incremental
 > GC algorithm).
 >
 > Running ruby process with marking phase, it is possible to introduce
 > critical bug because marked objects can points un-marked objects (on
the
 > incremental GC terminology, marked objects are called "Black" objects
 > and un-marked objects are called "White" objects). Such un-marked
 > objects can be left in un-marking and be swept.
 >
 > ```
 > # if `ary' is already marked, and obj is not marked
 > ary[0] = obj
 > obj = nil # no-body except `ary' refers `obj'
 > ```
 >
 > To prevent such destructive bug, we can use write barriers to detect
 > such "marked objects" to "un-marked objects". We can care about such
 > case.
 >
 > Yes, MRI/CRuby has "WB-unprotected" objects such objects does not
have
 > write barriers because of compatibility or implementation issues. To
 > care about such WB-unprotected objects, we need to traverse all of
 > living WB-unprotected objects at a time in the last of incremental
 > marking. This is new extending idea against traditional incremental
GC
 > algorithm (at least I surveyed).
 >
 > Deisgn and implementation details are here:
 > http://www.atdot.net/fp_store/f.p61can/file.data-i...
 >
 > Maybe a diagram at page 10 will help you to understand the flow of
all
 > GC process.
 >
 > Code is here:
 > https://github.com/ko1/ruby/tree/rincgc
 >
 > Compare with trunk:
 > https://github.com/ko1/ruby/compare/rincgc
 >
 > # Implementation note
 >
 > ## WB-unprotected bitmap
 >
 > As I said, we need to check all of living WB-unprotected objects at
the
 > last of incremental marking phase. To do it lightweight, introduce
 > WB-unprotected bitmap intead of specific bit in RBasic::flags.
 >
 > We can get all living WB-unprotected objects with the following
pseudo
 > code:
 >
 > ```
 > bits = mark_bits[i] & wb_unprotected_bits[i];
 > while (bits) {
 >   if (bits & 1) do something.
 >   bits >>= 1;
 > }
 > ```
 >
 > ## 4 age promotion
 >
 > Because we don't need to use WB-protected bit in RBasic::flags, we
have
 > another 1 bit in RBasic::flags. To utilize this bit, we add *age* of
an
 > object with exsiting promoted bit. Rename FL_WB_PROTECTED to
 > FL_PROMOTED0 and FL_PROMOTED to FL_PROMOTED1.
 >
 > These two bits represent object's age (0 to 3) and 3 means OLD
objects.
 >
 > ## Write barriers
 >
 > We extend write barriers to detect such [marked objects -> un-marked
 > objects] reference in incremental GC. It introduces some overhead.
 >
 > # Evaluation
 >
 > Benchmark results on real Linux machine (*1)
 > http://www.atdot.net/sp/raw/yekban
 > *1: Intel(R) Xeon(R) CPU E5335 @ 2.00GHz, 4GB memory, 2.6.32-5-amd64
(Debian squeeze)
 >
 > In most of case, there are only a few (~5%) performance down.
 > Incremental GC introduces some overhead.  But I think they are
 > acceptable speed-down.
 >
 > Discouse benchmark (only on Virtualbox VM, so accuracy is not good)
 > http://www.atdot.net/sp/raw/g9uban
 >
 > We can recognize reducing worst case seconds.
 >
 > # TODO
 >
 > (1) Clean up codes
 >
 > Now, we can not disable incremental GC codes and generational GC
codes.
 > We need to add ability to enable/disable features with macros.
 >
 > (2) Tuning parameters
 >
 > Now the parameters are fixed in codes. mruby already have tuning
 > parameters for incremental GC (matz said they are from Lua),
 > "GC.interval_ratio" and "GC.step_ratio". We can import these
functions
 > (or making another interface to tell).
 >
 > (3) Enter GC/ Exit GC internal events
 >
 > This patch also includes function "gc_enter()" and "gc_exit()" which
set
 > and reset a "doing GC" flag.
 >
 > If we introduce internal event to hook these functions, we can
measure
 > exact GC pause time (and mutators time).
 >
 > # Summary
 >
 > This feature proposal is to introduce incremental GC algorithm with
working code.
 > Incremental GC algorithm reduce application's pause time of major GC.
 >
 > Any feedback are welcome!
 >
 > Thanks,
 > Koichi
 >
 >
 >
 >


 --
 // SASADA Koichi at atdot dot net

----------------------------------------
Feature #10137: Introducing Incremental GC algorithm
https://bugs.ruby-lang.org/issues/10137#change-48386

* Author: Koichi Sasada
* Status: Open
* Priority: Normal
* Assignee: Koichi Sasada
* Category: core
* Target version: current: 2.2.0
----------------------------------------
# Abstract

Introduce *incremental GC* algorithm to reduce pause time of major/full
GC. This ticket includes design and implementation note and a working
patch.

