----- Original Message -----
From: "Dennis Jacobfeuerborn" <dennisml@conversis.de> To: "Andrew Cathrow" <acathrow@redhat.com> Cc: engine-devel@ovirt.org Sent: Tuesday, December 18, 2012 7:59:26 PM Subject: Re: [Engine-devel] CPU Overcommit Feature
On 12/18/2012 06:33 PM, Andrew Cathrow wrote:
----- Original Message -----
From: "Dennis Jacobfeuerborn" <dennisml@conversis.de> To: engine-devel@ovirt.org Sent: Tuesday, December 18, 2012 12:30:34 PM Subject: Re: [Engine-devel] CPU Overcommit Feature
On 12/17/2012 07:13 PM, Simon Grinberg wrote:
----- Original Message -----
From: "Greg Padgett" <gpadgett@redhat.com> To: "engine-devel" <engine-devel@ovirt.org> Sent: Monday, December 17, 2012 4:37:57 PM Subject: [Engine-devel] CPU Overcommit Feature
Hi,
I've been working on a feature to allow CPU Overcommitment of hosts in a cluster. This first stage allows the engine to consider host cpu threads as cores for the purposes of VM resource allocation.
This wiki page has further details, your comments are welcome! http://www.ovirt.org/Features/cpu_overcommit
Basically looking good. Hyperthread though is vendor specific.
For AMD it's Clustered Multi-Thread while for Intel it's Hyper-Thread Official name is simultaneous multithreading (SMT) but no one outside of the academy will recognize that.
in libvirt if I read it right it's <attribute name='thread_siblings'>
So why not just call it threads. We'll have cpuSockets, cpiCores, and cpuThreads, should be clear when in CPU context.
In the GUI just change hyperthreads to CPU threads. While in the tool tip explain that it's either AMD Clustered Multi-Thread or Intel Hyperthread
Does this affect only the number of potential vCpus for the guests or does this also have an impact on the actual scheduling? So far I always disabled HT out of fear that a 2 vCpu guest might actually be scheduled to run in 2 threads of the same core but now I'm not so sure anymore. In the HT case does KVM know that two threads belong to the same core and will it only schedule its vCpus on distinct cores? Is there some documentation about this somewhere?
This is about the maximum number of vCPUs we can give to a VM. If the machine has 32 Physical cores that are hyperthreaded then do we say the max number of vCPUs for a single VM is 32 or 64.
If the actual scheduling of vCPUs cannot distinguish between threads and cores then why would you even want to limit yourself to 32 in you example? In that case the scheduling might end up being inefficient no matter how many vCPUs you assign to a guest so why restrict the user? (You might of course want to limit the user for policy reasons but that has nothing to to with the thread/core topic.)
On the other hand if KVM does only schedule the vCPUs on distinct cores and extending the count from 32 to 64 implies that this distinction is to be disabled then this will have a performance impact for the guest. In that case I might want to limit the guests to just the 32 physical cores so two vCPUs of a single guest don't get scheduled on two threads of the same core.
I've never really looked that closely into the scheduling issue but it might play a role here so I asked if someone had any pointers to infos about how exactly KVM makes its scheduling decisions.
Regards, Dennis
Dennis, first of all every virtual cpu is basically a qemu-thread which can run on any cpu-thread. The scheduling is done by the kernel scheduler, while we may control it using cpu pinning. ie- you may ask for specific vcpu to run on a specific thread which is from the OS point of view another core. Indeed there are cases where this is not recommended, but other cases where this will actually give you a performance boost, as L1 cache is being shared by the sibling threads. So it's really up to you to test your workload and decide id you wish to utilize it or not. We're giving you powerful tools, and you can decide if and how to use it. Doron