EECS 252 Graduate Computer Architecture Lec 17 – Advanced Memory Hierarchy 2Review [1]Review [2/2]OutlineVirtual Machine Monitors (VMMs)VMM Overhead?Requirements of a Virtual Machine MonitorSlide 8ISA Support for Virtual MachinesImpact of VMs on Virtual MemoryISA Support for VMs & Virtual MemoryImpact of I/O on Virtual MemoryExample: Xen VMXen changes for paravirtualizationXen and I/OCS252: AdministriviaXen PerformanceXen Performance, Part IIXen Performance, Part IIIXen Performance, Part IVProtection and Instruction Set Architecture80x86 VM ChallengesIntel/AMD address 80x86 VM ChallengesSlide 24AMD Opteron Memory HierarchyOpteron Memory Hierarchy PerformanceCPI breakdown for Integer ProgramsCPI breakdown for Floating Pt. ProgramsPentium 4 vs. Opteron Memory HierarchyMisses Per Instruction: Pentium 4 vs. OpteronFallacies and Pitfalls“Virtual Machine Monitors: Current Technology and Future Trends” [1/2]“Virtual Machine Monitors: Current Technology and Future Trends” [2/2]And in ConclusionEECS 252 Graduate Computer Architecture Lec 17 – Advanced Memory Hierarchy 2David PattersonElectrical Engineering and Computer SciencesUniversity of California, Berkeleyhttp://www.eecs.berkeley.edu/~pattrsnhttp://vlsi.cs.berkeley.edu/cs252-s0601/18/19CS252 s06 Adv. Memory Hieriarchy 22Review [1]•Memory wall inspires optimizations since so much performance lost there–Reducing hit time: Small and simple caches, Way prediction, Trace caches–Increasing cache bandwidth: Pipelined caches, Multibanked caches, Nonblocking caches–Reducing Miss Penalty: Critical word first, Merging write buffers–Reducing Miss Rate: Compiler optimizations–Reducing miss penalty or miss rate via parallelism: Hardware prefetching, Compiler prefetching•“Auto-tuners” search replacing static compilation to explore optimization space?•DRAM – Continuing Bandwidth innovations: Fast page mode, Synchronous, Double Data Rate01/18/19CS252 s06 Adv. Memory Hieriarchy 23Review [2/2]•VM Monitor presents a SW interface to guest software, isolates state of guests, and protects itself from guest software (including guest OSes)•Virtual Machine Revival–Overcome security flaws of large OSes–Manage Software, Manage Hardware–Processor performance no longer highest priority01/18/19CS252 s06 Adv. Memory Hieriarchy 24Outline•Virtual Machines•Xen VM: Design and Performance•Administrivia•AMD Opteron Memory Hierarchy•Opteron Memory Performance vs. Pentium 4•Fallacies and Pitfalls•Discuss “Virtual Machines” paper•Conclusion01/18/19CS252 s06 Adv. Memory Hieriarchy 25Virtual Machine Monitors (VMMs)•Virtual machine monitor (VMM) or hypervisor is software that supports VMs•VMM determines how to map virtual resources to physical resources•Physical resource may be time-shared, partitioned, or emulated in software •VMM is much smaller than a traditional OS; –isolation portion of a VMM is 10,000 lines of code01/18/19CS252 s06 Adv. Memory Hieriarchy 26VMM Overhead?•Depends on the workload•User-level processor-bound programs (e.g., SPEC) have zero-virtualization overhead –Runs at native speeds since OS rarely invoked•I/O-intensive workloads OS-intensive execute many system calls and privileged instructions can result in high virtualization overhead –For System VMs, goal of architecture and VMM is to run almost all instructions directly on native hardware•If I/O-intensive workload is also I/O-bound low processor utilization since waiting for I/O processor virtualization can be hidden low virtualization overhead01/18/19CS252 s06 Adv. Memory Hieriarchy 27Requirements of a Virtual Machine Monitor•A VM Monitor –Presents a SW interface to guest software, –Isolates state of guests from each other, and –Protects itself from guest software (including guest OSes)•Guest software should behave on a VM exactly as if running on the native HW –Except for performance-related behavior or limitations of fixed resources shared by multiple VMs•Guest software should not be able to change allocation of real system resources directly•Hence, VMM must control everything even though guest VM and OS currently running is temporarily using them–Access to privileged state, Address translation, I/O, Exceptions and Interrupts, …01/18/19CS252 s06 Adv. Memory Hieriarchy 28Requirements of a Virtual Machine Monitor•VMM must be at higher privilege level than guest VM, which generally run in user mode Execution of privileged instructions handled by VMM•E.g., Timer interrupt: VMM suspends currently running guest VM, saves its state, handles interrupt, determine which guest VM to run next, and then load its state –Guest VMs that rely on timer interrupt provided with virtual timer and an emulated timer interrupt by VMM•Requirements of system virtual machines are same as paged-virtual memory: 1. At least 2 processor modes, system and user2. Privileged subset of instructions available only in system mode, trap if executed in user mode–All system resources controllable only via these instructions01/18/19CS252 s06 Adv. Memory Hieriarchy 29ISA Support for Virtual Machines•If plan for VM during design of ISA, easy to reduce instructions executed by VMM, speed to emulate–ISA is virtualizable if can execute VM directly on real machine while letting VMM retain ultimate control of CPU: “direct execution”–Since VMs have been considered for desktop/PC server apps only recently, most ISAs were created ignoring virtualization, including 80x86 and most RISC architectures•VMM must ensure that guest system only interacts with virtual resources conventional guest OS runs as user mode program on top of VMM–If guest OS accesses or modifies information related to HW resources via a privileged instruction—e.g., reading or writing the page table pointer—it will trap to VMM•If not, VMM must intercept instruction and support a virtual version of sensitive information as guest OS expects01/18/19CS252 s06 Adv. Memory Hieriarchy 210Impact of VMs on Virtual Memory•Virtualization of virtual memory if each guest OS in every VM manages its own set of page tables?•VMM separates real and physical memory –Makes real memory a separate, intermediate level between virtual memory and physical memory–Some use the terms virtual memory, physical memory, and machine memory to name the 3 levels–Guest OS maps virtual memory to real memory via its page tables, and
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