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1612 discussions

How does KVM handle multiple cores?
by MargoAndTodd 22 Nov '10

22 Nov '10

Hi All, How does KVM handle multiple cores. I have an x5650 with 6 real cores that presents itself to my OS as 12 virtual cores (hyperthreading). Does KVM see 6 or 12 cores. And, can I tell KVM how many cores I want it to use? Am I misunderstanding how KVM works? What I am after is if my guest is 100% busy, I still want some power left over for my host. Many thanks, -T

3 4

KVM and Windows "/use pmtimer"
by Eric Searcy 20 Nov '10

20 Nov '10

http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5/html/Virtualiz… Want to get some more people's opinion on this: the above doc says to use the boot parameter "/use pmtimer" to use the RTC "instead of the TSC for all time sources which resolves guest timing issues". One: does this have any bearing on whether the host has the constant_tsc flag (i.e. are all the sections that follow "Configuring hosts without a constant Time Stamp Counter" subordinate to it, or does that just have bearing on power management on that one CPU listed---AMD rev F)? Other question: http://support.microsoft.com/kb/938448 implies that for 2003 SP2 "/use pmtimer" shouldn't be needed as it will use the platform timer (RTC) if you have ACPI or APIC present. ("By default, Windows Server 2003 Service Pack 2 (SP2) uses the PM timer for all multiprocessor Advanced Programmable Interrupt Controller (APIC) Hardware Abstraction Layers (HALs) or Advanced Configuration and Power Interface (ACPI) HALs".) Anybody have any experience as to whether this (using ACPI feature in KVM) resolves the timing issues without needing pmtimer explicitly set? Thanks, Eric

2 1

What are KVM guest cores?
by MargoAndTodd 19 Nov '10

19 Nov '10

Hi All, What are guest cores in KVM? Are they fake, like everything else in the guest? Just another process running on the host emulating a core? Or are the guest cores actually connected directly to the physical cores on my motherboard? Many thanks, -T

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what scheduling algorithm does KVM use?
by Nick 19 Nov '10

19 Nov '10

This may not be the best place to ask, but I was prompted by a question about guest cores on KVM. We currently use VMWare Server (v1.0) on CentOS5. It supports up to two virtual CPUs, but not very well, as I understand it. VMWare Server 2.0 might do better at supporting the same maximum of 2 CPUs, but if my research is correct, they both use what is called "strict co-scheduling". Which means, if a two virtual-CPU VM is waiting for a time slice on the physical host, there needs to be two physical CPUs available to be allocated. This is because amount of time scheduled to each virtual CPU has to stay equal, else assumptions in the virtualised software may become false and things start going wrong. The problem with this is that it is obviously more of a constraint to have two physical CPUs available. Therefore adding virtual CPUs to a co-scheduled VM can actually make performance worse if the physical CPUs are under any sort of contention. Performance degrades rapidly under load - which is exactly what you don't want for a webserver. Therefore advice seems to be to avoid using multi-virtual CPU VMs. [1] VMWare ESX uses something supposedly better, called relaxed co-scheduling. [2]. We're not likely to use that any time soon, since we are using 1&1 root servers running CentOS. I did try and research the algorithm(s) used by KVM. Apparently it doesn't use co-scheduling, but possibly something called the "completely fair scheduler", but I'm no an expert in VM scheduling and I didn't manage to discover what this means, nor specifically what the implications are for performance were in terms of scaling up the number of virtual CPUs on KVM. (I understand that the number of virtual CPUs supported by KVM is be quite high.) Anyway, I've been asked how to scale a VM up utilise more - say four or more - physical CPUs. Is KVM better at this than VMware Server, or does the same basic problem persist? Thanks, Nick 1. http://communities.vmware.com/thread/169323 http://cs.gmu.edu/~hfoxwell/cs671projects/southern_v12n.pdf > "The SMP > scheduling algorithms used by ESX server and by Xen > have a low overhead for single-threaded compute inten- > sive workloads. However, they do not scale as well for > multi-threaded workloads on a system with overcommit- > ted CPU resources." 2. http://communities.vmware.com/thread/157849 http://communities.vmware.com/docs/DOC-4960 http://communities.vmware.com/docs/DOC-5501

2 4

SPICE Benchmark
by Alexey Vasyukov 18 Nov '10

18 Nov '10

Hi folks. We finally finished our work on benchmarking SPICE and would like to share the results. Detailed report in English: http://www.bureausolomatina.ru/sites/default/files/SPICE%20Benchmark%20-%20… The report provides benchmark results of SPICE network load for different types of workload. SPICE operation on limited low-speed network connection was also tested. The results were compared with similar tests for RDP. Testing was based on SPICE version 0.4.3. This version is not the latest one but it is used currently in RHEV-D 2.2 and also in RHEL6. So, we guess, the results are pretty useful. We are going to test SPICE 0.6 in nearest future because it has very interesting WAN improvements. So, we welcome any comments, critics or advices. Best regards, Alexey Vasyukov

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[HOWTO] Running Xen 4.0 host (dom0) with Redhat Enterprise Linux 6 (RHEL6)
by Pasi Kärkkäinen 18 Nov '10

18 Nov '10

Hello, If you're interested in running Xen 4.0 hypervisor/dom0 on RHEL6, take a look at here: http://wiki.xen.org/xenwiki/RHEL6Xen4Tutorial It explains steps needed to rebuild Xen 4.0.1 src.rpm from Fedora on RHEL6, and how to fetch dom0 capable 2.6.32.x kernel from upstream git repository. It also shows how to get libvirt/virt-manager working with Xen on RHEL6. Hopefully it helps :) -- Pasi

2 1

Gilberto Nunes wants to stay in touch on LinkedIn
by Gilberto Nunes 17 Nov '10

17 Nov '10

LinkedIn ------------ Discussion, I'd like to add you to my professional network on LinkedIn. - Gilberto Nunes Gilberto Nunes Owner at Konnecta Consulting Itajaí Area, Brazil Confirm that you know Gilberto Nunes https://www.linkedin.com/e/anzi98-ggmu3q30-6f/isd/1918541538/YqPt7-wa/ -- (c) 2010, LinkedIn Corporation

1 0

Re: [CentOS-virt] KVM: where are the directions?
by jayeola＠gmail.com 15 Nov '10

15 Nov '10

rpm -ql kvm rpm -qa | grep kvm /sys/john == /dev/BlackBerry

7 21

Can I mix kvm and virtual box?
by MargoAndTodd 14 Nov '10

14 Nov '10

Hi All, On my customer's CentOS 5.5 x64 server, I have Virtual Box installed. To make a side by side comparison, I would like to also run KVM. I read somewhere that this can not be done, but I can not remember where. Am I able to mix the two so I can demonstrate the differences to my customer? Many Thanks, -T

3 5

Does KVM have a shared clipboard?
by MargoAndTodd 11 Nov '10

11 Nov '10

Hi All, Does KVM have a shared clipboard, such that I can copy and paste between guest and host? Many thanks, -T

2 1

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