[pve-devel] [PATCH pve-docs 2/2] Document CPU and Memory Input panel

Emmanuel Kasper e.kasper at proxmox.com
Mon Jun 6 12:23:18 CEST 2016

 qm.adoc | 86 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 86 insertions(+)

diff --git a/qm.adoc b/qm.adoc
index f3e0887..342966d 100644
--- a/qm.adoc
+++ b/qm.adoc
@@ -175,6 +175,92 @@ With this enabled, Qemu uses one thread per disk, instead of one thread for all,
 so it should increase performance when using multiple disks.
 Note that backups do not currently work with *IO Thread* enabled.
+A *CPU socket* is a physical slot on a PC motherboard where you can plug a CPU.
+This CPU can then contain one or many *cores*, which are independent
+processing units. Whether you have a single CPU socket with 4 cores, or two CPU
+sockets with two cores is mostly irrelevant from a performance point of view.
+However some software is licensed depending on the number of sockets you have in
+your machine, in that case it makes sense to set the number of of sockets to
+what the license allows you, and increase the number of cores. +
+Increasing the number of virtual cpus (cores and sockets) will usually provide a
+performance improvement though that is heavily dependent on the use of the VM.
+Multithreaded applications will of course benefit from a large number of
+virtual cpus, as for each virtual cpu you add, Qemu will create a new thread of
+execution on the host system. If you're not sure about the workload of your VM,
+it is usually a safe bet to set the number of *Total cores* to 2.
+NOTE: It is perfectly safe to set the _overall_ number of total cores in all
+your VMs to be greater than the number of of cores you have on your server (ie.
+4 VMs with each 4 Total cores running in a 8 core machine is OK) In that case
+the host system will balance the Qemu execution threads between your server
+cores just like if you were running a standard multithreaded application.
+However {pve} will prevent you to allocate on a _single_ machine more vcpus than
+physically available, as this will only bring the performance down due to the
+cost of context switches.
+Qemu can emulate a number different of *CPU types* from 486 to the latest Xeon
+processors. Each new processor generation adds new features, like hardware
+assisted 3d rendering, random number generation, memory protection, etc ...
+Usually you should select for your VM a processor type which closely matches the
+CPU of the host system, as it means that the host CPU features (also called _CPU
+flags_ ) will be available in your VMs. If you want an exact match, you can set
+the CPU type to *host* in which case the VM will have exactly the same CPU flags
+as your host system. +
+This has a downside though. If you want to do a live migration of VMs between
+different hosts, your VM might end up on a new system with a different CPU type.
+If the CPU flags passed to the guest are missing, the qemu process will stop. To
+remedy this Qemu has also its own CPU type *kvm64*, that {pve} uses by defaults.
+kvm64 is a Pentium 4 look a like CPU type, which has a reduced CPU flags set,
+but is guaranteed to work everywhere. +
+ In short, if you care about live migration and moving VMs between nodes, leave
+the kvm64 default. If you don’t care about live migration, set the CPU type to
+host, as in theory this will give your guests maximum performance.
+You can also optionally emulate a *NUMA* architecture in your VMs. The basics of
+the NUMA architecture mean that instead of having a global memory pool available
+to all your cores, the memory is spread into local banks close to each socket.
+This can bring speed improvements as the memory bus is not a bottleneck
+anymore. If your system has a NUMA architecture footnote:[if the command
+`numactl --hardware | grep available` returns more than one node, then your host
+system has a NUMA architecture] we recommend to activate the option, as this
+will allow proper distribution of the VM resources on the host system. This
+option is also required in {pve} to allow hotplugging of cores and RAM to a VM.
+If the NUMA option is used, it is recommended to set the number of sockets to
+the number of sockets of the host system.
+For each VM you have the option to set a fixed size memory or asking
+{pve} to dynamically allocate memory based on the current RAM usage of the
+When choosing a *fixed size memory* {pve} will simply allocate what you
+specify to your VM.
+// see autoballoon() in pvestatd.pm
+When choosing to *automatically allocate memory*, {pve} will make sure that the
+minimum amount you specified is always available to the VM, and if RAM usage on
+the host is below 80%, will dynamically add memory to the guest up to the
+maximum memory specified. +
+When the host is becoming short on RAM, the VM will then release some memory
+back to the host, swapping running processes if needed and starting the oom
+killer in last resort. The passing around of memory between host and guest is
+done via a special `balloon` kernel driver running inside the guest, which will
+grab or release memory pages from the host.
+footnote:[A good explanation of the inner workings of the balloon driver can be found here https://rwmj.wordpress.com/2010/07/17/virtio-balloon/]
+All Linux distributions released after 2010 have the balloon kernel driver
+included. For Windows OSes, the balloon driver needs to be added manually and can
+incur a slowdown of the guest, so we don't recommend using it on critical
+// see https://forum.proxmox.com/threads/solved-hyper-threading-vs-no-hyper-threading-fixed-vs-variable-memory.20265/
+When allocating RAMs to your VMs, a good rule of thumb is always to leave 1GB
+of RAM available to the host.
 Managing Virtual Machines with 'qm'

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