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DATE 2015-04-01

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MESSAGE
DATE 2015-04-02
FROM Ruben Safir
SUBJECT Subject: [LIU Comp Sci] Fwd: omp pthread madness
From owner-learn-outgoing-at-mrbrklyn.com Thu Apr 2 13:22:01 2015
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From: Ruben Safir
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From: ruben
Newsgroups: comp.unix.programmer
Subject: omp pthread madness
Date: Wed, 1 Apr 2015 04:40:44 +0000 (UTC)
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I'm doing this test program with OMP threads and I'm getting a very odd
result which I have no idea where this is happening.

i set up a shared workspace for the threads called wordspace[MAX_SPACE]
and what should happen is that it gets filled with coordinate points. PI
can be estimated by a formula based on how many at random as they are
picked, end up within a circle.

When I construct it without threads, it does a decent job of predicting
about 3.14 etc.

With OMP, it is giving numbers in the 6 zone. I don't see anything that
would make my random selection change, nor do I see anything else, so I'm
at a loss, and admittedly can use some mentoring here, not only to
understand my mistake, but to understand the theory so I don't repeat it.


pi_lib.h

#ifndef MONTI_PI
#define MONTI_PI

#define MEMORY_SPACE 10000

struct point{
double x;
double y;
};

struct point * random_point(void);
double calc_pi(void);
int count_inside(void);


#endif


pid_lib.c

/* Question 4.18 from the operating systems HW from Text
* OS Ed 9
* Class Operating SYstems at LIU
* Prog M Ghriger
*
* Ruben Safir - Student
* ********************************************************/
#include
#include
#include "pi_lib.h"
#include
#include

struct point workspace[MEMORY_SPACE];
long seed = 0;
long inside_the_circle = 0;
struct timespec ts;
struct point sample;

struct point * random_point(void)
{
int i;
double x,y;

if(seed == 0){
clock_gettime(CLOCK_REALTIME, &ts);
seed = ts.tv_nsec;
srand(seed);
}
for (i = 0; i < MEMORY_SPACE; i++){
//fprintf( stderr, "Error %d\n", __LINE__);
//x = ((double)rand()) % (double)1000000 ;
x = rand() % 1000000 ;
x = x / (double)1000000;

// printf ("x==> %F", x);
//y = ((double)rand())% (double)1000000 ;
y = rand()% 10000000 ;
y = y / (double)10000000;
// printf(" y==> %F\n", y);
workspace[i].x = x;
workspace[i].y = y;
}

return &workspace[0];
}

int count_inside(void)
{
int i = 0;
double d;
for(i = 0; i < MEMORY_SPACE; i++){
d = sqrt(pow(workspace[i].x, 2.0) + pow(workspace[i].y,
2.0));
// printf ("distance => %f\n", d);
if (d <= 1.0)
inside_the_circle++;
}
return inside_the_circle;
}

double calc_pi(){
return 4.0 * (( double)inside_the_circle/ (double)MEMORY_SPACE) ;
}



monty_pi_omp.c

#include
#include
#include "pi_lib.h"
#include
#include

void * wrapper(void*);
double pi;

int main(int argc, char * argv[]){
double pi;
#pragma omp parallel
{
random_point();
count_inside();
}

pi = calc_pi();
printf("Pi is estimated at ==> %f\n\n", pi);

return 1;
}


and just for comparison

this is monty_pi.c

#include
#include
#include "pi_lib.h"
#include

void * wrapper(void*);

int main(int argc, char * argv[]){
double pi;
pthread_attr_t attr;
pthread_attr_init(&attr);
pthread_t tid;
void * arg = NULL;
pthread_create(&tid, &attr, wrapper, arg);
pthread_join(tid, NULL);

pi = calc_pi();
printf("Pi is estimated at ==> %f\n\n", pi);

return 1;
}

void * wrapper(void * arg){
random_point();
count_inside();
pthread_exit(arg);
}

and this was the makefile

XX:=gcc
CXXFLAGS:=-Wall -g -pg
LDFLAGS:= -pg -pthread -fopenmp

monty_pi_omp : monty_pi_omp.o pi_lib.o
$(CXX) $(CXXFLAGS) $(LDFLAGS) -o monty_pi_omp.exe monty_pi_omp.o
pi_lib.o

monty_pi : monty_pi.o pi_lib.o
$(CXX) $(CXXFLAGS) $(LDFLAGS) -o monty_pi.exe monty_pi.o pi_lib.o

