4. Memory virtualization and management
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Memory virtualization allows virtual machines to access virtual memory addresses that are translated to physical memory addresses. Hardware support for memory virtualization reduces overhead by offloading page table management to the CPU. Memory sharing between virtual machines reduces memory usage by identifying identical pages and having them share physical memory. Virtual memory overcommitment allocates more virtual memory than physical memory available by swapping out unused memory to disk. Techniques for memory sharing and overcommitment aim to improve memory utilization in virtualized systems.
![Memory Virtualization
• VMM: “Virtualizing virtual memory”
• Virtual Physical Machine
Level 2
Page
table
Page
table
Page
table
Page
table
Level 1
Page
table
.
.
.
Machine memory
Virtual address
Physical
to
Machine
Pseudo physical
memory
[Goal] Secure memory isolation
A VM is NOT permitted to access
another VM’s memory region
A VM is NOT permitted to manipulate
“physical-to-machine” mapping
All mapping to machine memory MUST be verified by VMM
3/30](https://image.slidesharecdn.com/4-150328170405-conversion-gate01/85/4-Memory-virtualization-and-management-3-320.jpg)
![HW Memory Virtualization
• AMD RVI (formerly Nested Page Tables (NPT))
• Two page table roots: gCR3 and nCR3
Accelerating Two-Dimensional Page Walks for Virtualized Systems [ASPLOS’08]
11/30](https://image.slidesharecdn.com/4-150328170405-conversion-gate01/85/4-Memory-virtualization-and-management-11-320.jpg)
![HW Memory Virtualization
• Advantages
• Significantly simplifying VMM
• Just informing MMU of a P2M root
• No shadow page tables
• No synchronizing overheads and memory overheads
• No OS modification
• Disadvantages
• Not always outperforming SW-based methods
• Page walking overheads on a TLB miss
• SW solution: SW-HW hybrid scheme [VEE’11], Large pages
• HW solution: Caching page walks [ASPLOS’08], Flat page tables
[ISCA’12]
12/30](https://image.slidesharecdn.com/4-150328170405-conversion-gate01/85/4-Memory-virtualization-and-management-12-320.jpg)
![VM Memory Management
• Memory sharing
• No parent-child relationship
• But, a research project finds this relationship in a useful case
• Virtual based honeyfarm [SOSP’05]
• Honeypot VMs CoW-share a reference image
• General memory sharing
• Block-based sharing
• Content-based sharing
• Memory overcommitment
• Σ VM memory allocation > Machine memory
• Dynamic memory balancing
VMM
Logging & Analysis
Parent
Honeypot
VM
Honeypot
VM
Machine
Memory
Scalability, Fidelity, and Containment in
the Potemkin Virtual Honeyfarm [SOSP’05]
17/30](https://image.slidesharecdn.com/4-150328170405-conversion-gate01/85/4-Memory-virtualization-and-management-17-320.jpg)
![Memory Sharing
• Block-based page sharing
• Transparent page sharing of Disco [SOSP’97]
• Sharing-aware block devices [USENIX’09]
• On reading a common block from shared disk, only
one memory copy is CoW-shared
+ Finding identical pages is lightweight
- Sharing only for shared disk
Disco: Running Commodity
Operating Systems on
Scalable Multiprocessors [SOSP’97]
19/30](https://image.slidesharecdn.com/4-150328170405-conversion-gate01/85/4-Memory-virtualization-and-management-19-320.jpg)
![Memory Sharing
• Content-based page sharing
• Sharing pages with identical contents
• VMWare ESX server and KSM for KVM
…2bd806af
4. Byte-by-byte
comparison
1. Periodic scan
2. Hashing page contents
3. Hash
collision
5. CoW sharing &
reclaiming a redundant page
Memory Resource Management in
VMware ESX Server [OSDI’02]
+ High memory utilization
- Finding identical pages is nontrivial
PA
MA
20/30](https://image.slidesharecdn.com/4-150328170405-conversion-gate01/85/4-Memory-virtualization-and-management-20-320.jpg)
![Memory Sharing
• Subpage sharing
• Difference Engine: Harnessing Memory Redundancy
in Virtual Machine [OSDI’08]
• Patching similar pages
• Compressing idle pages
• Reference & dirty bit tracking to find idle pages
PA
MA
Reference page
+ Much higher memory utilization
- Computationally intensive
Put it all together!
