Re: clock_gettime_ns

[H. Peter Anvin]

I think it would be crazy encoding UTC with a non-POSIX scheme.

Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>On Wed, Sep 4, 2013 at 3:29 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>> On 09/04/2013 01:54 PM, John Stultz wrote:
>>>>
>>>> I'd advocate for going whole hog and returning, atomically:
>>>>
>>>>  - TAI (nanoseconds from epoch)
>>>>  - UTC - TAI (seconds or nanoseconds) *
>>>>  - TAI - CLOCK_MONOTONIC (nanoseconds)
>>>>  - a leap second flag.
>>>>
>>>> * There are various ways to define this.  My fancy UTC - TAI
>wouldn't
>>>> actually need the leap-second flag, since the UTC time would
>indicate
>>>> leap seconds directly.
>>
>> Not so (see below).
>>
>>>  With the conventional approach, someone would
>>>> have to decide whether the leap second count increments at the
>>>> beginning or the end of the leap second.
>>>
>>> Well, adjtimex() gives you UTC & tai offset & leapsecond flag in one
>go.
>>>
>>
>> But not fractional-second information,right?  I believe it would be
>> desirable if we can create a small structure (<= 16 bytes) for this.
>>
>> UTC - TAI is always an integral number of seconds, possibly negative
>> (unlikely, but...)
>>
>> Something like:
>>
>>         struct time_ns {
>>                 u64 tai_s;
>>                 u32 tai_ns;
>>                 s16 utcdelta;   /* TAI - UTC */
>>                 u8 leap;        /* Positive leap second in progress
>*/
>>                 u8 pad;         /* Something useful here maybe? */
>>         };
>>
>> Why the leap second flag?  It is necessary to represent the 61st
>second
>> in a minute during a positive leap second.  Consider the below
>> (artificial) cases:
>>
>> (leap second)
>> TAI     31536000        31536001        31536002        31536003
>> Delta   2               2               ?               3
>> UTC     23:59:58        23:59:59        23:59:60        00:00:00
>>
>> (no leap second)
>> TAI     31536000        31536001        31536002        31536003
>> Delta   2               2               2               2
>> UTC     23:59:58        23:59:59        00:00:00        00:00:01
>>
>> (no leap second)
>> TAI     31536000        31536001        31536002        31536003
>> Delta   3               3               3               3
>> UTC     23:59:57        23:59:58        23:59:59        00:00:00
>>
>> There simply is no sufficiently meaningful value that can be put on
>the
>> delta during a positive leap second.  Both 2 and 3 would be wrong in
>the
>> above example, giving UTC of either 00:00:00 or 23:59:59.
>>
>> There is a way to do without the leap second flag by making UTC the
>main
>> time; this does have the advantage of higher compatibility with
>time_t,
>> struct timespec, etc:
>>
>>         struct timespecx {
>>                 time_t tx_sec;          /* POSIX UTC seconds */
>>                 u32 tx_ns;              /* Nanoseconds */
>>                 s32 tx_taidelta;        /* TAI - UTC */
>>         };
>>
>> The trick here is that tx_ns can grow all the way up to 1,999,999,999
>> during a positive leap second.
>>
>> (Note that while planning these sorts of things it is worth noting
>that
>> it is at least theoretically possible that another shift in the
>rotation
>> of the Earth could one day mean needing multiple leap seconds, so at
>> least allowing for them would be a good idea.  Both proposals above
>> would handle that -- up to 255 leap seconds for the former and 4 leap
>> seconds for the latter, either of which should be way more than
>necessary.)
>
>I suspect that nearly every program will screw this up -- leap second
>are rare, and the amount of branchy logic needed here is large.
>
>Let me clarify my proposal:
>
>A UTC time is year,month,day,hour,minute,second,fractional seconds.
>So 2013/12/31 23:59:60.100 is a valid UTC time, assuming that there's
>a leap second then.
>
>Suppose the epoch is 2013/12/31 00:00:00 UTC.  Then time 86399.000 is
>2013/12/31 23:59:59.000 UTC.  Time 86400.000 is 2013/12/31
>23:59:60.000 UTC, 86400.100 is 2013/12/31 23:59:60.000 UTC, and
>86401.000 is 2014/01/01 00:00:00.000 UTC.  This encoding happens
>regardless of whether 2013/12/31 actually has a leap second.
>
>So, for the purposes of the encoding, the last day of each month is
>86401 seconds long.  One of those seconds will most likely not occur.
>
>The benefits are that every possible UTC time has a unique
>representation as a single number.  That number increases
>monotonically with time.  The special case happens *every month*, so
>any program that screws it up will be obviously wrong.
>
>The main downside I can see is that it's a little strange.
>
>--Andy

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