Hi,

I have just installed Cent OS 6.0 on a x3850 X5 with 4 x Xeon 8C cpus.

Is it correct that the OS sees 64 cpus and not 32 ?

Also in virt-manager, I can assign up to 64 cpus for a guest.

I thought it was 4 times 8 = 32 cpus.

Regards.

Maybe the system has hyper-threading enabled, so that each core is seen as 2 virtual cores. You can easily verify it using
for each core you should see the following:

Sounds like hyperthreading is enabled. I don't know the spec's of the x3850, so I don't know whether this estimate is correct.

Hyperthreading (if supported by the CPU) can be enabled/disabled in the BIOS menu.

You might want to give some thought to whether you want it enabled.

If you have enough active software threads, then hyperthreading makes your system act like it has twice as many cores each roughly half as fast, which is not at all a win for a large number of cores (reduces latency on systems with very few cores).

With some mixes of instruction use within the threads, each virtual core runs a little faster than half as fast as a true core, so you get a net performance boost of twice as many cores each a little more than half as fast.

For some memory access patterns, using both halves of a core at the same time gives a dramatically higher cache miss rate, making each half core far slower than half of a real core.

With a hyperthreading aware OS, when you run fewer software threads than you have virtual cores, the OS should first idle one half of each core before completely idling any core. With one half of a core idle, the other half speeds up to nearly the speed it would have if hyperthreading had not been enabled.

If you virtualize a hyperthreading aware OS within another hyperthreading aware OS, I'm not sure how much of the hyperthreading awareness gets lost in the virtualization. I think you won't be able to avoid situations in which both halves of one core are in use at half speed while another core is wasted unused.

Yes, correct.

16 siblings and 8 cpu cores for each CPU, 0-63.

Is it okay if hyberthreading is enabled ?

The server shall act as a host for 3-4 guests.

Thanks, johnsfine. I replied before I read your answer.

But I'm not certain if i quite understand the point.

But the server came with hyberthreading enabled by default !

Another thing is, that it says 4 network cards. eth0 - eth3. But there is only 2 physically ports in the server.

I was still editing details about that into my earlier reply while you were posting that question.

I think it is more likely in your case that leaving hyperthreading enabled will reduce the performance of the system. But it is very hard to make an accurate estimate of whether hyperthreading will increase or reduce performance.

Do you have any specific questions? I was trying to describe the reasons (depending on use) why hyperthreading might give you more or less total compute power vs. disabling hyperthreading.

I still think it is just an option in BIOS setup. During a reboot, you can go into BIOS setup and change it if you want to.

I don't know about that one.