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Old 11-13-2008, 05:14 PM
Yong Huang
 
Default NTP problem for virtual RHEL 4 server on VmWare

> "The 2.4 kernels have HZ set to 100, while 2.6 kernels have HZ set to
> 1000. This value affects the frequency of clock interrupts that are
> generated, and those are per-CPU (that's why the problem gets worse
> when you go SMP). This change in frequency is the main reason for the
> clock problems for Linux guests. That is why your RH 7.3 works OK,
> while RHEL 4 doesn't. Another drawback of this change in value of HZ
> is that 2.6 kernels will waste more CPU cycles than 2.4 kernels when
> idling. Much more. When you have several SMP Linux 2.6 guests, just
> to keep the kernels running will waste a lot of the available CPU time.
>
> One recommendation from VmWare is to recompile the 2.6 kernel with HZ
> reduced back to 100 (as it was in 2.4 kernels). I haven't yet
> attempted this, but I heard reports that it works nicely.

That's only an issue for Linux in VMWare. Correct? My 2.6 kernel RH Linux uses 100 clock tick frequency:

$ uname -r
2.6.9-55.ELsmp
$ cat /etc/redhat-release
Red Hat Enterprise Linux AS release 4 (Nahant Update 7)
$ getconf CLK_TCK
100

I also have RH Linux 5.2 (kernel 2.6.18-92.e15) installed inside Microsoft Virtual PC and it doesn't use 1000.

Yong Huang




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Old 11-18-2008, 07:02 AM
"Kenneth Holter"
 
Default NTP problem for virtual RHEL 4 server on VmWare

Actually, most of our VM are running kernel version 2.6, and are causing
little or no problem with clock drift. But thank you for your useful input
on the subject - I'll take this into account the next time I come across a
drifting clock.

On 11/13/08, Eric Sisler <lbylnxgek@gmail.com> wrote:
>
> Kenneth Holter <kenneho.ndu@gmail.com> wrote:
>
> > What puzzles me is why this VM drifts so much, while most of our other
> VMs
> > doens't (at least not more than NTPd can handle).
>
> The Linux 2.6.x kernel is largely the culprit. Are your other VMs 2.4
> kernels? The kernel frequency HZ changed between the 2.4.x & 2.6.x
> kernels. A snippet of a *much* earlier posting. Unfortunately I
> don't have the name to give credit where it's due:
>
> "The 2.4 kernels have HZ set to 100, while 2.6 kernels have HZ set to
> 1000. This value affects the frequency of clock interrupts that are
> generated, and those are per-CPU (that's why the problem gets worse
> when you go SMP). This change in frequency is the main reason for the
> clock problems for Linux guests. That is why your RH 7.3 works OK,
> while RHEL 4 doesn't. Another drawback of this change in value of HZ
> is that 2.6 kernels will waste more CPU cycles than 2.4 kernels when
> idling. Much more. When you have several SMP Linux 2.6 guests, just
> to keep the kernels running will waste a lot of the available CPU time.
>
> One recommendation from VmWare is to recompile the 2.6 kernel with HZ
> reduced back to 100 (as it was in 2.4 kernels). I haven't yet
> attempted this, but I heard reports that it works nicely.
>
> Apperently the behaviour of Windows is much closer to 2.4 Linux
> kernels. Hence you don't see the problem that much there."
>
> As I mentioned earlier, adding a small patch to set the kernel clock
> HZ back to 100 & building custom RPMs does work, but can be time
> consuming if you have many servers to maintain. I *think* I have notes
> about the process somewhere if anyone's interested.
>
> -Eric
>
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Old 11-19-2008, 03:30 PM
"Eric Sisler"
 
Default NTP problem for virtual RHEL 4 server on VmWare

Yong Huang <yong321@yahoo.com> wrote:

>> One recommendation from VmWare is to recompile the 2.6 kernel with HZ
>> reduced back to 100 (as it was in 2.4 kernels). I haven't yet
>> attempted this, but I heard reports that it works nicely.

> That's only an issue for Linux in VMWare. Correct? My 2.6 kernel RH Linux uses 100 clock tick frequency:

> $ uname -r
> 2.6.9-55.ELsmp
> $ cat /etc/redhat-release
> Red Hat Enterprise Linux AS release 4 (Nahant Update 7)
> $ getconf CLK_TCK
> 100

AFAIK, it doesn't make any difference if the server is physical or
virtual - they all use the same kernel. Not sure what value 'getconf
CLK_TCK' is actually reporting. All of my RHEL 3, 4 & 5 servers
(physical & virtual) report the same thing. I don't know if the
setting I'm talking about is reported by getconf or in the /proc or
/sys filesystems. Information about the kernel compile setting is
elsewhere at the moment, I'll check on it later.

-Eric

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