Windows 10: VM's (any) should you set processors / cores to same as host

Hi folks
Whatever software you use for your VM's - is there any advantage in setting the virtual vCPU to having less cores / threads / processors than the Host.

AFAIK if a VM is idle it isn't consuming any host resources so the best way to run it as far as I can see is to give it the resources it needs rather than try and run it in "Starvation Mode".

@Kari

I assume this is true also if you run in HYPER-V . It seems (at least for me) to be quite difficult to get real dynamic details of resources a VM uses once you've sorted out RAM and I/O systems.

I've found though intuitively running VM's (at least with VMware) with the vCPU set to same number of processors / cores / threads as the Host and always assuming sufficient RAM and proper disk I/O they run better - especially when loaded quite heavily using the VM config parameters set that way rather than minimising to default vCPU options.

Any real "Geeky" opinions here =-- I'm actually quite interested in this topic if people have time.

Cheers
jimbo

:)

 

Quad Core Host -- should I assign 2 cores to a VM

The thing with VM and how many cores it's using is that however strange it sounds, adding more cores can actually do just that, slow down the system. The VM can only use cores assigned to it when they are all free to be used, this forces VM occasionally just to wait that host frees the cores.

 

Quad Core Host -- should I assign 2 cores to a VM


Hi there

I can't quite work out whether I should assign 2 cores out of a 4 core processor (AMD OPTERON X3421 APU} to a VM running a fairly intensive DB system (MiniSAP or SAP ABAP 7.50 minisap) which has to run on a Linux machine - so I've got it on a Centos VM.

This application is accessed via a front end GUI on a Windows Host which doesn't do much apart from some email, and EXCEL so the VM actually needs in theory to have the resources.

I 'm using Physical HDD's for the SAP DB in the VM. I can't run the VM basically as HOST on this machine as I can't get hold of any AHCI / SATA Linux drivers (yet) for this machine and the standard drivers use a HIDEOUS "Fake RAID" system with lousy performance so I've skipped that.

The windows host simply boots the boot loader from an INTERNAL USB 2 device (/boot on the USB card) which then loads the rest of the OS on to an SSD which works fine.

I assume that when the VM is idle the cpu cores are available to the Host but I don't know. As far as RAM is concerned -- I have plenty in the machine but new VM software only reserves RAM in use and releases it when not required so the RAM setting is for MAX that the VM can reserve.

so VM has for physical devices 1 SSD for the OS and 1 2.7 TB for databases etc.


appreciate any input from gurus here --I can muck about with VM's but things like cpu cores etc is where my knowledge is light.

Here's the CPU data the VM sees






Cheers
jimbo

 

Quad Core Host -- should I assign 2 cores to a VM


Hi there

Thanks to you both
@Kari
I've found on testing the VM seems to work best with the following settings

CPU - nr of processors : 1
nr of Cores : 2

for HDD for my main Database : Raw HDD --- not SCSI but as SATA and formatted in the VM with native file system -- chose XFS rather than EXT4 and HDD size 2.7 TB (3 TB RED WD NAS Disk), RAM alloc 4GB.

Although the VM advised using SCSI the I/O worked far better when I set it up as a physical HDD using SATA - probably because I could use the physical SATA driver from the Host rather than a Virtualized SCSI driver. The SAP system has a DB server and an Application server so the whole software is quite intensive and is accessed from the HOST (or other remote clients) by a standard GUI.

When upping processor or core count to more than 2 I found the VM would hang sometimes -- and a few minutes later so would the Host (W10 X 64 Pro).

1 Processor with 2 Cores performed the best. Was quite an improvement from using 1 processor and 1 core in the VM config. This might be for the particular CPU I've got -- AMP Opteron X3421. Other ones such as Intel I5/7 might yield totally different results.

I'm not sure whether that was a problem with how the SAP software handled the multi processing - I suspect it was the underlying VM system.

Unfortunately the only other server machines I can test this on have a Celeron which doesn't have enough poke to drive this software at any decent sort of response. I can't test this stuff on your average laptop - of course I can log on from a laptop (front end GUI) but the DB server and application server is on the VM.


I'm happy with the system now as it works fine -- although the SAP DB system causes a huge amount of I/O at VM Power on time -- it settles down after around 2 - 3 mins and as I'm the only user that's OK for me. After this period response time is fine and the Host system barely seems to notice there's this VM running.

It's very difficult to find decent documentation on optimising processor usage for VM's -- there's plenty on I/O and RAM but virtually nothing on processor usage - probably because most people tend to do "Office" type stuff on VM's or fairly easy things like file / print serving and multi-media streaming - none of which require a lot of CPU processing. Multi-media for example is just sending data like any other file sharing - all the decoding is done on the RECEIVING devices whether phone / TV / Amazon TV / Apple TV / Roku TV / PC etc etc.

Cheers
jimbo