[CentOS-virt] tick divider bugs

Mon May 5 13:41:11 UTC 2008
Johnny Hughes <johnny at centos.org>

Allen Tsang wrote:
> Also just to review, clocksource=acpi_pm should be used in conjunction 
> with the tools.synctime = "true" flag in your vmx file.  The combination 
> of the two settings prevents time from going into the future from too 
> many ticks, and synctime corrects slow clocks, which leads to a much, 
> much better clock sync.

That (clocksource=acpi_pm) is ONLY the best method if your clock is 
running SLOWER than normal .. and in fact, a "clocksource=pit" is 
probably the best solution for a clock that is running FASTER than 
normal.  If the clock is running FASTER than normal, also getting the 
max frequency (host.cpukHz) set per these instructions is important:


The /proc/ location in that article is not correct for centos4 or 
centos5, but you can probably get the correct info for frequency most of 
the time from /proc/cpuinfo, dmesg, or dmidecode.

> We'll have to wait until someone figures out a clever way to tie VM 
> clock ticks to a multiplexed physical clock source;
> until then, clocksync will always be a problem without a complete 
> solution (read up on it).  This is as close as it gets!

There is a potential fix in the Real Time Kernel () that Red Hat has 
released as part of their beta MRG release in that it might the support 
tickless option.  I have not had time to look at this solution yet, but 
the kernel SRPM is here if someone wants to:



Johnny Hughes

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