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Re: [Qemu-discuss] Why do additional cores reduce performance?

From: Oleg Ovechko
Subject: Re: [Qemu-discuss] Why do additional cores reduce performance?
Date: Tue, 16 Dec 2014 18:22:22 +0200

> What is your benchmark?

I've tried different ways (CrystalDiskMark 3.0.3 x64, ATTO Disk
Banchmark v2.47) all give same result.
The numbers I've provided in 1st mail are for 100G file copied over. I
simply subtract stop and start times. 50 seconds is so huge difference
(three sigma rule gives 10 secs for 10 tries), I can even use wall

When everything is enabled in BIOS it is 6:23 on real Windows versus
9:03 on virtualized...

Phil Ehrens has sent me link
If I don't misunderstand, it means kvm/qemu simply is not designed for
I guess I need to try different hypervisor. 50% performance is too
high price especially when VT-x and VT-d are meant to make it 0%

> Windows sometimes has scalability problems due to the way it does
> timing.  Try replacing "-cpu host" with "-no-hpet -cpu
> host,hv_time,hv_vapic".

Does not change results.

> It will be a mix.  Do not specify HT in the guest, unless you have HT in
> the host _and_ you are pinning the two threads of each guest core to the
> two threads of a host core.

Do you mean "-smp 4,sockets=1,cores=2,threads=2" for 2 cores with HT
enabled? Gives even worth result - 9:17

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