Here's an example where we allocate a large (4K) stack:
>>> bt
#0 allocateMightFail (cap=0x7f366808cfc0 <MainCapability>,
n=4096) at rts/sm/Storage.c:876
#1 0x00007f3667e4a85d in allocate (cap=0x7f366808cfc0
<MainCapability>, n=4096) at rts/sm/Storage.c:849
#2 0x00007f3667e16f46 in threadStackOverflow (cap=0x7f366808cfc0
<MainCapability>, tso=0x4200152a68) at rts/Threads.c:600
#3 0x00007f3667e12a64 in schedule
(initialCapability=0x7f366808cfc0 <MainCapability>, task=0x78c970) at
rts/Schedule.c:520
#4 0x00007f3667e1215f in scheduleWaitThread (tso=0x4200105388,
ret=0x0, pcap=0x7ffef40dce78) at rts/Schedule.c:2533
#5 0x00007f3667e25685 in rts_evalLazyIO (cap=0x7ffef40dce78,
p=0x736ef8, ret=0x0) at rts/RtsAPI.c:530
#6 0x00007f3667e25f7a in hs_main (argc=16, argv=0x7ffef40dd0a8,
main_closure=0x736ef8, rts_config=...) t rts/RtsMain.c:72
#7 0x00000000004f738f in main ()
This is based on an old tree so source locations may not be correct, it's this
code in threadStackOverflow():
// Charge the current thread for allocating stack. Stack usage is
// non-deterministic, because the chunk boundaries might vary from
// run to run, but accounting for this is better than not
// accounting for it, since a deep recursion will otherwise not be
// subject to allocation limits.
cap->r.rCurrentTSO = tso;
new_stack = (StgStack*) allocate(cap, chunk_size);
cap->r.rCurrentTSO = NULL;
SET_HDR(new_stack, &stg_STACK_info, old_stack->header.prof.ccs);
TICK_ALLOC_STACK(chunk_size);
Ömer
Ömer Sinan Ağacan <omeraga...@gmail.com>, 21 Haz 2018 Per, 13:42
tarihinde şunu yazdı:
>
> > Large objects can only be primitive objects, like MUT_ARR_PTRS, allocated by
> > the RTS, and none of these have SRTs.
>
> Is is not possible to allocate a large STACK? I'm currently observing this in
> gdb:
>
> >>> call *Bdescr(0x4200ec9000)
> $2 = {
> start = 0x4200ec9000,
> free = 0x4200ed1000,
> link = 0x4200100e80,
> u = {
> back = 0x4200103980,
> bitmap = 0x4200103980,
> scan = 0x4200103980
> },
> gen = 0x77b4b8,
> gen_no = 1,
> dest_no = 1,
> node = 0,
> flags = 1027, <-- BF_LARGE | BF_EVACUTED | ...
> blocks = 8,
> _padding = {[0] = 0, [1] = 0, [2] = 0}
> }
>
> >>> call printClosure(0x4200ec9000)
> 0x4200ec9000: STACK
>
> >>> call checkClosure(0x4200ec9000)
> $3 = 4096 -- makes sense, larger than 3277 bytes
>
> So I have a large STACK object, and STACKs can refer to static objects. But
> when we scavenge this object we don't scavenge its SRTs because we use
> scavenge_one(). This seems wrong to me.
>
> Ömer
>
> Simon Marlow <marlo...@gmail.com>, 20 Haz 2018 Çar, 14:32 tarihinde şunu
> yazdı:
> >
> > Interesting point. I don't think there are any large objects with SRTs, but
> > we should document the invariant because we're relying on it.
> >
> > Large objects can only be primitive objects, like MUT_ARR_PTRS, allocated
> > by the RTS, and none of these have SRTs.
> >
> > We did have plans to allocate memory for large dynamic objects using
> > `allocate()` from compiled code, in which case we could have large objects
> > that could be THUNK, FUN, etc. and could have an SRT, in which case we
> > would need to revisit this. You might want to take a look at Note [big
> > objects] in GCUtils.c, which is relevant here.
> >
> > Cheers
> > Simon
> >
> >
> > On 20 June 2018 at 09:20, Ömer Sinan Ağacan <omeraga...@gmail.com> wrote:
> >>
> >> Hi Simon,
> >>
> >> I'm confused about this code again. You said
> >>
> >> > scavenge_one() is only used for a non-major collection, where we aren't
> >> > traversing SRTs.
> >>
> >> But I think this is not true; scavenge_one() is also used to scavenge large
> >> objects (in scavenge_large()), which are scavenged even in major GCs. So it
> >> seems like we never really scavenge SRTs of large objects. This doesn't
> >> look
> >> right to me. Am I missing anything? Can large objects not refer to static
> >> objects?
