BUG: Is it a bug in Linux time() function or Linux OS calls?

BUG: Is it a bug in Linux time() function or Linux OS calls?

[Srinivas Nayak]

Dear All,

I wrote a small program, that creates files at an interval of 1 minute. But the time at which the file is created and last written and the last modification time of the file as shown by ls command differs by 1 second. The code and the output is presented below. please let me know where could be the bug?

	root@new:/home/srinivas# cat b.c 
	#include <time.h> 
	#include <stdio.h> 
	#include <sys/stat.h> 
	#include <dirent.h> 
	#include <fcntl.h> 
	int main () 
	{ 
	    int fd; 
	    int i=0; 
	    time_t initial_time = time(NULL); 
	    time_t interval = 60; 
	    time_t curr_time = time(NULL); 

	    fd=open ("test1.txt", O_WRONLY | O_CREAT | O_TRUNC, 0666); 
	    write(fd,"abcd1",5); 
	    while(1) 
	    { 
		curr_time = time(NULL); 
		if(curr_time >= initial_time) 
		{ 
		    if(i==0) 
		    { 
			close(fd); 
			printf("\ntime before test2.txt fileopen= %d\n", time(NULL)); 
			fd=open ("test2.txt", O_WRONLY | O_CREAT | O_TRUNC, 0666); 
			write(fd,"abcd2",5); 
			printf("time after test2.txt filewrite= %d\n", time(NULL)); 
			system("ls -l --time-style=+%s test2.txt"); 
			initial_time += interval; 
			i=1; 
		    } 
		    else 
		    { 
			close(fd); 
			printf("\ntime before test1.txt fileopen= %d\n", time(NULL)); 
			fd=open ("test1.txt", O_WRONLY | O_CREAT | O_TRUNC, 0666); 
			write(fd,"abcd1",5); 
			printf("time after test1.txt filewrite= %d\n", time(NULL)); 
			system("ls -l --time-style=+%s test1.txt"); 
			initial_time += interval; 
			i=0; 
		    } 
		} 
		usleep(1000); 
	    } 
	    return 0; 
	} 
	root@new:/home/srinivas# gcc b.c 
	root@new:/home/srinivas# ./a.out 

	time before test2.txt fileopen= 1268203133 
	time after test2.txt filewrite= 1268203133 
	-rw-r--r-- 1 root root 5 1268203133 test2.txt 

	time before test1.txt fileopen= 1268203193 
	time after test1.txt filewrite= 1268203193 
	-rw-r--r-- 1 root root 5 1268203192 test1.txt 

	time before test2.txt fileopen= 1268203253 
	time after test2.txt filewrite= 1268203253 
	-rw-r--r-- 1 root root 5 1268203252 test2.txt 

	time before test1.txt fileopen= 1268203313 
	time after test1.txt filewrite= 1268203313 
	-rw-r--r-- 1 root root 5 1268203312 test1.txt 

	time before test2.txt fileopen= 1268203373 
	time after test2.txt filewrite= 1268203373 
	-rw-r--r-- 1 root root 5 1268203372 test2.txt 

	root@new:/home/srinivas# ls -ltr --time-style=+%s 
	total 40 
	-rwxrwxrwx  1 root     root      1095 1268202457 b.c 
	-rwxr-xr-x  1 root     root     10300 1268202459 a.out 
	-rw-r--r--  1 root     root         5 1268203312 test1.txt 
	-rw-r--r--  1 root     root         5 1268203372 test2.txt 
	root@new:/home/srinivas# 


Some system info:
===========
Linux new 2.6.24-19-server #1 SMP Wed Jun 18 14:44:47 UTC 2008 x86_64 GNU/Linux
Gnu C                  4.2.4
Linux C Library        2.7
binutils               2.18.0.20080103
util-linux             2.13.1
Console-tools          0.2.3
Sh-utils               6.10
===========


With best regards,
 
Yours sincerely,
Srinivas Nayak
 
My Home page: http://www.mathmeth.com/sn/
My Blog: http://srinivas-nayak.blogspot.com


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Re: BUG: Is it a bug in Linux time() function or Linux OS calls?

[David Newall]

Dear Srinivas Nayak,

I don't suppose you're writing to a network mounted disk, with the 
network device's clock about half a second behind your test machine?  
Otherwise -- and I'm just guessing here, and will leave it to you to 
UTSL -- this could be explained by use of two different algorithms for 
converting a higher-resolution time source to the one-second 
resolution.  A truncation algorithm versus a rounding algorithm could 
produce the result you demonstrate.

