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SubjectRe: Finegrained a/c/mtime was Re: Directory notification problem
Hi Gerhard.

>>>> For stat is also requires a changed glibc ABI -- the glibc/2.4
>>>> stat64 structure reserved an additional 4 bytes for every
>>>> timestamp, but these either need to be used to give more seconds
>>>> for the year 2038 problem or be used for the ms fractions. y2038
>>>> is somewhat important too.

>>> The fields are meant for nanoseconds. The y2038 will definitely be
>>> solved by time-shifting or making time_t unsigned. In any way
>>> nothing of importance here and now. Especially since there won't be
>>> many systems which are running today and which have a 32-bit time_t
>>> be used then. For the rest I'm sure that in 37 years there will be
>>> the one or the other ABI change.

>> Right. Given current uptimes and being optimistic the fix for y2038
>> is probably needed by 2030 or just a little later. But in any case
>> 64 bit systems should be maxing out by then, and the conversion to
>> 128 bit systems should have already happened on the server side.
>> 32 bit systems will likely be limited to embedded and legacy systems
>> by then.

> Why do I get the feeling no one has learned from the problems the
> computer industry had with 2 digit date fields?

Precicely my feeling. Let's see what the various field widths do for the
y2038 problem, assuming a signed field and that we retain the current
date origin of Jan 1 00:00:00 UTC 1970 for the new routines:

Field Width Rollover Date Time
~~~~~~~~~~~ ~~~~~~~~~~~~~ ~~~~~~~~
32 19 Jan 2038 3:14:08
33 7 Feb 2106 6:28:16
34 16 Mar 2242 12:56:32
35 30 May 2514 1:53:04
36 26 Oct 3058 3:46:08
37 20 Aug 4147 7:32:16
38 8 Apr 6325 15:04:32
39 14 Jul 10680 6:09:04
40 25 Jan 19391 12:18:08
41 20 Feb 36812 0:36:16
42 10 Apr 71654 1:12:32
43 19 Jul 141338 2:25:04
44 4 Feb 280707 4:50:08
45 8 Mar 559444 9:40:16

I somehow don't see the need to go any further with this table...

We can get some really decent rollover dates by expanding the field
width, and the basic question comes down to how far ahead we wish to
push the problem - noting that the WinXX Y2K problem has only been
pushed back to be the Y10K problem now.

The other side of the equation is that we need to increase the
resolution with which we give out timestamps, and it appears to me that
the simplest means would be to change the kernel to use a smaller unit
to record timestamps. The current set of calls would then convert this
to seconds, and we would provide a new set of calls that returned the
raw values as used in the kernel.

Assuming the field widths have to be a complete number of bytes, we need
to determine what the minimum resolution is to allow us to record times
up to 00:00:00 GMT on the 1st of January in whatever year we wish to be
able to record up to. Here's what we would need to use for the given
field sizes to handle up to the following years:

Field Year Year Year Year Year Year Year Year Year
Width 2038 2500 5000 10000 25000 50000 100000 250000 500000
~~~~~ ~~~~~~ ~~~~~~ ~~~~~~ ~~~~~~ ~~~~~~ ~~~~~~ ~~~~~~ ~~~~~~ ~~~~~~
32 1 s
40 4 ms 31 ms 174 ms 461 ms
48 16 us 119 us 680 us 1.8 ms 5.1 ms 11 ms 22 ms 56 ms 112 ms
56 60 ns 465 ns 2.7 us 7.1 us 21 us 43 us 86 us 218 us 437 us
64 233 ps 1.8 ns 11 ns 28 ns 79 ns 165 ns 336 ns 849 ns 1.8 us
72 909 fs 7.1 ps 41 ps 108 ps 308 ps 642 ps 1.4 ns 3.4 ns 6.7 ns
80 3.6 fs 28 fs 159 fs 420 fs 1.2 ps 2.6 ps 5.2 ps 13 ps 27 ps
~~~~~ ~~~~~~ ~~~~~~ ~~~~~~ ~~~~~~ ~~~~~~ ~~~~~~ ~~~~~~ ~~~~~~ ~~~~~~

I note that with the recent Y2K changes, WinXX software will next hit
rollover in case (C), and we don't want to be worse than that. Also, to
keep the conversion routines for the current functions simple, we need
to choose an interval that divides exactly into one second.

I would therefore conclude that we could aim for any of the following:

Field Width Unit of Time Rollover Month
~~~~~~~~~~~ ~~~~~~~~~~~~ ~~~~~~~~~~~~~~

40 bits 500 ms May 10680
1 s Sep 19390

48 bits 2500 us Apr 13119
5 ms Jul 24268 *
10 ms Jan 46567
25 ms Jul 113462
125 ms Sep 559432

56 bits 10 us Nov 13386
25 us Feb 30512 *
50 us Mar 59054
100 us May 116138
500 us Nov 572811

64 bits 50 ns Jul 16583
100 ns Feb 31197 *
250 ns Oct 75037
500 ns Jul 148105
1000 ns Jan 294241
2500 ns Jul 732647

72 bits 125 ps Sep 11322
250 ps May 20675
500 ps Sep 39380 *
1 ns May 76791
10 ns Oct 750183

80 bits 500 fs Feb 11547
1000 fs Apr 21124
1250 fs Nov 25912 *
2500 fs Sep 49855
5 ps May 97741
10 ps Sep 193512
25 ps Nov 480826

Allowing that WinXX software is now only susceptible to the Y10K
problem, we can't afford to do worse than that, and the sooner we
sort this out, the better for all concerned as far as I can tell.

My personal choices at each field width would be those marked with an
asterisk, and this is based on the principle of using the shortest time
interval possible that is consistant with being able to record up to
around AD 25000 in a signed field.

My overall preference would be to go straight to 64 bit date fields and
define them as storing the time in units of 100 nanoseconds, but it has
apparently been decided that we will use 48 bit fields, if what I've
seen on this list is correct.

> Odds are legacy systems will be running something people for
> whatever reason couldn't replace.

Most probably...

Best wishes from Riley.

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