Re: Time nonlinearity (gettimeofday vs. mtime)

[john stultz]

On Fri, Nov 27, 2009 at 5:31 AM, Matthias Urlichs <matthias@xxxxxxxxxx> wrote:
> Lately I've seen this ugliness:
>
> 13:39:06.000313 clock_gettime(CLOCK_REALTIME, {1259325546, 341196}) = 0 <0.000010>
> 13:39:06.000685 mkdir("/var/tmp/CP_FileTest_TempFolder_d0AOiP/tempFolder1", 0777) = 0 <0.000043>
> 13:39:06.000973 stat64("/var/tmp/CP_FileTest_TempFolder_d0AOiP/tempFolder1", {st_dev=makedev(252, 2), st_ino=1919104, st_mode=S_IFDIR|0755, st_nlink=2, st_uid=501, st_gid=501, st_blksize=4096, st_blocks=8, st_size=4096, st_atime=2009/11/27-13:39:05, st_mtime=2009/11/27-13:39:05, st_ctime=2009/11/27-13:39:05}) = 0 <0.000015>
>
> This strace says that st.st_mtime is smaller than time.tv_sec even though the time was acquired earlier.
> Apparently, the problem is that ext3 uses a cached time value for performance.
>
> Question: Is there a reason that the cached time is not updated every time somebody calls gettimeofday() or clock_gettime()?
> Or did just that nobody notice this problem yet?

This behavior is expected. For performance reasons (some clocksources
take 1.3us per read), most filesystem code uses current_kernel_time()
which provides the time at the last timer tick.

If you check mainline, there's a new CLOCK_REALTIME_COARSE clockid,
that provides the same behavior as the filesystem timestamps.

thanks
-john
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