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Subject: Re: [OMPI users] printf and scanf problem of C code compiled with Open MPI
From: Yevgeny Kliteynik (kliteyn_at_[hidden])
Date: 2011-03-31 04:44:41


You can explicitly specify the type of buffering
that you want to get with setvbuf() C function.
It can be block-buffered, line-buffered and unbuffered.

Stdout is line-buffered by default.
To make it un-buffered, you need something like this:

setvbuf(stdout, NULL, _IONBF, 0)

-- YK

On 30-Mar-11 1:13 PM, Jeff Squyres wrote:
> I don't remember this being a function of the C language or the compiler; I'm pretty sure you can change whether stdout is line buffered or not at the OS level.
>
> As for OMPI, we deliberately set each MPI process' stdout to be line buffered before relaying it up to the mpirun process. This was because we wanted to limit non-MPI network traffic (i.e., bundle more output into fewer, longer messages, rather than more, shorter messages -- i.e., less overall network and CPU overhead). I don't know if that makes anything more or less friendly to interactive computing; I don't think using fflush() or \n is onerous.
>
> Indeed, stdout / stdin processing likely isn't part of the high performance section of your application, so we figured it was an easy tradeoff to make (i.e., less network overhead leading to potentially better MPI message performance).
>
>
> On Mar 30, 2011, at 3:31 AM, Meilin Bai wrote:
>
>> You remind me. I now realize that it is not an matter of compiler, but an issue of C language. The printf() function in C doesn't print messages onto the standard ouput immediately, but instead stores them in a buffer. Only in some cases does the standard output work, defined in standard C:
>> 1. The buffer is full or freshed compulsively (like use fflush()).
>> 2. When a newline comes (\n).
>> 3. Needs reading from buffer (such as scanf()).
>>
>> Some compilers may deal with this function, and compiled by Mpich2 this code seems be well-off.
>>
>> Thanks very much.
>>
>>
>> On Wed, Mar 30, 2011 at 3:39 PM, Pascal Deveze<Pascal.Deveze_at_[hidden]> wrote:
>> Maybe this could solve your problem: Just add \n in the string you want to display:
>> printf("Please give N= \n");
>>
>> Of course, this will return, but the string is displayed. This run by me without the fflush().
>>
>> On the other hand, do you really observe that the time of the scanf () and the time to enter "N" be insignificant ?
>>
>> Pascal
>>
>> Meilin Bai a écrit :
>>> So it means that MPI doesn't suit to interactive programming? Though we can really use fflush(stdout) to get the right order, it takes too more time, and it's said that using fflush() is not a good progrmming style in C.
>>>
>>> On the other hand, in Fortran language, this situation won't exist. Maybe it is because I/O implement is a built-in part of Fortran, while in C/C++ it is realized only through function like scanf, printf, et al?
>>>
>>>
>>>
>>> On Wed, Mar 30, 2011 at 2:38 AM, Prentice Bisbal<prentice_at_[hidden]> wrote:
>>> On 03/29/2011 01:29 PM, Meilin Bai wrote:
>>>> Dear open-mpi users:
>>>>
>>>> I come across a little problem when running a MPI C program compiled
>>>> with Open MPI 1.4.3. A part of codes as follows:
>>>>
>>>> MPI_Init(&argc,&argv);
>>>> MPI_Comm_size(MPI_COMM_WORLD,&numprocs);
>>>> MPI_Comm_rank(MPI_COMM_WORLD,&myid);
>>>> MPI_Get_processor_name(processor_name,&namelen);
>>>> if (myid == 0) {
>>>> printf("Please give N= ");
>>>> //fflush(stdout);
>>>> scanf("%d",&n);
>>>> startwtime = MPI_Wtime();
>>>> }
>>>>
>>>> If comment out the sentence of "fflush(stdout);", it doesn't print out
>>>> the message till I input an integer n. And if I add the fflush function
>>>> between them, it works as expected, though comsumming time obviously.
>>>>
>>>> However, when I compiled it with Mpich2-1.3.2p1, without fflush function
>>>> in the code, it works correctly.
>>>>
>>>> Can anyone know what the matter is.
>>>>
>>>
>>> The Open MPI Developers (Jeff, Ralph, etc) can confirm this:
>>>
>>> The MPI standard doesn't have a lot of strict requirements for I/O
>>> behavior like this, so implementations are allowed to buffer I/O if they
>>> want. There is nothing wrong with requiring fflush(stdout) in order to
>>> get the behavior you want. In fact, if you check some text books on MPI
>>> programming, I'm pretty sure they recommend using fflush to minimize
>>> this problem.
>>>
>>> MPICH behaves differently because its developers made different design
>>> choices.
>>>
>>> Neither behavior is "wrong".
>>>
>>> --
>>> Prentice
>>> _______________________________________________
>>> users mailing list
>>> users_at_[hidden]
>>> http://www.open-mpi.org/mailman/listinfo.cgi/users
>>>
>>>
>>>
>>> --
>>> Meilin Bai
>>>
>>>
>>> _______________________________________________
>>> users mailing list
>>>
>>> users_at_[hidden]
>>> http://www.open-mpi.org/mailman/listinfo.cgi/users
>>
>>
>> _______________________________________________
>> users mailing list
>> users_at_[hidden]
>> http://www.open-mpi.org/mailman/listinfo.cgi/users
>>
>>
>>
>> --
>> Meilin Bai
>> School of Electronic Engineering and Computer Science, Peking University
>> Beijing 100871, China
>> E-Mail: meilin.bai_at_[hidden]
>> meilin_at_[hidden]
>> MSN: meilin.bai_at_[hidden]
>> Cellular: +86-1342-619-8430
>>
>> _______________________________________________
>> users mailing list
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>> http://www.open-mpi.org/mailman/listinfo.cgi/users
>
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