21/30](https://image.slidesharecdn.com/4-150328170405-conversion-gate01/85/4-Memory-virtualization-and-management-21-320.jpg)
![Memory Sharing
• Kernel Samepage Merging (KSM)
• Open source!!
• Content-based page sharing in Linux
• Increasing memory density by using KSM [OLS’09]
• Linux kernel service
• Applicable to all Linux processes including KVM
• Target memory regions can be registered via madvise()
system call
• Content comparison is done by memcmp()
• Red-black tree
22/30](https://image.slidesharecdn.com/4-150328170405-conversion-gate01/85/4-Memory-virtualization-and-management-22-320.jpg)
![Memory Overcommitment
• Two types of memory overcommitment
• Using surplus memory reclaimed by sharing
• Providing to memory-hungry VMs
• Creating more VMs
• When is memory pressured?
• Shared pages are CoW-broken
• Balancing memory between VMs
• Providing idle memory to memory-hungry VMs
• When is memory pressured?
• Idle memory becomes busy
Research issues
• How to detect memory-hungry VMs
• How to detect idle memory in VMs
• How to effectively move memory from a VM to another
Working set estimation techniques
Satori: Enlightened page sharing [USENIX’09]
Sharing cycle
23/30](https://image.slidesharecdn.com/4-150328170405-conversion-gate01/85/4-Memory-virtualization-and-management-23-320.jpg)
![How to Detect Memory-hungry VMs
• Monitoring memory pressure of VMs
• Swap I/O traffic
• Simple method, but only for anonymous pages (e.g., heap)
• How much memory is required?
• Feedback-driven method
• Allocate more memory monitor swap traffics …
• Buffer cache monitoring (Geiger [ASPLOS’06])
• Monitoring the use of unified buffer cache based on
• Page faults, page table updates, and disk I/Os
• How much memory is required?
• LRU miss curve ratio (MRC)
Disk
VM
Unified
buffer
cache
Associate memory and disk locations
Detect page reuse as cache eviction
• Reused by CoW and demand paging
24/30](https://image.slidesharecdn.com/4-150328170405-conversion-gate01/85/4-Memory-virtualization-and-management-24-320.jpg)
![How to Detect Idle Memory
• Idle memory
• Inactive memory
• Not recently used memory
• Monitoring page access frequency
• Nontrivial
• Page access is done solely by HW
• Using memory protection of MMU
• Sampling-based idle memory tracking
• Memory Resource Management in VMware ESX Server [OSDI’02]
• Invalidating access privilege of sample pages Access to a
sample page generates page fault to VMM VMM estimates
the size of idle memory
25/30](https://image.slidesharecdn.com/4-150328170405-conversion-gate01/85/4-Memory-virtualization-and-management-25-320.jpg)
![How to Detect Idle Memory
• Para-virtualized approach
• Ghost buffer with hypervisor exclusive cache
• Paravirtualized paging
• Transcendent memory (tmem)
• Providing OS with explicit interface for hypervisor cache
• When a page is evicted, put the page in hypervisor cache
• Oracle’s project
• https://oss.oracle.com/projects/tmem/
Virtual Machine Memory Access Tracking With Hypervisor Exclusive Cache [USENIX’07]
MRC
<Original> <Hypervisor cache>
26/30](https://image.slidesharecdn.com/4-150328170405-conversion-gate01/85/4-Memory-virtualization-and-management-26-320.jpg)
![How to Move Memory
• Memory ballooning
• Para-virtualization
• OS is responsible for reclaiming pages to be moved
Memory Resource Management in
VMware ESX Server [OSDI’02]
+ OS knows the best target of victim pages
+ VMM doesn’t need to track guest memory
- Guest OS support is required
Popular solution now!
