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-incremental-gc.pdf

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