[Vm-dev] Garbage Collection (was Re: [Pharo-dev] Discussing the roadmap)

Clément Bera bera.clement at gmail.com
Mon Dec 4 07:47:48 UTC 2017


The mail is very long so I did not read all of it.

Here are some ideas/things to say on the tip of my head:
- Changing an object to a forwarding object is non atomic (we need to
maintain at least stack invariant)
- To decrease the pauses in full GC I have 2 plans:
-- incremental marking (split the mark pause in multiple small pauses): Not
implemented right now.
-- selective compaction (compacts only part of the heap instead of all the
heap and sweeps the rest, similar to G1, but uses forwarders instead of
lots of card marking metadata): I implemented SpurSweeper which only sweeps
but works very well.
- Currently the marking phase removes all forwarders and I would like
incremental marking to maintain the same invariant (forwarders are always
- In general, Concurrent marking and sweeping have been implemented
everywhere, but no concurrent compaction. For compaction you can make it
selective (compact only part of the heap and the part which needs it the
most) like I suggest and like in G1, which decreases considerably
compaction pause time. Work on concurrent compaction is state of the art
and not in production everywhere, see for example
Shenandoah Garbage Collector
*A. Shipilev*
Pause-Less GC for Improving Java Responsiveness

*C. Gracie*
And I will watch at talk on this topic tomorrow for the Android GC.
- Some runtime, especially now with small servers being rent, are running
on single core machines. So we need the low-pause GC to work incrementally
aside from concurrently. So step 1 incremental GC. Step 2 concurrent
marking and sweeping with low-pause for scavenge/compaction.

No more time right now.

On Sun, Dec 3, 2017 at 6:33 AM, Ben Coman <btc at openinworld.com> wrote:

