Re: Problem with function select on kernel 2.6.29.6-rt23

[Sujit K M <sjt.kar@gmail.com>]

Hi,

One thing at the onset I would like you to check is that what happens
to the program when the loop
count is made more like 1000/10,000/100000 - 1 Million/10 Million.
Does the Time Graph Increase.
Try Plotting the Difference with actual time start. Try Making Use of
Some scripting language like TCL/TK.

There is some info regarding the select system call. I think it is
pertaining to this problem.
http://linux.die.net/man/2/syscalls. Basically It is an Optimization
that the Current Kernels Look Into.

Thanks,
Sujit

On Sat, Sep 19, 2009 at 1:10 AM, Jürgen Mell <mell@hedrich-winders.com> wrote:
> Meanwhile I have dug a little deeper into this problem. The problem
> occurs under the following conditions:
> - the BIOS clock must be slow
> - the NTP daemon is used to adjust the system time
> The problem can be reproduced on real hardware as well as on a virtual
> machine running under VMware. Set the BIOS clock back about ten minutes
> against the 'real' time. Then start the NTP daemon and then run the
> little test program:
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <time.h>
> #include <errno.h>
> #include <sys/select.h>
>
> int main(int argc, char *argv[])
> {
>   time_t t;
>   struct timeval timeout;
>   int i;
>   int ret;
>
>   t = time (NULL);
>   printf ("Current time before = %s", ctime (&t));
>
>   for (i = 0; i < 20; i++)
>   {
>      timeout.tv_sec  = 1;
>      timeout.tv_usec = 0;
>
>      if ((ret = select (FD_SETSIZE, NULL, NULL, NULL, &timeout)) < 0)
>      {
>         printf ("select returned %d, errno = %d\n", ret, errno);
>         return EXIT_FAILURE;
>      }
>   }
>   t = time (NULL);
>   printf ("Current time after = %s", ctime (&t));
>   return EXIT_SUCCESS;
> }
>
> On a virtual machine under VMware I got the following result after some
> minutes of system run time:
>
> hws@cwc-vmware:/home/hws > /space/software/select_test/debug/src/select_test
> Current time before = Fri Sep 18 20:05:51 2009
> Current time after = Fri Sep 18 20:06:11 2009
> hws@cwc-vmware:/home/hws > /space/software/select_test/debug/src/select_test
> Current time before = Fri Sep 18 20:14:29 2009
> Current time after = Fri Sep 18 20:14:33 2009
> hws@cwc-vmware:/home/hws > /space/software/select_test/debug/src/select_test
> Current time before = Fri Sep 18 20:14:57 2009
> Current time after = Fri Sep 18 20:14:57 2009
> hws@cwc-vmware:/home/hws > /space/software/select_test/debug/src/select_test
> Current time before = Fri Sep 18 20:15:20 2009
> Current time after = Fri Sep 18 20:15:40 2009
> hws@cwc-vmware:/home/hws >
>
> Normally, the time distance between 'before' and 'after' should be 20
> seconds as in the first and last run of the program. For the second run
> the time difference is only 4 seconds and for the third run it is even zero.
>
> On the real hardware I have also some other time-related issues when the
> problem occurs. Keyboard input will often 'bounce' - key presses are
> detected two or more times and some delay times are prolonged (!). I
> could not yet reproduce this in the virtual machine.
>
> The problem will not always occur immediately after the system is
> started but it may take several minutes until the first effects occur. I
> have not tested this issue with other kernels yet but I will do so
> during the weekend.
>
> Are there any ideas what to do about this (beside buying a better BIOS
> clock)? I would really like to have the NTP daemon running to keep the
> system time accurate, but somehow it seems to effect wait queues in the
> kernel pretty badly.
>
> Bye,
>           Jürgen
>
> Jürgen Mell schrieb:
>> Hi,
>>
>> I have an application which connects via a network socket to a server
>> running on the same machine (IP 127.0.0.1) This application uses the
>> function 'select' to wait for new data from the server or until a two
>> seconds timeout. This works well until there is network traffic on the
>> external network interfaces (eth* or WLAN). When there is network
>> traffic on the external interfaces, the select function does not wait
>> anymore but it returns with a return code of zero, indicating not data
>> available on the socket. This happens nearly immediately (after 8 to 9
>> microseconds) and not after the specified two seconds interval. The
>> timeout parameter of select is updated accordingly (it shows eg. 1 s
>> 999991 us).
>> Up to now I could not test this with another kernel but I will try to
>> do it this afternoon. Are there any known problems with select? Is
>> there any way to circumvent this?
>>
>> Any help would be greatly appreciated!
>>
>>        Jürgen
>>
>
> --
> Jürgen Mell (Software-Entwicklung)       mell@hedrich-winders.com
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-- 
-- Sujit K M

blog(http://kmsujit.blogspot.com/)
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