monty_pi_omp.o : monty_pi_omp.c pi_lib.h
$(CXX) $(CXXFLAGS) $(LDFLAGS) -c monty_pi_omp.c

monty_pi.o : monty_pi.c pi_lib.h
$(CXX) $(CXXFLAGS) $(LDFLAGS) -c monty_pi.c

pi_lib.o : pi_lib.h pi_lib.c
$(CXX) $(CXXFLAGS) $(LDFLAGS) -c pi_lib.c

include make.deps
make.deps: *.c; ${CXX} ${CXXFLAGS} -M *.c >$-at-

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Path: reader1.panix.com!panix!goblin3!goblin.stu.neva.ru!bolzen.all.de!fu-berlin.de!uni-berlin.de!not-for-mail
From: jt-at-toerring.de (Jens Thoms Toerring)
Newsgroups: comp.unix.programmer
Subject: Re: omp pthread madness
Date: 1 Apr 2015 10:04:59 GMT
Organization: Freie Universitaet Berlin
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ruben wrote:
> I'm doing this test program with OMP threads and I'm getting a very odd
> result which I have no idea where this is happening.

> i set up a shared workspace for the threads called wordspace[MAX_SPACE]
> and what should happen is that it gets filled with coordinate points. PI
> can be estimated by a formula based on how many at random as they are
> picked, end up within a circle.

> When I construct it without threads, it does a decent job of predicting
> about 3.14 etc.

> With OMP, it is giving numbers in the 6 zone. I don't see anything that
> would make my random selection change, nor do I see anything else, so I'm
> at a loss, and admittedly can use some mentoring here, not only to
> understand my mistake, but to understand the theory so I don't repeat it.

Let me guess: you have a 2-core processor (or have set the
OMP_NUM_THREADS environment variable set to 2).

The obvious problem is that you have (at least) 2 threads which
uncoordinatedly write into shared, global variables. That's
always a recipe for disaster, with or without OpenMP. Consider
what's going on here: you have two threads that both write to
your 'workspace' (which isn't the major problem, but only be-
cause what gets written in there are random numbers, and it
doesn't help either) and you have these threads, when done with
filling 'workspace', both increment 'count_inside'. So, if you
have two threads, you will end up with a value of 'count_inside'
that is roughly twice the value it would have had when there
would only have been a single thread (since you did the coun-
ting twice). And thus you end up with a value for pi that is
too large by roughly a factor of 2.

If your intention is to do the whole calculation twice (or
as many times as you have cores or processors) and then e.g
average over the results from the different threads to in-
crease the precision, the way to go might be to have a single
function, with 'workspace' and 'inside_count' etc. as local
variables, that does the calculations and, when finished,
adds the calculated value of pi to a global variable, which
you protect from concurent accesses with e.g. a mutex. Then,
when all threads are done, divide that variable's value by
the number of threads (the omp_get_num_threads() function will
tell you how many you're using). And use rand_r() instead of
rand() since, as the man page explicitely states, rand() is
not thread-safe! (When using threads you better check for each
function you use if it's thread-safe and if it's not, either
use the replacement function with '_r' appended to the name
or, if such a replacement function does not exist, do not use
that function at all.)
Regards, Jens
--
\ Jens Thoms Toerring ___ jt-at-toerring.de
\__________________________ http://toerring.de

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From: ruben
Newsgroups: comp.unix.programmer
Subject: Re: omp pthread madness
Date: Thu, 2 Apr 2015 06:24:10 +0000 (UTC)
Organization: PANIX Public Access Internet and UNIX, NYC
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On Wed, 01 Apr 2015 10:04:59 +0000, Jens Thoms Toerring wrote:


> The obvious problem is that you have (at least) 2 threads which
> uncoordinatedly write into shared, global variables.



who is it uncoordinated. I couldn't think of anything that is shared
that would upset the final result.

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Path: reader1.panix.com!panix!not-for-mail
From: ruben
Newsgroups: comp.unix.programmer
Subject: Re: omp pthread madness
Date: Thu, 2 Apr 2015 06:29:53 +0000 (UTC)
Organization: PANIX Public Access Internet and UNIX, NYC
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On Wed, 01 Apr 2015 10:04:59 +0000, Jens Thoms Toerring wrote:

> The obvious problem is that you have (at least) 2 threads which
> uncoordinatedly write into shared, global variables.

How is it uncoordinated. I couldn't think of anything that is shared
that would upset the final result.

