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Huge Pages Support for Ganeti |
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=============================== |
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This is a design document about implementing support for huge pages in |
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Ganeti. (Please note that Ganeti works with Transparent Huge Pages i.e. |
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THP and any reference in this document to Huge Pages refers to explicit |
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Huge Pages). |
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|
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Current State and Shortcomings: |
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------------------------------- |
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The Linux kernel allows using pages of larger size by setting aside a |
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portion of the memory. Using larger page size may enhance the |
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performance of applications that require a lot of memory by improving |
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page hits. To use huge pages, memory has to be reserved beforehand. This |
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portion of memory is subtracted from free memory and is considered as in |
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use. Currently Ganeti cannot take proper advantage of huge pages. On a |
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node, if huge pages are reserved and are available to fulfill the VM |
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request, Ganeti fails to recognize huge pages and considers the memory |
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reserved for huge pages as used memory. This leads to failure of |
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launching VMs on a node where memory is available in the form of huge |
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pages rather than normal pages. |
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|
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Proposed Changes: |
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----------------- |
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The following components will be changed in order for Ganeti to take |
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advantage of Huge Pages. |
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|
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Hypervisor Parameters: |
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---------------------- |
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Currently, It is possible to set or modify huge pages mount point at |
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cluster level via the hypervisor parameter ``mem_path`` as:: |
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|
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$ gnt-cluster init --no-lvm-storage --no-drbd-storage \ |
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>--enabled-hypervisors=kvm -nic-parameters link=br100 \ |
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> -H kvm:mem_path=/mount/point/for/hugepages |
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|
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This hypervisor parameter is inherited by all the instances as |
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default although it can be overriden at the instance level. |
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|
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The following changes will be made to the inheritence behaviour. |
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|
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- The hypervisor parameter ``mem_path`` and all other hypervisor |
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parameters will be made available at the node group level (in |
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addition to the cluster level), so that users can set defaults for |
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the node group:: |
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|
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$ gnt-group add/modify\ |
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> -H hv:parameter=value |
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|
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This changes the hypervisor inheritence level as:: |
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|
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cluster -> group -> OS -> instance |
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|
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- Furthermore, the hypervisor parameter ``mem_path`` will be changeable |
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only at the cluster or node group level and users must not be able to |
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override this at OS or instance level. The following command must |
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produce an error message that ``mem_path`` may only be set at either |
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the cluster or the node group level:: |
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|
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$ gnt-instance add -H kvm:mem_path=/mount/point/for/hugepages |
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|
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Memory Pools: |
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------------- |
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Memory management of Ganeti will be improved by creating separate pools |
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for memory used by the node itself, memory used by the hypervisor and |
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the memory reserved for huge pages as: |
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- mtotal/xen (Xen memory) |
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- mfree/xen (Xen unused memory) |
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- mtotal/hp (Memory reserved for Huge Pages) |
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- mfree/hp (Memory available from unused huge pages) |
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- mpgsize/hp (Size of a huge page) |
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|
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mfree and mtotal will be changed to mean "the total and free memory for |
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the default method in this cluster/nodegroup". Note that the default |
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method depends both on the default hypervisor and its parameters. |
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|
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iAllocator Changes: |
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------------------- |
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If huge pages are set as default for a cluster of node group, then |
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iAllocator must consider the huge pages memory on the nodes, as a |
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parameter when trying to find the best node for the VM. |
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Note that the iallocator will also be changed to use the correct |
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parameter depending on the cluster/group. |
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|
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hbal Changes: |
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------------- |
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The cluster balancer (hbal) will be changed to use the default memory |
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pool and recognize memory reserved for huge pages when trying to |
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rebalance the cluster. |
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.. vim: set textwidth=72 : |
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.. Local Variables: |
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.. mode: rst |
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.. fill-column: 72 |
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.. End: |