Re: svn_io_sleep_for_timestamps not sleeping long enough

--
Join WANdisco's free daily demo sessions on Scaling Subversion for the Enterprise
<http://www.wandisco.com/training/webinars>
----- Original Message -----
> From: Philip Martin <philip_at_codematters.co.uk>
> To: dev_at_subversion.apache.org
> Cc: 
> Sent: Monday, 24 March 2014, 17:27
> Subject: svn_io_sleep_for_timestamps not sleeping long enough
> 
> I'm seeing that checkout does not always sleep for long enough for
> timestamp-based change detection to be reliable:
> 
>   svnadmin create repo
>   svn -mm import repo/format file://`pwd`/repo/f
>   svn co file://`pwd`/repo wc
>   echo x > wc/f
>   svn st wc
> 
> For some checkouts status shows the modification of wc/f, for other
> checkouts it does not.
> 
> The problem is that my filesystem has about 4ms timestamp resolution
> while svn_io_sleep_for_timestamps() only sleeps for 1ms.  I can
> determine the resolution experimentally using:
> 
>   echo x > f && stat f | grep Modify && echo y > f 
> && stat f | grep Modify
> 
> often that prints the same timestamp twice, but when the timestamps
> differ they typically differ by about 4ms:
> 
>   Modify: 2014-03-24 16:40:17.566569826 +0000
>   Modify: 2014-03-24 16:40:17.570569785 +0000
> 
> In order to avoid missing changes I need to change
> svn_io_sleep_for_timestamps:
> 
> Index: subversion/libsvn_subr/io.c
> ===================================================================
> --- subversion/libsvn_subr/io.c    (revision 1580884)
> +++ subversion/libsvn_subr/io.c    (working copy)
> @@ -1315,7 +1315,7 @@ svn_io_sleep_for_timestamps(const char *path, apr_
> 
>            /* Sleep for at least 1 millisecond.
>               (t < 1000 will be round to 0 in apr) */
> -          apr_sleep(1000);
> +          apr_sleep(4000);
> 
>            return;
>          }
> 
> I'm running a standard 64-bit Debian/stable kernel with working copies
> on an ext4 filesystem on a local SSD.  I believe the 4ms is related to
> the kernel's CONFIG_HZ=250 setting (it also has CONFIG_NO_HZ=y).  Now I
> could build a custom kernel and increase the resolution but I think
> Subversion should support what is essentially a standard Linux setup.  I
> think we should increase the sleep to at least 10ms on the basis that
> Linux kernels built with CONFIG_HZ=100 are not unusual.  This does
> penalise systems with true nanosecond resolution as they will sleep 9ms
> more than necessary.
> 
> I've also got an ext3 filesystem on a spinning disk but that doesn't
> show the problem because timestamps are always whole seconds and so
> svn_io_sleep_for_timestamps uses a different code path.
Is it possible to query the system clock resolution at run time? That would seem best.
But, compared with the present 1 ms that's inadequate on many systems and the 1 second that's too much on some but not enough on others, I would be happy with simply increasing the 1 ms to 10 ms for a "quick fix" if we don't want to invest any more effort.
- Julian