performance of multithreading gets gradually worse under gdb

[Markus Alber]

 On Wed, 02 Feb 2011 13:43:50 -0800, Michael Snyder wrote:

>>  It allocates about 100kB per iteration.
>
> Hmmm, and that's for roughly 36 thread start/stops, so it could be
> losing roughly 3000 bytes per thread.  That's much bigger than my
> first guess would have been (a "struct thread_info" is only 336 
> bytes).
>
>>  One interesting finding might also be:
>> I terminated the process when an iteration took about 3 min (instead 
>> of 1 sec)
>>  and gdb had about 115MB allocated.
>
> I assume that at this point, your system still had plenty of ram to
> spare?  It wasn't simply swapping?

 The system had about 20 GB to spare.
 
>>  On starting the application again, it ran alright for a while and 
>> the gdb memory allocation
>>  stayed constant. When it finally started to grow again, the 
>> application slowed down, and became
>>  slower with every iteration - the usual picture.
>> I attached a sample file from the application where the computation 
>> bifurcates into
>>  the worker threads. This is one of three instances per iteration, 
>> but they all follow the
>>  same pattern.
>
> I was really hoping for a stripped-down sample that we could compile 
> and run.

 See the attached file. It shows a similar behaviour, although it only 
 allocates 8kB per iteration.
 You have to wait some time before this happens.

>> The machine has 2x6 cores x 3 instances per iteration = 36 worker 
>> threads per iteration.
>
> x86 architecture?
>
>>  On another note, I tried to compile gdb-6.5 on my machine (because 
>> it was the release I
>>  used to work with before, without problems) and configure comes 
>> back with an error that it cannot find a termcap lib. There is none on 
>> the SuSE. Which package would I need to install?
>
> That would be libncurses, I think.


 I compiled gdb-6.5 alright and it performs well as usual, without this 
 problem.


>
>
>>  On Wed, 02 Feb 2011 12:27:58 -0800, Michael Snyder wrote:
>>> Markus Alber wrote:
>>>>  Hello,
>>>> I have experienced the following problem:
>>>> I'm debugging a number-crunching application which spawns a lot 
>>>> (36) little
>>>>  worker threads per iteration. The system does typically OoM 200 
>>>> iterations.
>>>>  Although each of them should take about the same amount of time, 
>>>> the performance
>>>>  gets worse with every iteration and becomes excruciatingly slow.
>>>> A system monitor reveals that gdb allocates more memory with every 
>>>> iteration,
>>>>  i.e. with every 36 threads started and finished. The CPU load of 
>>>> GDB goes up, too.
>>>>  The CPU usage of the application goes down. Compared to the solo 
>>>> performance, it
>>>>  gets slower by a factor 20 and more, if run long enough.
>>>> The application behaves perfectly when run by itself. The 
>>>> multi-threaded part is not
>>>>  debugging compiled when this behaviour occurs.
>>>> The distribution is SuSE 11.3 / gdb 7.1.
>>>> Is there anything I can change about this behaviour, any options 
>>>> of gdb that need to
>>>>  be set in these circumstances?
>>> Interesting.
>>>
>>> By how much does gdb's memory allocation increase?
>>> In total or, if possible, per iteration?  This might
>>> give is a clue as to where to look.
>>>
>>> Do you think you could write a simple sample program that
>>> allocates threads in a manner similar to your application?
>>>
>>> Thanks,
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