Re: [DynInst_API:] Hung process

[Bill Williams <bill@xxxxxxxxxxx>]

On 02/16/2015 10:35 AM, Gerard wrote:
> Hi,
>
> I'm having another problem. I increased the MAX_THREADS variable to
> 10240 because I need to instrument a mutatee that creates around 10000
> threads. After increasing the variable I could instrument the mutatee
> without problems, but I added more snippets and the process hung again,
> this time after creating 3999 threads. Now, it doesn't matter if I
> increase even more the constant, it still hangs.
>
> Is there any other reason why the mutatee could hang or is there any
> other limit somewhere?
I'm not aware of other hardcoded limits, and if you already had tramp 
guards disabled, it seems likely that you're running into some form of 
bad behavior at scale that's independent of MAX_THREADS.
Is it possible for you to put together a simple reproducer 
(mutator/mutatee) and send that to me? Do you know whether the process 
is hanging at a particular point (thread creation/destruction, process 
exit)?
> Thanks,
>
> Gerard
>
> 2015-02-04 11:22 GMT+01:00 Gerard <nouboh@xxxxxxxxx
> <mailto:nouboh@xxxxxxxxx>>:
>
>     Thanks! That was exactly the problem. I have increased the constant
>     MAX_THREADS and now I don't have this problem anymore. Is there any
>     reason why the constant is set to 32 instead of a higher value?
>
>     I also have tried to find how to dynamically change
>     the DYNINST_max_num_threads value but I haven't found where is it
>     implemented (version 8.2.1). And about disabling tramp guards, I
>     already had them disabled so it seems that this workaround does not
>     solve the problem.
>
>     Gerard
>
>     2015-02-03 20:13 GMT+01:00 Barton Miller <bart@xxxxxxxxxxx
>     <mailto:bart@xxxxxxxxxxx>>:
>
>         Disabling tramp guards certainly works if you really know that
>         you're not recursing.  That can be subtle and error prone, which
>         is why tramp guards were invented.  Proceed cautiously here.
>
>         --bart
>
>
>         On 2/3/2015 12:02 PM, Matthew LeGendre wrote:
>
>             Another possible fix may be to disable tramp guards.  Tramp
>             guards are used to prevent recursive instrumentation.  For
>             example, if you instrument malloc() with instrumentation
>             that calls malloc(), then tramp guards will prevent you from
>             going into infinite recursion.
>
>             If you already know that your instrumentation can't
>             infinitely recurse,
>             then disabling tramp guards will give a big performance win
>             and may work around this hang.  To disable tramp guards, put
>             a call to BPatch::setTrampRecursive(__true) before you
>             insert instrumentation.
>
>             -Matt
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--
--bw

Bill Williams
Paradyn Project
bill@xxxxxxxxxxx