It is filling he space with random point. Even if it steps on itself, it
shouldn't cause a statistical bias.

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From: jt-at-toerring.de (Jens Thoms Toerring)
Newsgroups: comp.unix.programmer
Subject: Re: omp pthread madness
Date: 2 Apr 2015 07:54:23 GMT
Organization: Freie Universitaet Berlin
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Xref: panix comp.unix.programmer:234418

ruben wrote:
> On Wed, 01 Apr 2015 10:04:59 +0000, Jens Thoms Toerring wrote:

> > The obvious problem is that you have (at least) 2 threads which
> > uncoordinatedly write into shared, global variables.

> How is it uncoordinated. I couldn't think of anything that is shared
> that would upset the final result.

> It is filling he space with random point. Even if it steps on itself, it
> shouldn't cause a statistical bias.

But after filling 'workspace' with your random_point() function
you also have all threads run the count_insider() function. And
in that you increment the shared variable 'inside_the_circle'.
So this is now done twice (or as many threads you have) as often
as with a single-threaded program, so you will get a result for pi
that's about twice (or as many threads you have) as large as ex-
pected. Just move the line where you call count_inside() from the
omp block to directly after it and see what happens. Or divide the
result you get for pi by the number of threads.

Regards, Jens
--
\ Jens Thoms Toerring ___ jt-at-toerring.de
\__________________________ http://toerring.de

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From: jt-at-toerring.de (Jens Thoms Toerring)
Newsgroups: comp.unix.programmer
Subject: Re: omp pthread madness
Date: 2 Apr 2015 14:26:14 GMT
Organization: Freie Universitaet Berlin
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Xref: panix comp.unix.programmer:234420

ruben wrote:
> On Wed, 01 Apr 2015 10:04:59 +0000, Jens Thoms Toerring wrote:

> > The obvious problem is that you have (at least) 2 threads which
> > uncoordinatedly write into shared, global variables.

> How is it uncoordinated. I couldn't think of anything that is shared
> that would upset the final result.

It is uncoordinated because it's unpredictable in which sequence
the threads will access these global variables and, worse, they
may do it at the same time.

Are you aware of the 'sig_atomic_t' type in C? The reason
for its existence is that reads from or writes to even
something simple as an integer isn't "atomic", i.e. one
thread may have updated parts of that integer (say one of
the bytes it consists of) when another thread gets sche-
duled and tries to read this integer which is now in some
strange state. So the second thread will get some more or
less random value instead of what the programmer expected.
Or let's have thread 1 having started to update the integer,
then thread 2 comes along, updates all of it and then we're
back in thread 1 that writes the parts it hadn't got around
to before. Result? A random value in memory. What's special
about the 'sig_atomic_t' is that with that type this can't
happen, you're guaranteed that a thread of execution can't
be interupted while it writes or reads it.

Now, if this already can happen with integers, it clearly isn't
any better with floats or doubles. And your program assumes
obviously that the random values you write into the 'workspace'
have a well-definen upper limit (looks like it's 1). Unccor-
dinated writes to the same location may result in values that
doesn't satisfy this condition. And in other cases values
may be smaller. You probably won't notice it, but it can
happen and add some additional noise to the results of
your calculations.

Another place were things can go wrong is with your
'inside_the_circle' variable. There's a non-vanishing
chance that, for the line

inside_the_circle++;

the following happens: thread 1 reads its value to increment
it. Along comes thread 2 and possibly increments it several
times before thread 1 gets to finish what it set out to do.
Then it will increment ther value it had read and write it
into memory, thereby destroying all the work the other thread
had done in between.

Or you may end up with a situation where the compiler
optimizes your program in way that during the count_inside()
function the value of 'inside_the_circle' is kept in a CPU
register and only this copy is incremented. It's then only
written out at the very end of the function. And then, when
both threads run that function at the same time, they both
increment their copy in the CPU register and write it out at
the end, with the thread getting there last simply overwri-
ting whatever any other thread had put there before.

So you can't trust what this variable is set to when both
threads are done with it.

