Hi,
Markus Hochholdinger wrote:
> Here is my solution to this problem, lenny xen kernel:
> * dom0 with clocksource=jiffies and /proc/sys/xen/independent_wallclock=0
> * domU with clocksource=jiffies and /proc/sys/xen/independent_wallclock=0
Using jiffies as a clock source is not a "solution", it's a workaround,
because its resolution (CONFIG_HZ^1) is not good enough for reading
microseconds, that is, time with microseconds will become just monotonic.
This will cause problems for any program that wants its time readouts to
be strictly increasing, as real-world time usually is :)
In other words you will get this (real example from a while ago):
% i=0; while :; do i=$((i+1)); if [ $i = 20 ]; then break; fi; date
--rfc-3339=ns; done
2009-09-23 13:35:13.123400807+02:00
2009-09-23 13:35:13.127400857+02:00
2009-09-23 13:35:13.131400906+02:00
2009-09-23 13:35:13.135400956+02:00
2009-09-23 13:35:13.139401005+02:00
2009-09-23 13:35:13.143401055+02:00
2009-09-23 13:35:13.147401104+02:00
2009-09-23 13:35:13.151401154+02:00
2009-09-23 13:35:13.151401154+02:00
2009-09-23 13:35:13.155401203+02:00
2009-09-23 13:35:13.155401203+02:00
2009-09-23 13:35:13.159401253+02:00
2009-09-23 13:35:13.163401302+02:00
2009-09-23 13:35:13.167401352+02:00
2009-09-23 13:35:13.171401401+02:00
2009-09-23 13:35:13.171401401+02:00
2009-09-23 13:35:13.175401451+02:00
2009-09-23 13:35:13.179401500+02:00
2009-09-23 13:35:13.183401550+02:00
In my case this manifested itself when some PHP profiling via microtime()
suddenly became useless, and it also caused occasional PostgreSQL errors
with tables that had timestamp columns as keys, since it became possible
for two independent transactions to come in at the exact same time.
Having said that, if one doesn't switch to jiffies but wants to use live
migration, that ends up being hampered by the "Time went backwards" problem.
--
2. That which causes joy or happiness.
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