Performances an SMP

[zemotard]

Hi, I have done a small test to be sure my oracle performances.

Indeed, I have done a export (dump) of my oracle database with 1vcpu = 12 Minutes

With 2 vcpu = 15 minutes

With 4 vcpu = 23 minutes ...

So I ask me if smp is very usefull ....

Regards.

For information my host is a bi-quad cores

[TiBoReR]

Hi,

On that same ESX host you're running that VM, do you have any other VMs running ? If so, how many and if you could list vCPU used for them.

Maybe you could run in a CPU scheduler problem.

[RDPetruska]

No, most of the time multiple virtual CPUs are NOT useful. They can actually worsen performance, as it is orders of magnitude harder to keep clocks in sync on multiple physical CPUs which are virtualized to a single guest. The primary reason for multiple vCPU support is so you can test your multi-threaded Applications, and how they will run on a multi-CPU host.

[Rumple]

SMP can either help or hurt your systems.

If I understand correctly, your host is has a 2 Quad core processors in it.

When doing your tests, how many other VM's are present on this box.

If you put in a 4 vCPU server, then in order for this VM to do work (ie, export data) then 4 of the cores must be available at the same time (which means other VM's may be in turn idle.

Also remember that on each physical CPU there are 4 cores, but you will not get 100% performance out of 3/4 cores...more like 75-80% as an estimate.

Because you are losing performance as you increase Virtual CPU's then I'd guess you are running either multiple SMP VM's or at least 6-8 single CPU VM's on this server.

One thing to see is that you have actually tested the performance of each configuration to see what will perform better in your environment.

[CWedge]

What kind of Host were you using? How many cores/pCpus?

[Paul_Nothard]

This is completely expected.

Q: How many CPUs does a database dump make use of?

A: One, maybe a little more.

Q: What is the scheduling overhead to get all SMP cores running at the same time?

A: "not zero"

SMP is to be discouraged as a rule unless you have a proven need for it.

Run the same test but using hammerora and there will give you more meaningful real-world figures... but it will probably still not be linear.

Only true CPU intensive applications really show linear benefit from vSMP machines.

Paul.