# Background and problem

Ruby 2.1 uses generational GC algorithm (named RGenGC) to improve GC
throughput. Genrational GC algorithm separates existing objects into
young generation objects and old generation objects. At the most of GC
timing, GC only marks young generation objects (called minor GC). If
there are no enough memory, then mark all of objects (called major GC or
full GC). Minor GC is dramatically fast than major GC. So that total
throughput of application improves (10% improvement in my RDoc
benchmark,
[ruby-list:49896] reported that GC intensive application is 6 times
faster!).

However, major GC is needed periodically and it pauses same time as GC
on Ruby 2.0 and before. This problem hits response time intensive
application such as web application.

# Proposal

Introduce *Incremental GC* algorithm for major GC.

Incremental GC algorithm is well-known GC algorithm to reduce GC pause
time. Scatter GC (marking) process in several phases and run processes
interleaving with Ruby's execution process. This is similar to current
lazy sweep algorithm (in fact, lazy sweep is a half part of incremental
GC algorithm).

Running ruby process with marking phase, it is possible to introduce
critical bug because marked objects can points un-marked objects (on the
incremental GC terminology, marked objects are called "Black" objects
and un-marked objects are called "White" objects). Such un-marked
objects can be left in un-marking and be swept.

```
# if `ary' is already marked, and obj is not marked
ary[0] = obj
obj = nil # no-body except `ary' refers `obj'
```

To prevent such destructive bug, we can use write barriers to detect
such "marked objects" to "un-marked objects". We can care about such
case.

Yes, MRI/CRuby has "WB-unprotected" objects such objects does not have
write barriers because of compatibility or implementation issues. To
care about such WB-unprotected objects, we need to traverse all of
living WB-unprotected objects at a time in the last of incremental
marking. This is new extending idea against traditional incremental GC
algorithm (at least I surveyed).

Deisgn and implementation details are here:
http://www.atdot.net/fp_store/f.p61can/file.data-i...

Maybe a diagram at page 10 will help you to understand the flow of all
GC process.

Code is here:
https://github.com/ko1/ruby/tree/rincgc

Compare with trunk:
https://github.com/ko1/ruby/compare/rincgc

# Implementation note

## WB-unprotected bitmap

As I said, we need to check all of living WB-unprotected objects at the
last of incremental marking phase. To do it lightweight, introduce
WB-unprotected bitmap intead of specific bit in RBasic::flags.

We can get all living WB-unprotected objects with the following pseudo
code:

```
bits = mark_bits[i] & wb_unprotected_bits[i];
while (bits) {
  if (bits & 1) do something.
  bits >>= 1;
}
```

## 4 age promotion

Because we don't need to use WB-protected bit in RBasic::flags, we have
another 1 bit in RBasic::flags. To utilize this bit, we add *age* of an
object with exsiting promoted bit. Rename FL_WB_PROTECTED to
FL_PROMOTED0 and FL_PROMOTED to FL_PROMOTED1.

These two bits represent object's age (0 to 3) and 3 means OLD objects.

## Write barriers

We extend write barriers to detect such [marked objects -> un-marked
objects] reference in incremental GC. It introduces some overhead.

# Evaluation

Benchmark results on real Linux machine (*1)
http://www.atdot.net/sp/raw/yekban
*1: Intel(R) Xeon(R) CPU E5335 @ 2.00GHz, 4GB memory, 2.6.32-5-amd64
(Debian squeeze)

In most of case, there are only a few (~5%) performance down.
Incremental GC introduces some overhead.  But I think they are
acceptable speed-down.

Discouse benchmark (only on Virtualbox VM, so accuracy is not good)
http://www.atdot.net/sp/raw/g9uban

We can recognize reducing worst case seconds.

# TODO

(1) Clean up codes

Now, we can not disable incremental GC codes and generational GC codes.
We need to add ability to enable/disable features with macros.

(2) Tuning parameters

Now the parameters are fixed in codes. mruby already have tuning
parameters for incremental GC (matz said they are from Lua),
"GC.interval_ratio" and "GC.step_ratio". We can import these functions
(or making another interface to tell).

(3) Enter GC/ Exit GC internal events

This patch also includes function "gc_enter()" and "gc_exit()" which set
and reset a "doing GC" flag.

If we introduce internal event to hook these functions, we can measure
exact GC pause time (and mutators time).

# Summary

This feature proposal is to introduce incremental GC algorithm with
working code.
Incremental GC algorithm reduce application's pause time of major GC.

Any feedback are welcome!

Thanks,
Koichi
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