any advice to find root cause of "Falling back to HPET" ?

guy guy.choo.keren at
Sat May 22 14:42:14 CDT 2010

you can try putting s8's drives inside s7 and see what you get.

if you get errors, this can be one of:

1. the hard disk themselves are faulty.


2. there is some different driver on s8 that is not found on s7 (or the 
other way around) and it was installed not via the RPM system (so in RPM 
you won't see a difference) - which is causing the problems.


Bond Masuda wrote:
> Hello,
> I'd appreciate any help/advice anyone can provide regarding our issue. I've
> run out of ideas on this one...
> We have two identical PowerEdge 2950, one is called s7 and the other is s8.
> Both are web servers running Apache and PHP. We first noticed the problem
> because our benchmarking showed drastically different results between the
> two servers. With s7, we were able to get 180 requests/sec while on s8 we
> only get 35 request/sec (and now only 15 requests/sec - more on that below).
> After this, we became aware that almost all tasks on s8 were slower than s7,
> whether it is CPU bound or I/O bound, everything we tried was slower on s8
> than on s7 (untar'ing archives, running md5 hashes, etc).
> I started digging around. Both servers are identical in terms of software
> and configuration (other than things like hostname and IP addresses). Both
> servers are RHEL4U8, kernel-2.6.9-89.0.25.ELsmp, x86_64, exact same packages
> and exact same versions. I even ran 'rpm --verify' on all packages and
> didn't find anything unusual on both s7 and s8.
> The ONLY error message I'm seeing that is unique to s8 are the following
> messages in dmesg:
> Losing some ticks... checking if CPU frequency changed.
> warning: many lost ticks.
> Your time source seems to be instable or some driver is hogging interupts
> rip __do_softirq+0x4d/0xd0
> Falling back to HPET
> Some google searching found:
> which refers to:
> But that seems to refer to problems with virtualization. This is on real
> hardware.
> What we don't understand is that s7 does *not* exhibit any slowness nor the
> messags above, only s8. Again, both are identical.
> So, thinking this might be a hardware issue, we asked our hosting company to
> pull the drives out of s8 and replace the entire chassis. After replacing
> the entire chassis of s8, we are still getting the above messages in dmesg.
> Not only that, things have gotten worse... our benchmarking (using 'ab') now
> shows the server can only do 15 requests/sec (all these test were run
> locally on loopback to avoid any network related issue).
> Since the chassis was swapped, we feel that it probably isn't a hardware
> issue. But we have s7 which is configured identically to s8 that doesn't
> have this issue, so it is hard to say that it is a software issue.
> Any advice? What can I do to find the root cause?
> TIA,
> -Bond
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