On Fri, May 06, 2022 at 02:06:14PM +0000, Trapp, Michael wrote:
>
>
> > On 29. Apr 2022, at 13:46, Karel Zak <kzak@xxxxxxxxxx> wrote:
> >
> > On Fri, Apr 22, 2022 at 12:08:30PM +0200, Michael Trapp wrote:
> >> +static int get_clock_cont(uint32_t *clock_high,
> >> + uint32_t *clock_low,
> >> + int num)
> >> +{
> >> + /* 100ns based time offset according to RFC 4122. 4.1.4. */
> >> + const uint64_t reg_offset = (((uint64_t) 0x01B21DD2) << 32) + 0x13814000;
> >> + THREAD_LOCAL uint64_t last_clock_reg = 0;
> >> + uint64_t clock_reg;
> >> +
> >> + if (last_clock_reg == 0)
> >> + last_clock_reg = get_clock_counter();
> >> +
> >> + clock_reg = get_clock_counter();
> >> + clock_reg += MAX_ADJUSTMENT;
> >> +
> >> + if ((last_clock_reg + num) >= clock_reg)
> >> + return -1;
> >
> >
> > If I read your code correctly, it initializes the clock at uuidd start
> > and then continues (forever). Each short time of inactivity will
> > increase the difference between time stored in UUIDs and real-time.
> > For example, this difference will be huge for databases where users
> > don't allocate new UUIDs at night.
> >
> >
> > Maybe we can implement some hybrid model that resets the continuous
> > clock start point (last_clock_reg) from time to time, for example,
> > every minute (hour, ...). I don't think it will be a performance
> > problem when it does not use LIBUUID_CLOCK_FILE. The result will be
> > UUIDs that match with reality. Does it make sense?
>
> Of course, we can use a <max_offset_value> as optarg for the ‘-C’ option and update last_clock_reg accordingly.
> A custom value could be set with ‘-C <NUM>’ and the feature could also be deactivated by omitting ‘-C’.
> What do you think about that?
Sounds good.
The question is (from my point of view), do we need fully continuous
(= disconnected) time in uuidd by default for -C? I guess it will be
confusing for UUIDs consumers. Maybe the default should be reset every
N minutes and <max_offset_value> as optional argument to modify it.
-C -- default is reset every N minutes
-C=<num> -- overwrite the default
-C=0 or -C=noreset -- don't reset
But I have no strong opinion about it, the default could be -C without
reset as well.
Do you plan to use -C for SAP databases? I guess it will resolve all
the performance issues for SAP use-cases.
My plan to keep /var/lib/libuuid/clock.txt in uuidd hands will be
unnecessary (or less important) if -C will be good enough.
Karel
--
Karel Zak <kzak@xxxxxxxxxx>
http://karelzak.blogspot.com
