1. Guest Transparent Dynamic Memory Balancing
in Virtual Machines
Changwoo Min, Inhyuk Kim, Taehyoung Kim, Young Ik Eom
Motivation Monitoring Guest Swapping
Memory could be a bottleneck in server consolidation. Why do we need it?
- It is difficult to share among guest, - In order to estimate memory demand larger
because it is a non-renewable resource. than the current memory allocation
- Efficient memory management scheme is needed  Guest physical access monitoring cannot reflect demand
to balance memory allocation according to demand beyond its allocated memory size.
of each guest.
How to monitor swapping in a guest transparent way
Previous studies have limitations. - Use para-virtualized swap device
- Estimation of memory demand is limited.  Backend driver in QEMU handles actual swapping operation.
- Cannot work with hardware nested paging, Intel EPT. - Update page list and LRU histogram to reflect memory
- Need to modify guest OS. demand larger than the current allocation size.
Guest Kernel QEMU
Swap Subsystem Block Device
Goal Swap
Emulation
Swap
Front-end Driver Back-end Driver
Dynamic memory balancing scheme that does not require guest Host Kernel/Hypervisor
modifying guest OS. swap in/out
block
VMM
swap in/out I/O
VMM
Key Techniques Swap File
Working Set Size Estimation VMM swapping without double paging
- Estimation Model: LRU histogram - VMM swapping is need to cope with delayed memory
- Monitoring Page Access reclamation of ballooning technique.
 hardware nested paging - Double paging could decrease performance significantly
- Monitoring Guest Swapping when a guest tries to swap out a page that is already
 paravirtualized swap device swapped out by VMM. A page should be swapped in
by VMM for a guest to swap out.
Rebalancing Memory Allocation  Swap back-end device is shared between guest and VMM.
- Guest Page Reclaiming: ballooning technique In such double paging case, swap back-end driver omits
block I/O and just updates sector mapping table.
Guest Kernel QEMU
Monitoring Page Access Guest Page Table
Guest Swap Radix Tree
<Guest Page Slot Number,
Physical Page Slot Number>
GFN Hash Table
Guest Physical Page Access <Guest Physical Frame Number,
… Physical Page Slot Number>
- Turns off presence bit in a nested page table entry for a page …
PTE of swapped-out
- Monitors a nested page fault and update LRU histogram page is marked as
swapped out page ID.
Allocation Bitmap
 Update page list and LRU histogram Page Slot Allocation Status
Host Kernel/Hypervisor
Reducing Monitoring Overhead Nested Page Table Page List
- Do NOT monitor some number of hot pages
that could be accessed soon …
PTE of swapped-out Hot List Warm List
page is invalidated.
Cold List
 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
Rebalancing Memory Allocation
nested page fault 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
Hot List Warm List Cold List
For Further Information
(not monitored) (monitored) (monitored, swapped-out)
Please, contact Changwoo Min (multics69@ece.skku.ac.kr)