Re: Proposal: Use hi-res clock for file timestamps

Neil Brown
2010-08-19T00:52:33Z

On Thu, 19 Aug 2010 09:41:36 +1000
Neil Brown <neilb@suse.de> wrote:

> So I agree that this is probably more of an issue for directories than for
> files, and that implementing it just for directories would be a sensible
> first step with lower expected overhead - just my reasoning seems to be a bit
> different.

Just to be sure we are on the same page:
file_update_time would always refer to current_nfsd_time, but nfsd would
only update current_nfsd_time when a directory was examined (and the other
conditions were met).

So my current thinking on how this would look - names have been changed:

- global timespec 'current_fs_precise_time' is zeroed when
current_kernel_time moves backwards and is protected by a seqlock

- current_fs_time would be
now = max(current_kernel_time(), current_fs_precise_time)
return timespec_trunc(now, sb->s_time_gran)
(with appropriate seqlock protection)

- new function in fs/inode.c
get_precise_time(timestamp)
cft = current_fs_time()
if (timestamp == cft)
write_seqlock()
if cft == current_fs_precise_time
current_fs_precise_time.tv_nsec++
else if cft > current_fs_precise_time
current_fs_precise_time = cft
write_sequnlock()
return timestamp

- nfsd xdr response routine does
ts = inode->i_mtime
if (S_ISDIR(inode->i_mode))
ts = get_precise_time(ts)
xdr_encode_timespec(ts)

get_precise_time() probably needs a bit more subtlety to handle different
s_time_gran values and possible races, but I think it is fairly close.

Then if we ever had an xstat or similar that could ask for precise
timestamps, it just makes a similar call to get_precise_time.
Also if we added code later to use a hires timer on hardware where it was
efficient, get_precise_time could test for that and become a no-op

Yes, I should probably turn this into a patch ... maybe another day.

NeilBrown