> >>
> >> Thanks
> >>
> >> Ömer
> >>
> >> Ömer Sinan Ağacan <omeraga...@gmail.com>, 2 May 2018 Çar, 09:03
> >> tarihinde şunu yazdı:
> >> >
> >> > Thanks Simon, this is really helpful.
> >> >
> >> > > If you look at scavenge_fun_srt() and co, you'll see that they return
> >> > > immediately if !major_gc.
> >> >
> >> > Thanks for pointing this out -- I didn't realize it's returning early
> >> > when
> >> > !major_gc and this caused a lot of confusion. Now everything makes sense.
> >> >
> >> > I'll add a note for scavenging SRTs and refer to it in relevant code and
> >> > submit
> >> > a diff.
> >> >
> >> > Ömer
> >> >
> >> > 2018-05-01 22:10 GMT+03:00 Simon Marlow <marlo...@gmail.com>:
> >> > > Your explanation is basically right. scavenge_one() is only used for a
> >> > > non-major collection, where we aren't traversing SRTs. Admittedly this
> >> > > is a
> >> > > subtle point that could almost certainly be documented better, I
> >> > > probably
> >> > > just overlooked it.
> >> > >
> >> > > More inline:
> >> > >
> >> > > On 1 May 2018 at 10:26, Ömer Sinan Ağacan <omeraga...@gmail.com> wrote:
> >> > >>
> >> > >> I have an idea but it doesn't explain everything;
> >> > >>
> >> > >> SRTs are used to collect CAFs, and CAFs are always added to the oldest
> >> > >> generation's mut_list when allocated [1].
> >> > >>
> >> > >> When we're scavenging a mut_list we know we're not doing a major GC,
> >> > >> and
> >> > >> because mut_list of oldest generation has all the newly allocated
> >> > >> CAFs,
> >> > >> which
> >> > >> will be scavenged anyway, no need to scavenge SRTs for those.
> >> > >>
> >> > >> Also, static objects are always evacuated to the oldest gen [2], so
> >> > >> any
> >> > >> CAFs
> >> > >> that are alive but not in the mut_list of the oldest gen will stay
> >> > >> alive
> >> > >> after
> >> > >> a non-major GC, again no need to scavenge SRTs to keep these alive.
> >> > >>
> >> > >> This also explains why it's OK to not collect static objects (and not
> >> > >> treat
> >> > >> them as roots) in non-major GCs.
> >> > >>
> >> > >> However this doesn't explain
> >> > >>
> >> > >> - Why it's OK to scavenge large objects with scavenge_one().
> >> > >
> >> > >
> >> > > I don't understand - perhaps you could elaborate on why you think it
> >> > > might
> >> > > not be OK? Large objects are treated exactly the same as small objects
> >> > > with
> >> > > respect to their lifetimes.
> >> > >
> >> > >>
> >> > >> - Why we scavenge SRTs in non-major collections in other places (e.g.
> >> > >> scavenge_block()).
> >> > >
> >> > >
> >> > > If you look at scavenge_fun_srt() and co, you'll see that they return
> >> > > immediately if !major_gc.
> >> > >
> >> > >>
> >> > >> Simon, could you say a few words about this?
> >> > >
> >> > >
> >> > > Was that enough words? I have more if necessary :)
> >> > >
> >> > > Cheers
> >> > > Simon
> >> > >
> >> > >
> >> > >>
> >> > >>
> >> > >> [1]: https://github.com/ghc/ghc/blob/master/rts/sm/Storage.c#L445-L449
> >> > >> [2]: https://github.com/ghc/ghc/blob/master/rts/sm/Scav.c#L1761-L1763
> >> > >>
> >> > >> Ömer
> >> > >>
> >> > >> 2018-03-28 17:49 GMT+03:00 Ben Gamari <b...@well-typed.com>:
> >> > >> > Hi Simon,
> >> > >> >
> >> > >> > I'm a bit confused by scavenge_one; namely it doesn't scavenge
> >> > >> > SRTs. It
> >> > >> > appears that it is primarily used for remembered set entries but
> >> > >> > it's
> >> > >> > not at all clear why this means that we can safely ignore SRTs
> >> > >> > (e.g. in
> >> > >> > the FUN and THUNK cases).
> >> > >> >
> >> > >> > Can you shed some light on this?
> >> > >> >
> >> > >> > Cheers,
> >> > >> >
> >> > >> > - Ben
> >> > >> >
> >> > >> > _______________________________________________
> >> > >> > ghc-devs mailing list
> >> > >> > ghc-devs@haskell.org
> >> > >> > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
> >> > >> >
> >> > >> _______________________________________________
> >> > >> ghc-devs mailing list
> >> > >> ghc-devs@haskell.org
> >> > >> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
> >> > >
> >> > >
> >
> >
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