Regards,

David

Re: BUG: Is it a bug in Linux time() function or Linux OS calls?

[Srinivas Nayak]

Dear David,

I am not writing to network mounted disk. This is locally mounted and filesystem is ext3.

With best regards,
 
Yours sincerely,
Srinivas Nayak
 
My Home page: http://www.mathmeth.com/sn/
My Blog: http://srinivas-nayak.blogspot.com


--- On Wed, 10/3/10, David Newall <davidn@davidnewall.com> wrote:

> From: David Newall <davidn@davidnewall.com>
> Subject: Re: BUG: Is it a bug in Linux time() function or Linux OS calls?
> To: "Srinivas Nayak" <sinu_nayak2001@yahoo.co.in>
> Cc: linux-kernel@vger.kernel.org
> Date: Wednesday, 10 March, 2010, 2:17 PM
> Dear Srinivas Nayak,
> 
> I don't suppose you're writing to a network mounted disk,
> with the network device's clock about half a second behind
> your test machine?  Otherwise -- and I'm just guessing
> here, and will leave it to you to UTSL -- this could be
> explained by use of two different algorithms for converting
> a higher-resolution time source to the one-second
> resolution.  A truncation algorithm versus a rounding
> algorithm could produce the result you demonstrate.
> 
> Regards,
> 
> David
> 


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Re: BUG: Is it a bug in Linux time() function or Linux OS calls?

[john stultz]

On Tue, Mar 9, 2010 at 11:59 PM, Srinivas Nayak
<sinu_nayak2001@yahoo.co.in> wrote:
> I wrote a small program, that creates files at an interval of 1 minute. But the time at which the file is created and last written and the last modification time of the file as shown by ls command differs by 1 second. The code and the output is presented below. please let me know where could be the bug?
>
[snip]
> Some system info:
> ===========
> Linux new 2.6.24-19-server #1 SMP Wed Jun 18 14:44:47 UTC 2008 x86_64 GNU/Linux
> Gnu C                  4.2.4
> Linux C Library        2.7
> binutils               2.18.0.20080103
> util-linux             2.13.1
> Console-tools          0.2.3
> Sh-utils               6.10
> ===========

Hrm. Is CONFIG_NO_HZ enabled?  If so this is a known issue. The
filesystem timestamps use xtime, which is normally updated every HZ.
However to reduce overhead with NO_HZ we changed the update to be a
half-second, which can cause issues like what you describe.

The issue has since been resolved, so I'd be interested to hear if you
can still reproduce it on a more recent kernel.

thanks
-john

Re: BUG: Is it a bug in Linux time() function or Linux OS calls?

[Srinivas Nayak]

Dear John,

Thanks for your answer. This is informative.

With best regards,
 
Yours sincerely,
Srinivas Nayak
 
My Home page: http://www.mathmeth.com/sn/
My Blog: http://srinivas-nayak.blogspot.com


--- On Wed, 10/3/10, john stultz <johnstul@us.ibm.com> wrote:

> From: john stultz <johnstul@us.ibm.com>
> Subject: Re: BUG: Is it a bug in Linux time() function or Linux OS calls?
> To: "Srinivas Nayak" <sinu_nayak2001@yahoo.co.in>
> Cc: linux-kernel@vger.kernel.org
> Date: Wednesday, 10 March, 2010, 10:49 PM
> On Tue, Mar 9, 2010 at 11:59 PM,
> Srinivas Nayak
> <sinu_nayak2001@yahoo.co.in>
> wrote:
> > I wrote a small program, that creates files at an
> interval of 1 minute. But the time at which the file is
> created and last written and the last modification time of
> the file as shown by ls command differs by 1 second. The
> code and the output is presented below. please let me know
> where could be the bug?
> >
> [snip]
> > Some system info:
> > ===========
> > Linux new 2.6.24-19-server #1 SMP Wed Jun 18 14:44:47
> UTC 2008 x86_64 GNU/Linux
> > Gnu C                  4.2.4
> > Linux C Library        2.7
> > binutils               2.18.0.20080103
> > util-linux             2.13.1
> > Console-tools          0.2.3
> > Sh-utils               6.10
> > ===========
> 
> Hrm. Is CONFIG_NO_HZ enabled?  If so this is a known
> issue. The
> filesystem timestamps use xtime, which is normally updated
> every HZ.
> However to reduce overhead with NO_HZ we changed the update
> to be a
> half-second, which can cause issues like what you
> describe.
> 
> The issue has since been resolved, so I'd be interested to
> hear if you
> can still reproduce it on a more recent kernel.
> 
> thanks
> -john
> 


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