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Perspectives on Virtualized Resource Management
Carl Waldspurger June 26, 2013
10th International Conference on Autonomic Computing
USENIX Federated Conference Week, San Jose
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Resource Management
� Map workloads onto physical resources
� Varying importance
� Diverse resources, granularities
� Complex interactions
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Virtualization
� Hypervisor: extra level of indirection
� Powerful new capabilities
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All problems in computer science can be solved by another level of indirection… — David Wheeler
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Virtualization: Wildly Successful 4
Source: IDC Server Virtualization Forecast
% W
orkl
oads
in
VMs
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Indirection: Double-Edged Sword
� Performance isolation
� Semantic gap
� Complexity
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… but that usually will create another problem. — David Wheeler
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My Vantage Point
� Research and product development
� Systems I’ve helped build Spawn (PARC), lottery/stride scheduling (MIT), DCPI and Itsy (DEC), ESX and DRS (VMware), …
� Challenges building autonomic systems
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No Silver Bullet
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Recurring Themes
� Randomization and sampling
� Indirection and interposition
� Semantic gap and transparency
� Hardware/software co-evolution
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Path to Autonomic Systems
1. Measurement
2. Modeling
3. Mechanisms
4. Policies
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1. Accurate Measurement Profiling, accounting, virtualized timekeeping
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If you can’t measure something, you can’t understand it. If you can’t understand it, you can’t control it. — H. James Harrington
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Measurements Gone Wrong
� Blind spots, distortions
� Statistical profiling
� CPU accounting
� Virtualized time-keeping
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Virtualized Timekeeping
� Maintain illusion of dedicated system
� Periodic guest timer interrupts � Track passage of real time � Statistical process accounting
� What happens when VM descheduled?
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Timer Interrupt Backlog 13
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Timer Interrupt Backlog 14
descheduled
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descheduled
Less Distortion: Timer Sponge 22
descheduled
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descheduled
Less Distortion: Timer Sponge 23
descheduled
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descheduled
Less Distortion: Timer Sponge 24
descheduled
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Hazards of Warping Time
� Distorting guest time measurements
� Degrading network throughput
� Exposing guest bugs
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Future Research Directions: Measurement
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� Descheduled time distortion — still!
� Guest access to hardware counters
� Distributed measurements
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2. Practical Modeling Cache locality, MRCs, big data
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Essentially, all models are wrong, but some are useful. — George Box
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Modeling Goals
� Predict effect of change � Resource allocation � Reconfiguration
� Inform higher-level policies � Determine if satisfiable � Both reactive and proactive
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Cache Modeling
� Inform cache sizing policy � Performance non-linear in allocation � Marginal utility
� Mattson stack algorithm (1970) � Computes misses for all possible sizes � Very powerful, single pass � Still expensive
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Mattson Algorithm Example 31
references B … A D C
distances ∞ 3 7 4 …
� Reuse distance � Unique refs since last access � Distance from top of LRU-ordered stack
� Hit if distance < cache size, else miss
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Mattson Algorithm Example 32
A references B … A D C
distances ∞ 3 7 4 …
✓
� Reuse distance � Unique refs since last access � Distance from top of LRU-ordered stack
� Hit if distance < cache size, else miss
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Mattson Algorithm Example 33
A B references B … A D C
distances ∞ 3 7 4 …
✗ ✓
� Reuse distance � Unique refs since last access � Distance from top of LRU-ordered stack
� Hit if distance < cache size, else miss
1
✓
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Mattson Algorithm Example 34
A C B references B … A D C
distances ∞ 3 7 4 …
✗ ✓
� Reuse distance � Unique refs since last access � Distance from top of LRU-ordered stack