• Module-based implementation
• Simple implementation
• Balloon drivers for KVM and Xen are
maintained in Linux mainline
• Windows versions are also available
28/30](https://image.slidesharecdn.com/4-150328170405-conversion-gate01/85/4-Memory-virtualization-and-management-28-320.jpg)
4. Memory virtualization and management
- 3. Memory Virtualization • VMM: “Virtualizing virtual memory” • Virtual Physical Machine Level 2 Page table Page table Page table Page table Level 1 Page table . . . Machine memory Virtual address Physical to Machine Pseudo physical memory [Goal] Secure memory isolation A VM is NOT permitted to access another VM’s memory region A VM is NOT permitted to manipulate “physical-to-machine” mapping All mapping to machine memory MUST be verified by VMM 3/30
- 4. SW-Based Memory Virtualization • x86 was virtualization-unfriendly w.r.t. memory • Memory management unit (MMU) has only a page table root for “virtual-to-machine (V2M)” mapping Level 2 Page table Page table Page table Page table Level 1 Page table . . . Machine memory Virtual address Physical to Machine Pseudo physical memory MMU CR3 “Pseudo” means SW, not HW This P2M table is used to establish V2M, not recognized by HW 4/30
- 5. Full- vs. Para-virtualization • How to maintain V2M mapping • Full-virtualization • No modification to V2P in a guest OS • Secretly modifying binary violates OS semantic • “Shadow page tables” • V2M made by referring to V2P and P2M • + No OS modification • - Performance overheads for maintaining shadow page tables • Para-virtualization • Direct modification to V2P in a guest OS using hypercall • V2P V2M • + High performance (batching optimization is possible) • - OS modification 5/30
- 6. Full- vs. Para-virtualization • How to maintain V2M mapping MMU Hardware Page directory Page table Page table Page table Page table Page directory Page table Page table Page table Page table VMM Guest OS Shadow Page table Shadow mode (full virtualization) Direct mode (para-virtualization) V2P V2M sync MMU Page directory Page table Page table Page table Page table V2M Read Write Read Write Page fault Page fault handler Verify that the machine page to be updated is owned by the domain? 6/30
- 7. Linux Virtual Memory (x86-32) Kernel (1G) User (3G) Virtual memory Page directory Page table Page table Page table Page table cr3 PFN N PFN N-1 . . . PFN 4 PFN 3 PFN 2 PFN 1 PFN 0 PAGE_OFFSET PFN N’s descriptor PFN N-1’s descriptor . . . PFN 2’s descriptor PFN 1’s descriptor PFN 0’s descriptor mem_map struct page _count flags mapping lru Physical memory high_memory Buddy system allocator Slab allocator __alloc_pages __free_pages 7/30
- 8. Xen Memory Virtualization • Para-virtualization Xen(64M) Kernel User (3G) Page directory Page table Page table Page table Page table cr3 MFN N MFN N-1 . . . MFN 4 MFN 3 MFN 2 MFN 1 MFN 0 Xen(64M) Kernel User (3G) Page directory Page table Page table Page table Page table Virtual memory Virtual memory Machine memory MFN N’s descriptor MFN N-1’s descriptor . . . MFN 2’s descriptor MFN 1’s descriptor MFN 0’s descriptor frame_table struct page_info list count_info _domain type_info Buddy system allocator __alloc_heap_pages __free_heap_pages 8/30
- 9. Page Table Identification • Auditing page table updates • Following mapping from a page table root (CR3) to identify page tables • Once identified, page table updates are carefully monitored and verified Page directory Page table Page table . . . . . . Page Type PD PT RW Pin request validated Page Page . . . 9/30
- 10. HW Memory Virtualization • What if nested page table walking is supported by HW? • Eliminating SW overheads to maintain V2M • HW-assisted memory virtualization • Intel Extended Page Tables (EPT) • AMD Rapid Virtualization Indexing (RVI) VMM Guest OS V2P P2M V2M Shadow page tables (SPT) MMU SPT VMM Guest OS V2P P2M Extended page tables (EPT) EPT MMU V2M EPT GPT 1st walking 2nd walking GPT 10/30
