memprof / GC dynamics

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Steven Parkes

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Feb 23, 2010, 6:46:18 PM2/23/10

to ruby...@ruby-lang.org, johnso...@googlegroups.com, dam...@alumni.cmu.edu, Aman Gupta

Wow, memprof is great stuff.

I have a case I don't really understand, though. (On 1.8.7-p249)

I have

<code>
Memprof.start # optionally
GC.start
<code>

Whether I have the Memprof.start call in there or not changes the results of GC.start. [GC.start should be a complete mark/sweep, shouldn't it?]

My context is Johnson, which is linking to Spidermonkey, the Mozilla JS interpreter. Johnson has C ext Ruby objects that are roots to Spidermonkey's GC. If I run GC.start w/o Memprof.start, it appears Ruby refs are getting through the GC and staying roots to Spidermonkey. Simply adding Memprof.start changes this: the GC.start does (apparently) free those roots.

I don't know that this is related to Memprof per-se: just happened to be what I was using. Is this simply stack shadowing, with Memprof dirtying the stack enough that Ruby doesn't see any false references?