Prior to dual core I typically monitored cpu load and essentially got concerned if the load was greater than 1 x number of processes (e.g. > 3 on a 3 processor system).

However I've seen several of my Dual Core systems (Xeon 3 or 3.2 GHz) go very high on the load measurement without apparent impact the applications/databases running on the servers.

My research yesterday said each core of a dual core processor would show up in tools like top as a separate processor. It also said all Xeons greater 2 GHz were dual core. In a system where I have two Xeon 3.2 GHz processors I see only 2 processors in top (should see 4) and on one where I have four Xeon 3.0 Ghz processors I see 4 processors in top (should see 8). This makes it seem like the statement about each core showing up as a separate processor is incorrect.

What I'm really trying to find is what is the "rule of thumb" for monitoring cpu load on dual core systems? Does disabling hyperthreading in the BIOS change the way they should be monitored (i.e. does it change a dual core from appearing a 2 processors to 1 processor)?

Please do NOT reply that the monitoring should be based on experience with the system as that is already what I'm doing. What I'm trying to understand is exactly how to know at a glance whether a system is too heavily loaded.

It's definitely not true that all Xeons > 2GHz are dual core.
They SHOULD be hyper-threading; and if you disable hyper-
threading you'll only see one CPU.


As for the load: it really depends on what the box is meant to
be doing; but I'm not concerned if the load is over 3 per CPU
over an extended period of time.


Cheers,
Tink

Thanks.

If I only see one CPU bceause hyperthreading is off does the dual-core handle using the second core internally (hidden from the OS) or does it just use one core? I'm wondering why the deployment guide for Oracle RAC would specifically have stated hyperthreading should be turned off.

Also the load of 3 you mentioned - is that assuming hyperthreading is on? If so would you say the same if it were off?

I cannot tell you what oracles rationale is; but you still keep
confusing cores and hyperthreading.

As for the load. I've seen reasonably happy wintel dual-cpu machines
(4 "CPUs" according to top) running Oracle with a load of 15 if the IO
subsystem is sensible enough.

I've seen a spectacular 1000 on a Sun T2000 with 8 cores :} and STILL
can't believe that I could login to the machine via ssh with only 5
seconds delay for the login and another 4 to get past the password entry.



Cheers,
Tink

Your first quote stated if I disable hyper-threading I'll only see one CPU. I assumed you meant "see one CPU per dual-core processor". Your latter quote confuses me though as it suggests that hyperthreading has nothing to do with dual-core processors. If however you meant I'd only see one CPU on an SMP system with multiple single core processors that is not the case. I see a CPU for EACH Xeon I have but have hyperthreading turned off (in the BIOS).

It appears I am missing something between these two quotes as they appear to be contradictory to me for the reasons stated.

I did understand you saying the Xeons may not be dual-core at all. In fact my service tags don't mention dual-core and it is only the fact that my boss seemed convinced they were dual-core that made me think they were. Is there a way to query the CPU to see if it is dual-core. /proc/cpuinfo doesn't say it is but also doesn't say it isn't. Does lack of mention mean it isn't?

P.S. I had done some further reading on the Oracle RAC. It appears there were issues with RedHat AS 2.1 and 3.x hyperthreading implementations so Oracle recommended disabling it. This may be fixed in AS 4.x but we're using 3.x here so will stick with the recommendation.

In brief:

Hyperthreading has NOTHING to do with dual core. It's just a
trick to make use of cycles that wouldn't be used of the CPU
on a single-CPU environment. Dual-Core means "two CPUs
on one wafer".


Cheers,
Tink

I know what dual-core is. Hyperthreading not so much.

I understood your prior post (and the most recent one) to say hyperthreading have nothing to do with dual-core. My question was based on the quote in the post before that prior post.

What you just wrote doesn't explain the first quote in my last post. Restating the question:

Why would disabling hyperthreading make it see only one CPU?

Because a hyperthreading CPU pretends to the OS to be
two CPUs (unless you disable hyperthreading in BIOS). The DUAL
CORE is two physical CPUs, there's no switching that on or off.


Cheers,
Tink

After further reading and research it appears these are NOT dual core Xeons at all which is why each one shows as a single CPU rather than a 2 CPUs.

Since they are single core they do allow for hyperthreading which would have shown each as 2 CPUs in the OS but as we had turned that off during install they show only as single CPUs.

Apparently lots of misinformation internal to my organization as well as on the web led to misconceptions. My co-worker on purchase of these systems had told me they were dual core but he now concurs that they appear to be single core and he must have misunderstood the original purchase discussion.

The comment about Xeons greater than 2 GHz all being dual core is not borne out by Intel's site. As noted I'd found a quote to that effect prior to my original post. Both single core and dual core Xeon processors exist but in typical vendor EOL fashion Intel doesn't bother to give details about past products so there is no way to confirm by part number these are single core. It is the fact that nothing in my system information printed at the time we got them mentions dual core and the fact they all show up in the OS the way a single core non-hyperthread enabled CPU would that confirms they are single core.

Thanks for your patience.

Of course I'm still weirded out by the fact that a load of 24 on a 2 CPU box is not a problem from the app standpoint. I guess I'll just change my monitoring to look at idle time instead.