- 11. HW Memory Virtualization • AMD RVI (formerly Nested Page Tables (NPT)) • Two page table roots: gCR3 and nCR3 Accelerating Two-Dimensional Page Walks for Virtualized Systems [ASPLOS’08] 11/30
- 12. HW Memory Virtualization • Advantages • Significantly simplifying VMM • Just informing MMU of a P2M root • No shadow page tables • No synchronizing overheads and memory overheads • No OS modification • Disadvantages • Not always outperforming SW-based methods • Page walking overheads on a TLB miss • SW solution: SW-HW hybrid scheme [VEE’11], Large pages • HW solution: Caching page walks [ASPLOS’08], Flat page tables [ISCA’12] 12/30
- 13. ARM Memory Virtualization • Two-stage address translation Applications OS Hardware Applications Guest OS VMM Hardware Virtual Address (VA) Physical Address (PA) Virtual Address (VA) Physical Address (PA) Intermediate Physical Address (IPA) Guest Kernel Guest User Stage 1 translation Stage 2 translation Virtual Address Space Physical Address Space Intermediate Physical Address Space 13/30
- 14. Summary • SW-based memory virtualization has been the most complex part in VMM • Before HW support, Xen continued optimizing its shadow page tables up to ver3 • Virtual memory itself is already complicated, but virtualizing virtual memory is horrible • HW-based memory virtualization significantly reduces VMM complexity • The most complex and heavy part is now offloaded to HW • But, energy issues on ARM HW memory virtualization? 14/30
- 16. Process Memory Management • Memory sharing • Parent-child copy-on-write (CoW) sharing • On fork(), a child CoW-shares its parent memory • On write to a shared page, copy and modify a private page • Advantages • Reducing memory footprint • Lightweight fork • Memory overcommitment • Giving a process larger memory space than physical memory • Paging or swapping out to backing storage when memory is pressured • Advantage • Efficient memory utilization 16/30
- 17. VM Memory Management • Memory sharing • No parent-child relationship • But, a research project finds this relationship in a useful case • Virtual based honeyfarm [SOSP’05] • Honeypot VMs CoW-share a reference image • General memory sharing • Block-based sharing • Content-based sharing • Memory overcommitment • Σ VM memory allocation > Machine memory • Dynamic memory balancing VMM Logging & Analysis Parent Honeypot VM Honeypot VM Machine Memory Scalability, Fidelity, and Containment in the Potemkin Virtual Honeyfarm [SOSP’05] 17/30
- 18. Why VM Memory Sharing? • Why memory? • Memory limitation inhibits high consolidation density • Other resources wastage • HW cost • Memory itself • Limited motherboard slot • Energy cost • RAM energy consumption matters! • Main goal • Reducing memory footprint as much as possible even with more CPU computation 18/30
- 19. Memory Sharing • Block-based page sharing • Transparent page sharing of Disco [SOSP’97] • Sharing-aware block devices [USENIX’09] • On reading a common block from shared disk, only one memory copy is CoW-shared + Finding identical pages is lightweight - Sharing only for shared disk Disco: Running Commodity Operating Systems on Scalable Multiprocessors [SOSP’97] 19/30
- 20. Memory Sharing • Content-based page sharing • Sharing pages with identical contents • VMWare ESX server and KSM for KVM …2bd806af 4. Byte-by-byte comparison 1. Periodic scan 2. Hashing page contents 3. Hash collision 5. CoW sharing & reclaiming a redundant page Memory Resource Management in VMware ESX Server [OSDI’02] + High memory utilization - Finding identical pages is nontrivial PA MA 20/30
- 21. Memory Sharing • Subpage sharing • Difference Engine: Harnessing Memory Redundancy in Virtual Machine [OSDI’08] • Patching similar pages • Compressing idle pages • Reference & dirty bit tracking to find idle pages PA MA Reference page + Much higher memory utilization - Computationally intensive Put it all together! 21/30