> Hi Eliot, Clement,
> On 7 July 2017 at 00:41, Eliot Miranda <eliot.miranda at gmail.com> wrote:
> >
> > > - Better support for large heaps (GC tuning API, incremental GC).
> > > Pharo 64 bit is now able to manage large heap. However better
> > > performance can be proposed by offering better settings for the
> > > different GC zone.
> >
> > The most important thing here is the incremental GC.  Spur has a
> generation
> > scavenger that collects garbage in newly created objects (new space),
> > and a mark-compact collector that collects and compacts garbage in old
> space.
> >
> > Right now on my 2.3GHz MacMini doing normal development, the generation
> > scavenger causes pauses of 1ms or less, and the mark-compact collector
> > than causes pauses of around 200ms.  Both account for about 0.75% of
> > entire execution time (1.5% total), so the scavenger is invoked
> frequently
> > and the pauses that it creates are not noticeable.  But the occasional
> > pauses created by the mark-compact collector /are/ noticeable,
> > especially in games and music.
> >
> > The idea for the incremental collector is to implement a mark-sweep or
> > mark-sweep-compact collector for old space that works incrementally,
> > doing a little bit of work on each invocation, probably immediately
> after a scavenge.
> > It is intended to avoid the long pauses caused by the non-incremental
> > mark-compact collector and make the system more suitable for games,
> music, etc.
> Reading http://www.mirandabanda.org/cogblog/2013/09/13/lazy-
> become-and-a-partial-read-barrier/
>   "An alternative implementation, oft-used in Lisp systems, is to add a
>   read barrier to all object access, and mark objects as forwarders.
>   This can be used to implement a >>>lazy copying garbage collection<<<<
>   where objects are copied from one semi-space to another in parallel to
> the
>   main program (the “mutator”).  To become, or move an object one replaces
> the
>   object’s header or first field with a forwarding pointer to the desired
> target
>   or copy in a new location, marking the “corpse” as forwarded.  The
> program
>   checks the forwarded flag on each access.  If there is hardware support,
>   as in a Lisp machine, this can work well.  But without hardware support,
>   and like the object table representation, it has costs, slowing down
>   program execution due to the scattering of forwarding checks and
>   forwarding pointer follows throughout program execution."
> I'm curious... Given we now have forwarders with Spur, are we
> already sufficiently paying the cost of forwarding checks that a lazy
> copying
> garbage collector might be a feasible form of incremental garbage
> collection?
> I presume "parallel to the main program" means garbage collection occuring
> in a separate thread to the main vm thread, potentially resulting in
> very low main program pause times for garbage collection.
> I found this a useful summary of the terminology...
> * https://www.dynatrace.com/resources/ebooks/javabook/
> reduce-garbage-collection-pause-time/
> and I'm curious how our planned Incremental CG fits those categories.
> That article got me contemplating our performance constraint of the VM only
> operating in only in a single native thread. Even though GC is a only
> a few percent of performance, I wondered what a concurrent GC might
> look like for us.
> I found this video describing concurrent GC in Go (ignore first 11:20
> and the second half was not so interesting)
> * https://pusher.com/sessions/meetup/the-realtime-guild/
> golangs-realtime-garbage-collector
> where they present some interesting charts of their concern with latency
> pauses.
> (btw they reference a multi-language GC latency benchmark
>   https://github.com/WillSewell/gc-latency-experiment)
> And for balance of that I found...
> https://blog.plan99.net/modern-garbage-collection-911ef4f8bd8e
> And then my mind wandered around implementation details of concurrent
> garbage collection.
> To organise and quiet my thoughts I needed to put pen to paper, so I
> thought sharing that
> might stir thoughts for others.  Probably naive and please excuse the
> brain dump format...
> Considering two threads...
> * Main program thread "MP"
> * Garbage collection thread "GC"
> with object-space shared between them, consisting of objects split in
> object-header & object-body...
> 1. Only "MP" mutates the object-body, updating slots creating new edges
>    in the object-graph, and relocating objects in memory using forwarders.
>    This rule avoids potential race conditions without needing to add
>    synchronisation code affecting performance of "MP".
> 2. Concurrently "GC" performs a Marking Phase by following the object-graph
>    tricolour tagging objects gray & black. it needs to mutate the
> object-header,
>    in a synchronised way something like...
>      a. Load object-header from shared object-space into local variable
>           H <== object-header
>      b. Modify header into another local variable
>           H' <== H + updated GC color bits
>      c. Atomic compare-and-swap H' back into object-space
>           object-header <== if H then H'
>      "MP" gets priority. Conflicts presumed rare.
> Then considering object mutation by "MP" concurrent/overlapped with
> "GC" marking...
> 3. TLDR; see 4.
>    In old-space if 'mutated' object is linked to a 'target' object
>      a. if 'mutated' isBlack, "MP" marks 'target' gray (to be later
> processed by "GC").
>      b. if 'mutated' isGray, overlapped 'mutated' gray->black by "GC"
> while 'target'
>         remains white would break tricolor invariant (is it a credible
> case?), options...
>           i. mark 'target' gray, the state anyway in case 'mutated'
> gray -> black
>           ii. re-mark 'mutated' gray, i.e. normally gray->gray, and
> rarely black->gray
>           iii. optimisticly just check after mutation if color changed
> (cached reads
>                cheaper than writes and conflicts presumed rare) then
> set color to gray.
>      c. if 'mutated' isWhite, options...
>           i. do nothing and let "GC" reach 'target' normally, as long
> as overlapped white->black cannot occur
>           ii. if white->black possible, do same as (3.b) "MP" marks
> 'target' gray, to be later processed by "GC".
> 4. Overall simplification of (3.) might be...
>      "MP" checks for any color change during mutation, and only then
> marks 'mutated' object gray.
>      How expensive would such a check be?  Presumed marking is infrequent
> and
>      can be done safely like (2.)
> 5. Eden/survivor space is ignored by "GC" thread. No point in adding
> work to the gray set
>     until survivors filtered. Stop the world scavenging done as normal by
> "MP",
>     only marking objects gray when they are moved to old-space.
>     Post scavenging options...
>        a. Resume the world and leave it for "GC" to process these
> recently grayed objects.
>        b. Keep world stopped for "MP" to complete marking, emptying gray
> set to
>            transition to Sweep Phase, at which point "MP" resumes the
> world.
>            It doesn't matter if subsequently objects are added to the gray
> set,
>            since existing white objects can never again be referenced by
> "MP".
> 6. Sweeping can be safely done by "GC" since white-objects are
> unreachable from "MP".
>    "GC" can also take time to determine an optimum page P to compact and
>    then (per 1.) passes to "MP" via a relocation-queue the objects to be
>    relocated using forwarding pointers. "GC" could even spend extra time
>    to determine the optimum relocation-destination without impacting the
>    performance of "MP".  When "MP" empties the relocation-queue, "GC"
> starts
>    on the next Marking Phase.
> 7. Now a question remains about multi-threaded flattening of
> forwarding pointers.
>     If two threads simultaneously perform an identical transform from...
>        someObject-slot --> forwarder-b --> finalObject-c
>     to...
>        someObject-slot --> finalObject-c
>     does it matter that these operation may be done twice overlapping?
>    Options...
>      a. One mitigation could be for "GC" to identify forwarders to be
> flattened
>         and queue them for "MP" to process (reuse the compaction-queue).
> This
>         is work that "MP" would need to do anyway, but brings it forward
> to be
>         dealt with at a convenient time.
>      b. I guess "MP" and "GC" could play nice together if when encountering
>         a slot containing with a forwarding pointer they both do
> something similar to (2.) like...
>           i. Load object-slot from shared object-space into local
> variable S <== slot
>          ii. Set local variable S' <== flattened/followed pointer
>         iii. Atomic compare-and-swap S' back into object-space,
> (object-slot <== if S then S')
>              - "MP" gets priority but presume conflicts rare anyway.
>              - While this violates rule (1.) the presumed frequency of
>                encountering forwarding pointers is low for "MP", so
> performance
>                should not be affected. Indeed by "GC" pre-emptively
> flattening forwarders
>                the frequency "MP" sees reduces.
> 8. The release of the compacted page back to the OS is held up by
> forwarding pointers.
>    Forwarders are part of the graph followed in the Marking Phase they get
>    marked gray/black just like objects if they are referenced.  After
> forwarders are
>    fully flattened they are skipped by Marking and end up marked white
> and released
>    just like any other object. Once all forwarders are released, the
> page is released
>    to the OS.   If "GC" can effectively flatten pointers concurrently
> with "MP" during its
>    normal Marking Phase, then pages would be released back to the OS
> in a timely manner.
> Now one thing I am curious about how the GC tri-color marking is
> implemented.
> At https://clementbera.wordpress.com/2014/01/16/spurs-new-object-format
> it describes how Spur's object header has three bits for GC.
> * isGray
> * isRemembered
> * isMarked  (I presume this means marked "black")
> Do the bits imply the gray set is not stored in a separate data
> structure on the heap, but rather distributed in-place, which I guess
> would require multiple passes through memory to grow the gray set?
> So this is not an area I'm set up to seriously work on, but I remain
> curious
> and hopefully its a useful seed discussion for others.
> cheers -ben
> P.S. At https://clementbera.wordpress.com/2017/09/19/vm-learning-
> memory-management/
> nice to hear that you have Sophie (I presume) continuing with the VM.

Clément Béra
Pharo consortium engineer
Bâtiment B 40, avenue Halley 59650 Villeneuve d'Ascq
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