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Guest Transparent Dynamic Memory Balancing in Virtual Machines
Changwoo Min, Inhyuk Kim, Taehyoung Kim, Young Ik Eom
Motivation Memory could be a bottleneck in server consolidation. - It is difficult to share among guest, because it is a non-renewable resource. - Efficient memory management scheme is needed to balance memory allocation according to demand of each guest. Previous studies have limitations. - Estimation of memory demand is limited. - Cannot work with hardware nested paging, Intel EPT. - Need to modify guest OS.
Goal Dynamic memory balancing scheme that does not require modifying guest OS.
Key Techniques Working Set Size Estimation - Estimation Model: LRU histogram - Monitoring Page Access hardware nested paging - Monitoring Guest Swapping paravirtualized swap device Rebalancing Memory Allocation - Guest Page Reclaiming: ballooning technique
For Further Information Please, contact Changwoo Min ([email protected])
Monitoring Page Access Guest Physical Page Access - Turns off presence bit in a nested page table entry for a page - Monitors a nested page fault and update LRU histogram Update page list and LRU histogram Reducing Monitoring Overhead - Do NOT monitor some number of hot pages that could be accessed soon Dynamically adjusting the number of hot pages considering tradeoff between overhead and accuracy
- Use extended red-black tree to calculate LRU stack distance in logarithmic time
Hot List (not monitored)
Cold List (monitored, swapped-out)
Warm List (monitored)
nested page fault
Monitoring Guest Swapping Why do we need it? - In order to estimate memory demand larger than the current memory allocation Guest physical access monitoring cannot reflect demand beyond its allocated memory size. How to monitor swapping in a guest transparent way - Use para-virtualized swap device Backend driver in QEMU handles actual swapping operation. - Update page list and LRU histogram to reflect memory demand larger than the current allocation size. VMM swapping without double paging - VMM swapping is need to cope with delayed memory reclamation of ballooning technique. - Double paging could decrease performance significantly when a guest tries to swap out a page that is already swapped out by VMM. A page should be swapped in by VMM for a guest to swap out. Swap back-end device is shared between guest and VMM. In such double paging case, swap back-end driver omits block I/O and just updates sector mapping table.
Guest Kernel QEMU
Host Kernel/Hypervisor
Swap File
Swap Subsystem
Block Device Emulation
guest swap in/out
VMM
VMM swap in/out
block I/O
Swap Front-end Driver
Swap Back-end Driver
Rebalancing Memory Allocation New Memory Allocation Size - A working set size is estimated from its LRU histogram. - Allocation size is proportionally determined over total estimated working set size for all guests. Reclaiming Memory from Guests - Send ballooning request for a guest to give free memory back
Guest Kernel QEMU
Host Kernel/Hypervisor
… …
Nested Page Table
PTE of swapped-out page is invalidated.
… …
Guest Page Table
PTE of swapped-out page is marked as swapped out page ID.
Allocation Bitmap
Page Slot Allocation Status
GFN Hash Table <Guest Physical Frame Number, Physical Page Slot Number>
Guest Swap Radix Tree <Guest Page Slot Number, Physical Page Slot Number>
Hot List Warm List Cold List
Page List
