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How the Nasal GC works: Difference between revisions
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= Status ( | = Status (02/2016) = | ||
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As of 06/2012, a new incremental Nasal GC is being worked on [http://www.mail-archive.com/flightgear-devel@lists.sourceforge.net/msg37579.html]: | |||
<pre> | |||
I have been working on a 4-color incremental mark/sweep collector with | |||
the intention of merging it into the Nasal interpreter. | |||
The work so far can be found at http://github.com/chrisforbes/incgc; | |||
There's still quite a lot to do, but the path is clear. | |||
</pre> | |||
--> | |||
The main Nasal repository (maintained by Andy Ross) can also be found at github: https://github.com/andyross/nasal | |||
The latest version in FlightGear is maintained in the SimGear repository which is to be found at gitorious: https://gitorious.org/fg/simgear/trees/next/simgear/nasal | |||
The garbage collector in Nasal is pretty self-contained and entirely implemented in the file [https://gitorious.org/fg/simgear/blobs/next/simgear/nasal/gc.c gc.c]. | |||
{{FGCquote | |||
|Nasal's garbage collector requires that no other Nasal threads are <br/> | |||
running, and achieves this by keeping a count of active Nasal threads <br/> | |||
(nThreads, simgear/nasal/code.h), and having the thread that needs GC <br/> | |||
ask all other threads to stop and wait until nThreads-1 say they have <br/> | |||
(bottleneck(), simgear/nasal/gc.c). This will hang if there are any <br/> | |||
threads that Nasal thinks are active but actually aren't, as they will <br/> | |||
never reply to the stop request. | |||
|{{cite web |url=http://sourceforge.net/p/flightgear/mailman/message/32969200/ | |||
|title=<nowiki>[Flightgear-devel] Nasal threading, naCall() vs naCallMethodCtx()</nowiki> | |||
|author=<nowiki>Rebecca N. Palmer</nowiki> | |||
|date=<nowiki>2014-10-25</nowiki> | |||
}} | |||
}} | |||
{{FGCquote | {{FGCquote | ||
|1= Such things may look simple at first. Easy to convert a simple "hello world". But it's very complex when supporting all the features of an interpreted script language. And the funny thing is: you'd still need to worry about automatic garbage collection and count references (though that'd be a lesser issue compared to many others then). So, time wake up... | |1= Such things may look simple at first. Easy to convert a simple "hello world". But it's very complex when supporting all the features of an interpreted script language. And the funny thing is: you'd still need to worry about automatic garbage collection and count references (though that'd be a lesser issue compared to many others then). So, time wake up... | ||
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| date = Mar 26th, 2011 | | date = Mar 26th, 2011 | ||
| added = Mar 26th, 2011 | | added = Mar 26th, 2011 | ||
| script_version = 0.23 | |||
}} | |||
}} | |||
=== Patches === | |||
{{FGCquote | |||
|1= there is still the issue that you can't execute Nasal while the GC is running (for that you'd need a concurrent GC which is most likely not so easily implemented). The implication is that if the time required for a collection exceeds the frame dealy you'll still notice it even when the GC runs in a different thread. I have updated my experimental patch from last year, which should also work on Windows now (and it has even been tested there): | |||
* http://sleipner.gidenstam.org/users/anders/FlightGear/sg-gc-4.diff | |||
* http://sleipner.gidenstam.org/users/anders/FlightGear/fg-gc-3.diff | |||
Again: It doesn't solve the bigger problem of the current Nasal GC needing to run uninterrupted and in mutual exclusion - but I see very few GC runs in the main loop here (the patch prints "** Nasal GC in main thread **" in the console when that happens) except a few during startup. | |||
|2= {{cite web | |||
| url = http://sourceforge.net/p/flightgear/mailman/message/29306592/ | |||
| title = <nowiki>Re: [Flightgear-devel] Nasal performance</nowiki> | |||
| author = <nowiki>Anders Gidenstam</nowiki> | |||
| date = May 23rd, 2012 | |||
| added = May 23rd, 2012 | |||
| script_version = 0.23 | |||
}} | |||
}} | |||
{{FGCquote | |||
|1= There are algorithms for incremental and/or concurrent and/or parallel garbage collection out there. They most likely not easy to implement and as far as I have seen so far would require (at least for concurrent and /or parallel GC) all writes of pointers to the Nasal heap (and possibly reads) to be redirected via wrapper functions (also known as (GC) read/write barriers). This will not be an easy task but in my opinion it would be a promising option. It might be possible to use a GC module from a GPL:d Java vm or similar. Btw, just running the normal (mutually exclusive) Nasal GC in another thread than the main loop is not hard - but since it is mutually exclusive to executing Nasal functions it doesn't help much when it comes to reducing the worst case latency. The small changes needed to add a separate GC thread are available here: | |||
* http://www.gidenstam.org/FlightGear/misc/test/sg-gc-2.diff | |||
* http://www.gidenstam.org/FlightGear/misc/test/fg-gc-1.diff | |||
|2= {{cite web | |||
| url = http://sourceforge.net/p/flightgear/mailman/message/27338998/ | |||
| title = <nowiki>Re: [Flightgear-devel] Nasal Garbage Collector (was Stuttering at 1 | |||
Hz rate)</nowiki> | |||
| author = <nowiki>Anders Gidenstam</nowiki> | |||
| date = Apr 10th, 2011 | |||
| added = Apr 10th, 2011 | |||
| script_version = 0.23 | | script_version = 0.23 | ||
}} | }} | ||
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| script_version = 0.23 | | script_version = 0.23 | ||
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}} | }} | ||