In any case, if you'd run your program several times with
the same seed for the random generator you'll rather likely
end up with different results. Your program has become in-
deterministic with its result depending on subtle timing
differences in how the threads get scheduled or access
your global variables. That's something one tries to avoid
like the plague.
Regards, Jens
--
\ Jens Thoms Toerring ___ jt-at-toerring.de
\__________________________ http://toerring.de

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Xref: panix comp.unix.programmer:234411

On 2015-04-01, ruben wrote:
> I'm doing this test program with OMP threads and I'm getting a very odd
> result which I have no idea where this is happening.
>
> i set up a shared workspace for the threads called wordspace[MAX_SPACE]
> and what should happen is that it gets filled with coordinate points. PI
> can be estimated by a formula based on how many at random as they are
> picked, end up within a circle.
>
> When I construct it without threads, it does a decent job of predicting
> about 3.14 etc.
>
> With OMP, it is giving numbers in the 6 zone. I don't see anything that
> would make my random selection change, nor do I see anything else, so I'm
> at a loss, and admittedly can use some mentoring here, not only to
> understand my mistake, but to understand the theory so I don't repeat it.
>
OpenMP often requires more then simply declaring a portion of code
parallel. I'm not an expert in its use but have looked into it a bit.
Below is an example where a variable, sum, is created that will be
unique in each thread then summed.

https://computing.llnl.gov/tutorials/openMP/#REDUCTION

#include
#include
#include


int main () {
static long num_steps = 100000;

double step = 1.0 / (double) num_steps;

double sum;

double start = omp_get_wtime ();

#pragma omp parallel
{
#pragma omp for reduction(+:sum) schedule(static)
for (int i = 0; i < num_steps; ++i) {
double x = (i + 0.5) * step;
sum += 4.0 / (1.0 + x * x);
}

}

double end = omp_get_wtime ();

double pi = step * sum;

printf ("PI = %.*f\n", DBL_DIG, pi);
printf ("time = %.*f\n", DBL_DIG, end - start);

return 0;

}


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Subject: Re: omp pthread madness
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Greg Martin writes:
>On 2015-04-01, ruben wrote:
>> I'm doing this test program with OMP threads and I'm getting a very odd
>> result which I have no idea where this is happening.
>>
>> i set up a shared workspace for the threads called wordspace[MAX_SPACE]
>> and what should happen is that it gets filled with coordinate points. PI
>> can be estimated by a formula based on how many at random as they are
>> picked, end up within a circle.
>>
>> When I construct it without threads, it does a decent job of predicting
>> about 3.14 etc.
>>
>> With OMP, it is giving numbers in the 6 zone. I don't see anything that
>> would make my random selection change, nor do I see anything else, so I'm
>> at a loss, and admittedly can use some mentoring here, not only to
>> understand my mistake, but to understand the theory so I don't repeat it.
>>
>OpenMP often requires more then simply declaring a portion of code
>parallel. I'm not an expert in its use but have looked into it a bit.
>Below is an example where a variable, sum, is created that will be
>unique in each thread then summed.
>
>https://computing.llnl.gov/tutorials/openMP/#REDUCTION
>
>#include
>#include
>#include
>
>
>int main () {
> static long num_steps = 100000;
>
> double step = 1.0 / (double) num_steps;
>
> double sum;
>
> double start = omp_get_wtime ();
>
>#pragma omp parallel
> {
>#pragma omp for reduction(+:sum) schedule(static)
> for (int i = 0; i < num_steps; ++i) {
> double x = (i + 0.5) * step;
> sum += 4.0 / (1.0 + x * x);
> }
>
> }
>
> double end = omp_get_wtime ();
>
> double pi = step * sum;
>
> printf ("PI = %.*f\n", DBL_DIG, pi);
> printf ("time = %.*f\n", DBL_DIG, end - start);
>
> return 0;
>
>}
>

And this should result in the compiler generating SIMD
instructions to provide parallelism; it's not multithreaded.

