VMWare and number of cores

[GojeDK]

Hi All,

I have a virtual Untangle installation running VMWare and at times the virtual server is a bit high on CPU usage. My thought is, that an easy fix would be to add another core to the virtual server through VMWware.

Would this work or would the Linux image not have multicore support since it was not installed with more than 1 core active?

Thanks in advance

[pazza3564]

Should be fine.

[sky-knight]

I don't think you can get a single core kernel out of Linux these days without hacking things up.

That said, I know from personal experience you can add cores at will. If you want to see for yourself after you've done it, just open up the terminal and run: cat /proc/cpuinfo

You should see both CPU's listed.

[GojeDK]

Thanks - it worked like a charm.

To be honest I haven't worked with Linux much since 2001 and back then it took equal parts of part black magic, human sacrifice and mad hacking to get SMP to work.

[far182]

This should be against VMware best practice, and in theory works against you. VMware takes all the CPU cycles from all the cores in the host and presents them as one super CPU to a guest. By adding a vcpu, you are adding overhead.

[RBoynton]

This should be against VMware best practice, and in theory works against you. VMware takes all the CPU cycles from all the cores in the host and presents them as one super CPU to a guest. By adding a vcpu, you are adding overhead.

That is true, but if the VM performance is suffering from lack of processor resources, then the additional processor will more than outweigh the additional overhead needed. The trick to VMWare is to balance the amount of hardware resources allocated to best serve the VM's needs. Once that "sweet spot" is found, it will run very well.

[far182]

That is true, but if the VM performance is suffering from lack of processor resources, then the additional processor will more than outweigh the additional overhead needed. The trick to VMWare is to balance the amount of hardware resources allocated to best serve the VM's needs. Once that "sweet spot" is found, it will run very well.

Not true form my understanding. Lets say you allocated 1 vCPU. Lets also say you have 8 cores at 2gz per core. That's 16gz total on the host. Your one virtual guest can consume the total of 16gz.

Adding a 2nd vCPU to the guest is still only going to provide you 16gz, but with more overhead.

[sky-knight]

It allows the guest OS to operate two threads simultaneously. The single vCPU configuration does not.

So what do you want? More GHZ, or more execution threads?

[RBoynton]

Not true form my understanding. Lets say you allocated 1 vCPU. Lets also say you have 8 cores at 2gz per core. That's 16gz total on the host. Your one virtual guest can consume the total of 16gz.

Adding a 2nd vCPU to the guest is still only going to provide you 16gz, but with more overhead.

I misread your other post. The guest VM will never see the aggregate total of the host's processor power. The VM will not get a massive 16ghz CPU! That is not how VM's work. I run ESXi 5 on a Dell T610 with two quad core 2.127ghz Xeons. When I do a software inventory of one of my VM servers, it shows up as a Xeon E5506 2.13ghz processor. Another one of my VM servers has two NIC's, and shows up as having two Xeon E5506 2.13ghz.

It is possible to give a particular VM a higher priority (reserve) on host CPU resources, but that is NOT normal... and very counterproductive when trying to run lots of VM's on a host. As Sky said, the benefit of multiple vCPUs in a guest is for multi-threaded applications that can take advantage of this, such as SQL, Exchange, etc. The neat thing about running a virtual system is you can experiment with adding memory, CPU's, etc.

Feel free to build a box, add, take away, and see what those changes do to your system's performance (both host and VM's). VMWare is quite mature now, and there is a lot of documentation out there you can draw from.

[far182]

Of course you don't see 16ghz. Though you can use all the processing power.

http://www.vmware.com%2Fpdf%2FPerf_B...MURmBw&cad=rja

Read this. It should clear things up for you.

Short story, most often it is against best practice to have more than one vCPU per guest. Only in very specific instances (high utilization of multithread) should you add a second vCpu. Things like SQL servers and such can benefit. Though, if they are not highly utilized on SQL operations, you could actually see the performance suffer with adding another vCpu.

This is just the way I understand it. So that takes us back to the original post. Does adding a 2nd vCpu help the performance of Untangle on ESX?