- 22. Memory Sharing • Kernel Samepage Merging (KSM) • Open source!! • Content-based page sharing in Linux • Increasing memory density by using KSM [OLS’09] • Linux kernel service • Applicable to all Linux processes including KVM • Target memory regions can be registered via madvise() system call • Content comparison is done by memcmp() • Red-black tree 22/30
- 23. Memory Overcommitment • Two types of memory overcommitment • Using surplus memory reclaimed by sharing • Providing to memory-hungry VMs • Creating more VMs • When is memory pressured? • Shared pages are CoW-broken • Balancing memory between VMs • Providing idle memory to memory-hungry VMs • When is memory pressured? • Idle memory becomes busy Research issues • How to detect memory-hungry VMs • How to detect idle memory in VMs • How to effectively move memory from a VM to another Working set estimation techniques Satori: Enlightened page sharing [USENIX’09] Sharing cycle 23/30
- 24. How to Detect Memory-hungry VMs • Monitoring memory pressure of VMs • Swap I/O traffic • Simple method, but only for anonymous pages (e.g., heap) • How much memory is required? • Feedback-driven method • Allocate more memory monitor swap traffics … • Buffer cache monitoring (Geiger [ASPLOS’06]) • Monitoring the use of unified buffer cache based on • Page faults, page table updates, and disk I/Os • How much memory is required? • LRU miss curve ratio (MRC) Disk VM Unified buffer cache Associate memory and disk locations Detect page reuse as cache eviction • Reused by CoW and demand paging 24/30
- 25. How to Detect Idle Memory • Idle memory • Inactive memory • Not recently used memory • Monitoring page access frequency • Nontrivial • Page access is done solely by HW • Using memory protection of MMU • Sampling-based idle memory tracking • Memory Resource Management in VMware ESX Server [OSDI’02] • Invalidating access privilege of sample pages Access to a sample page generates page fault to VMM VMM estimates the size of idle memory 25/30
- 26. How to Detect Idle Memory • Para-virtualized approach • Ghost buffer with hypervisor exclusive cache • Paravirtualized paging • Transcendent memory (tmem) • Providing OS with explicit interface for hypervisor cache • When a page is evicted, put the page in hypervisor cache • Oracle’s project • https://oss.oracle.com/projects/tmem/ Virtual Machine Memory Access Tracking With Hypervisor Exclusive Cache [USENIX’07] MRC <Original> <Hypervisor cache> 26/30
- 27. How to Move Memory • VMM-level swap (host swap) • Full-virtualization • VMM is responsible for reclaiming pages to be moved VM1 VM2 VMM Guest swap Guest swap Host swap Memory Memory Drawback • VMM cannot know which page is less important (VMM does not know OS policies) • Even if VMM chooses the same victim page as OS, double page fault occurs if OS tries to swap out a “host-swapped page” to guest swap swap-out 27/30
- 28. How to Move Memory • Memory ballooning • Para-virtualization • OS is responsible for reclaiming pages to be moved Memory Resource Management in VMware ESX Server [OSDI’02] + OS knows the best target of victim pages + VMM doesn’t need to track guest memory - Guest OS support is required Popular solution now! • Module-based implementation • Simple implementation • Balloon drivers for KVM and Xen are maintained in Linux mainline • Windows versions are also available 28/30
- 29. How to Move Memory • Memory ballooning • Overcommitted memory • Guest OS 2 requests six pages, but four pages are available VMM Guest OS 1 Guest OS 2 Balloon driver Guest OS 1’s Swap Memory allocator Request 6 pages Reclaim 2 pages U U U U U F Guest OS 1’s page Balloon page 29/30
- 30. Summary • Memory is precious in virtualized environments • Sharing and overcommitment contribute to high consolidation density • But, we should take care of memory efficiency vs. QoS • Insufficient memory can largely degrade QoS • VM memory management issues will be more focused in mobile virtualization The degree of consolidation High QoS Low memory utilization Low QoS High memory utilization 30/30