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Newsgroups: comp.unix.programmer
From: Greg Martin
Subject: Re: omp pthread madness
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On 2015-04-01, Scott Lurndal wrote:
> Greg Martin writes:
>>On 2015-04-01, ruben wrote:
>>> I'm doing this test program with OMP threads and I'm getting a very odd
>>> result which I have no idea where this is happening.
>>>
>>> i set up a shared workspace for the threads called wordspace[MAX_SPACE]
>>> and what should happen is that it gets filled with coordinate points. PI
>>> can be estimated by a formula based on how many at random as they are
>>> picked, end up within a circle.
>>>
>>> When I construct it without threads, it does a decent job of predicting
>>> about 3.14 etc.
>>>
>>> With OMP, it is giving numbers in the 6 zone. I don't see anything that
>>> would make my random selection change, nor do I see anything else, so I'm
>>> at a loss, and admittedly can use some mentoring here, not only to
>>> understand my mistake, but to understand the theory so I don't repeat it.
>>>
>>OpenMP often requires more then simply declaring a portion of code
>>parallel. I'm not an expert in its use but have looked into it a bit.
>>Below is an example where a variable, sum, is created that will be
>>unique in each thread then summed.
>>
>>https://computing.llnl.gov/tutorials/openMP/#REDUCTION
>>
>>#include
>>#include
>>#include
>>
>>
>>int main () {
>> static long num_steps = 100000;
>>
>> double step = 1.0 / (double) num_steps;
>>
>> double sum;
>>
>> double start = omp_get_wtime ();
>>
>>#pragma omp parallel
>> {
>>#pragma omp for reduction(+:sum) schedule(static)
>> for (int i = 0; i < num_steps; ++i) {
>> double x = (i + 0.5) * step;
>> sum += 4.0 / (1.0 + x * x);
>> }
>>
>> }
>>
>> double end = omp_get_wtime ();
>>
>> double pi = step * sum;
>>
>> printf ("PI = %.*f\n", DBL_DIG, pi);
>> printf ("time = %.*f\n", DBL_DIG, end - start);
>>
>> return 0;
>>
>>}
>>
>
> And this should result in the compiler generating SIMD
> instructions to provide parallelism; it's not multithreaded.

Where do you get that from? Everything I've read indicates that
#pragma omp parallel
will spawn threads.

--
http://www.softsprocket.com


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Subject: Re: omp pthread madness
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ruben , dans le message , a ?crit?:
> pid_lib.c

Apart from the obvious mistakes in thread handling, there are quite a few
stylistic issues in your code, I do not know if you are aware of them and
just wrote it that way to illustrate your problem or if they are genuine
mistakes.

> long seed = 0;
> long inside_the_circle = 0;

Do not use long (unless you are interacting with an old API that uses it),
it is just useless. Use types adapted for the use of your numbers (off_t for
file offsets, size_t for indices in memory objects) or their magnitude
(intXX_t, int_leastXX_t if you want to be sure).

> struct timespec ts;

Why does it need to be global?

> struct point sample;
>
> struct point * random_point(void)
> {
> int i;
> double x,y;
>
> if(seed == 0){
> clock_gettime(CLOCK_REALTIME, &ts);
> seed = ts.tv_nsec;
> srand(seed);
> }
> for (i = 0; i < MEMORY_SPACE; i++){
> //fprintf( stderr, "Error %d\n", __LINE__);
> //x = ((double)rand()) % (double)1000000 ;

> x = rand() % 1000000 ;

Do you know that some pseudo random numbers generators will perform very
badly with modulus. It could even be catastrophic with a power of two
modulus. Even nowadays, some libc still use theses PRNGs, for
compatibility's sake.

> x = x / (double)1000000;

Since you are dividing by a float anyway, do it the right way: RAND_MAX.

>
> // printf ("x==> %F", x);
> //y = ((double)rand())% (double)1000000 ;
> y = rand()% 10000000 ;
> y = y / (double)10000000;
> // printf(" y==> %F\n", y);
> workspace[i].x = x;
> workspace[i].y = y;
> }
>

> return &workspace[0];

Why are you returning a value that is never used?

> }
>
> int count_inside(void)
> {
> int i = 0;
> double d;
> for(i = 0; i < MEMORY_SPACE; i++){
> d = sqrt(pow(workspace[i].x, 2.0) + pow(workspace[i].y,
> 2.0));
> // printf ("distance => %f\n", d);
> if (d <= 1.0)
> inside_the_circle++;
> }
> return inside_the_circle;
> }
>
> double calc_pi(){
> return 4.0 * (( double)inside_the_circle/ (double)MEMORY_SPACE) ;
> }
>
>
>
> monty_pi_omp.c
>
> #include
> #include
> #include "pi_lib.h"
> #include
> #include
>
> void * wrapper(void*);
> double pi;
>
> int main(int argc, char * argv[]){
> double pi;
> #pragma omp parallel
> {

> random_point();
> count_inside();

Why do you feel the need of storing each point and then counting them?
Generate the point and count them directly, no need for a huge memory
buffer.

> }
>
> pi = calc_pi();
> printf("Pi is estimated at ==> %f\n\n", pi);
>

> return 1;

Did you program fail?

> }

--------------030607060602070805010502--

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