Minor page faults from pthread_create()

Minor page faults from pthread_create()

[Adrian Knoth]

Hi!

While playing with

 
https://rt.wiki.kernel.org/articles/t/h/r/Threaded_RT-application_with_memory_locking_and_stack_handling_example_f48b.html

I was wondering why pthread_create() should cause minor page faults when
there is a pre-allocated pre-faulted 100MB memory pool.

Simply running the example causes roughly 30 minor page faults. When I
manually allocate stack memory for the stack (after mlockall() and
mallopt have been called) and use pthread_attr_setstack() before calling
pthread_create(), minor page faults drop to 2.

Does anybody happen to know what is causing these remaining two page
faults? My only guess so far is kernel memory to hold some
organizational data for the new thread after clone() has been invoked,
but this could also be completely wrong.

Note that I don't need to get page faults down to 0, I'm only looking
for an explanation to understand the issue at hand to confirm that
pthread_create() must not be used from an RT context.


TIA

Re: Minor page faults from pthread_create()

[Remy Bohmer]

Hi,

2012/1/6 Adrian Knoth <adi@drcomp.erfurt.thur.de>:
> I was wondering why pthread_create() should cause minor page faults when
> there is a pre-allocated pre-faulted 100MB memory pool.

That pre-allocated block of memory is used by Glibc for its malloc routines.
While doing a pthread_create() you go to the kernel. Inside the kernel
many functions allocate memory as well. This is outside the control of
the C-library.
Probably some function inside the kernel allocates this memory and
uses it, resulting in these page-faults.
One example is allocating memory for the stack.

Kind regards,

Remy

Re: Minor page faults from pthread_create() aka still problems with 3.0.12-rt30

[Tim Sander]

Hi
> Simply running the example causes roughly 30 minor page faults. When I
> manually allocate stack memory for the stack (after mlockall() and
> mallopt have been called) and use pthread_attr_setstack() before calling
> pthread_create(), minor page faults drop to 2.
> 
> Does anybody happen to know what is causing these remaining two page
> faults? My only guess so far is kernel memory to hold some
> organizational data for the new thread after clone() has been invoked,
> but this could also be completely wrong.
> 
> Note that I don't need to get page faults down to 0, I'm only looking
> for an explanation to understand the issue at hand to confirm that
> pthread_create() must not be used from an RT context.
While unfortunatly i can't help i've got the impression that something is 
still strange with 3.0.12pmx-rt30. (This is on a arm i.mx35 with some 
platform adaptations and a cyclic clock interrupt.). I haven't build a newer 
version of 3.0-rt yet.

See my mail where i describe this problem here.
https://lkml.org/lkml/2011/12/7/657

I am ocasionally also seeing a ksoftirqd/0 running on as much cpu as it gets.
This is always when i see the message: sched: RT throttling activated

Not the best start into the new year :-(.

Best regards
Tim

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Re: Minor page faults from pthread_create()

[Arnaldo Carvalho de Melo]

Em Fri, Jan 06, 2012 at 09:46:29PM +0100, Adrian Knoth escreveu:
> While playing with
> 
> https://rt.wiki.kernel.org/articles/t/h/r/Threaded_RT-application_with_memory_locking_and_stack_handling_example_f48b.html
> 
> I was wondering why pthread_create() should cause minor page faults when
> there is a pre-allocated pre-faulted 100MB memory pool.
> 
> Simply running the example causes roughly 30 minor page faults. When I
> manually allocate stack memory for the stack (after mlockall() and
> mallopt have been called) and use pthread_attr_setstack() before calling
> pthread_create(), minor page faults drop to 2.
> 
> Does anybody happen to know what is causing these remaining two page
> faults? My only guess so far is kernel memory to hold some
> organizational data for the new thread after clone() has been invoked,
> but this could also be completely wrong.
> 
> Note that I don't need to get page faults down to 0, I'm only looking
> for an explanation to understand the issue at hand to confirm that
> pthread_create() must not be used from an RT context.

Try:

$ perf record --event minor-faults --call-graph your-program
$ perf report

Should answer that question if running on a -fno-omit-frame-pointer
binary world. Or at least provide the callchains for the kernel part of
the call chains.

32-bit distros should be OK in that respect.

- Arnaldo