[SOLVED] QEMU/KVM interesting behaviour
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  1. #1
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    QEMU/KVM interesting behaviour

    Hello guys,

    I am proceeding to test the following:
    Host: Centos7.4 (latest, 64bit), running on dual Epycs 7351Ps. (reason is, Fedora won't run in that server, it comes up with errors as I posted before)
    Guests:
    QEMU/KVM Fedora 27, 32bit, PAE i686 , 8GB ram, 4x CPUs (Nehalem)
    QEMU/KVM Fedora 27, 32bit, 4GB ram, 4x CPUs (Nehalem)

    The interesting thing is that when I load these puppies up with say 400 HTTP connections / second each (random list of 5 million or so URLs with HTML and large jpegs), so that I can see their usage.... the PAE (8GB RAM) guest consumes (almost) exactly twice the CPU (as seen in the host) as the 4GB RAM one.
    here attached 3 randomly taken screenies from the top on the host.
    Click image for larger version. 

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    So, apparently, either the dealing with the PAE, or the extra RAM, or both, generates the extra load.
    This leads to one specific conclusion for me, I will run these with 4GB ram in 32bit mode, as long as they can handle the load (memorywise).

    I have yet to test with a 64bit guest, will do that next.
    "monsters John ... monsters from the ID..."
    "ma vule teva maar gul nol naya"

  2. #2
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    Re: QEMU/KVM interesting behaviour

    I set the ram to 4GB on both, and voilla, both are now comparable in cpu load on the host.
    Very interesting.
    "monsters John ... monsters from the ID..."
    "ma vule teva maar gul nol naya"

  3. #3
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    Re: QEMU/KVM interesting behaviour

    Another EXTREMELY interesting behaviour:

    I have 2 Qemu guests (same OS), running webservers to the public, and with "internal" connection to another server with a database.

    One guest has the internal network NIC configured in virt-manager, as "virtio", and the other one is configured with "rtl8139" .

    I have a program that does some hefty database action to get some stats, and load into shm, and on the guest with the virtio internal NIC, it takes 15 minutes. Yet on the rtl8139 one it takes 26 minutes !!!!!!

    I will now change the rtl8139 guest to also have virtio NIC, and we shall see, I suspect it will work exactly like the other one.

    EDIT: update: sure enough !, the virtio NIC massively improves connection speed of the guest. (or should I say any other model slows it down?)
    Last edited by bobx001; 22nd May 2018 at 08:44 PM.
    "monsters John ... monsters from the ID..."
    "ma vule teva maar gul nol naya"

  4. #4
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    Re: QEMU/KVM interesting behaviour

    UPDATE:
    the new AMD dual-Epyc (Supermicro) server I have is running on Centos7 since their support guys told me FC27/28 is not certified on it, which I documented here before (https://forums.fedoraforum.org/showt...highlight=epyc).

    And I have also been having problems with it, random reboots, then the "no-IRQ ... (irq: -1)" kernel messages,
    so the Support guys at Supermicro have pointed me to this here:
    ===============
    Hello Robert,

    It looks like you want to enable the virtualization feature.
    Please go into BIOS,
    Advanced -> NB Configuration -> IOMMU (change to Enabled).
    Advanced -> PCIe/PCI/PnP Configuration -> SR-IOV Support (change to Enabled).

    Hope this takes care of your issue.
    ==================

    Soooo. since then, the machine is just "GOOD", and works perfectly as advertised. So much so, that I am willing to try and install FC28 in it, to see if it was that same issue !

    more to come on this.

    Oh, and BTW, the KVM/QEMU issues I was seeing, have disappeared since changing the settings the support guy told me to !!!.
    "monsters John ... monsters from the ID..."
    "ma vule teva maar gul nol naya"

  5. #5
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    Re: QEMU/KVM interesting behaviour

    I think this thread is related to:
    https://forums.fedoraforum.org/showt...t-is-this-UUID

    so I am closing it as solved.
    "monsters John ... monsters from the ID..."
    "ma vule teva maar gul nol naya"

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