Re: Scavenging SRTs in scavenge_one

21 Jun 2018

      When scavenge_one() sees a STACK, it calls scavenge_stack() which traverses
the stack frames, including their SRTs.

So I don't understand what's going wrong for you - how are the SRTs not
being traversed?

Cheers
Simon

On 21 June 2018 at 11:58, Ömer Sinan Ağacan  wrote:
...
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 , 21 Haz 2018 Per, 13:42
tarihinde şunu yazdı:
...
...
Large objects can only be primitive objects, like MUT_ARR_PTRS,
...
...
the RTS, and none of these have SRTs.
Is is not possible to allocate a large STACK? I'm currently observing
...
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 , 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
...
...
Cheers
Simon
On 20 June 2018 at 09:20, Ömer Sinan Ağacan 
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 , 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 :
...
Your explanation is basically right. scavenge_one() is only used
for a
non-major collection, where we aren't traversing SRTs. Admittedly
...
...
...
...
...
subtle point that could almost certainly be documented better, I
...
...
...
...
...
just overlooked it.
More inline:
On 1 May 2018 at 10:26, Ömer Sinan Ağacan 
wrote:
>
> I have an idea but it doesn't explain everything;
>
> SRTs are used to collect CAFs, and CAFs are always added to the
allocated by
this in
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.
this is a
probably
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 :
> > 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
> >
> > _______________________________________________
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> > ghc-devs@haskell.org
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> >
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