� Hit if distance < cache size, else miss
1
✓
2 3
✓ ✗
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Cache Utility Curves � How performance
varies with size
� MRC � miss ratio curve � miss rate curve
� Working set “knees”
� Many applications
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Allocation
Mis
ses
knee
knee
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Mattson Implementations
� Naïve Stack � N = total refs, M = unique refs � O(N � M) time, O(M) space
� Optimized � Balanced tree: compute reuse distance � Hash table: maps address to tree node � O(N log M) time, O(M) space
� Parallel algorithms
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MRC Approximations � Hardware Support � Qureshi and Patt (MICRO ’06)
� Temporal sampling � Bursty tracing, detect phase transitions � RapidMRC (ASPLOS ’09), Zhao et al. (ATC ’11)
� Spatial sampling � VMware memory MRCs (USPTO App ’10) � CloudPhysics I/O MRCs
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Sampled-Page MRCs
� Spatial sampling � Trace only small random subset of pages � Each sample represents many pages � Run full LRU-based Mattson on subset
� Rate-limit trace rearming for hot pages
� Extremely efficient � Excellent accuracy with < 1% overhead � Leave on continuously, online MRCs
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Sampled-IO MRCs
� New spatial sampling technique � CloudPhysics caching analytics � Detailed paper in preparation
� Huge performance wins � Orders of magnitude faster, smaller � Surprising accuracy with 1% sample
� Practical online construction
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Sampled-IO MRC (Small Trace) 40
Cache Size (MB)
Read
Mis
s Ra
tio
(%) 2 million IOs
4 GB reads 10 GB writes
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Sampled-IO MRC (Larger Trace) 41
Cache Size (GB)
Read
Mis
s Ra
tio
(%) 7 day trace
153 million IOs 1.2 TB reads 0.4 TB writes
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Modeling Complex Systems
� Many interacting components � E.g. cache, bandwidth to backing store � Huge state space: cpu × mem × net × io × …
� Approaches
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� Experimentation � Observation
� Analytical models � Simulation
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Active Experimentation
� Run many experiments on real system � Load testing tools, e.g. HP LoadRunner � VMware SDRS load injector (SOCC ’11)
� Experiment with cloned VMs � Fork using live migration, vary allocations � JustRunIt, Zheng et al. (ATC ’09) � Nondeterminism, external dependencies
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Passive Observation
� Observe many real systems � Diverse configurations, devices � Diverse workloads, demand patterns
� Reach critical mass of “big data” � Model-by-query: lookup similar scenarios � Interpolate to handle sparseness
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Future Research Directions: Modeling
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� MRC temporal dynamics � Behavior at different time scales � MRC “diffs” and “movies”
� General “microcosm” simulation?
� Multi-resource modeling
� Big data techniques
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3. Effective Mechanisms Co-scheduling, ballooning
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Rule of Separation: Separate policy from mechanism; separate interfaces from engines.
— Eric S. Raymond
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Co-scheduling vCPUs
� Semantic gap � What does 100% busy vCPU mean? � Useful work?
� Co-scheduling � Maintain illusion of dedicated hardware � Limit skew between vCPUs within VM
� Alternatives � Para-virtualization, e.g. Hyper-V � Hardware assist, e.g. Intel PLE
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Or spinning on lock?
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VM Memory Reclamation
� Transparent: demand paging � Hard meta-level page replacement decisions � Best data to guide decisions internal to guest � “Double paging” anomaly
� Alternative: implicit cooperation � Coax guest into doing page replacement � Avoid meta-level policy decisions
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Ballooning 50
Virtual disk Guest swap
VM Physical Memory Guest RAM
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Ballooning 51
Virtual disk Guest swap
VM Physical Memory Guest RAM
may page out Inflate: more pressure
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Ballooning 52
Virtual disk Guest swap
Deflate: less pressure
VM Physical Memory Guest RAM
may page in
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Ballooning Retrospective
� Exploits semantic gap � Complete transparency not always desirable � Coax guest into doing hard work
� Has worked well for a long time � Primary ESX memory reclamation mechanism � Now used by Hyper-V, Xen, KVM, EM4J, ...
� More recent issue: large pages
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Large Pages � Coarser mapping granularity � Single x86 large page covers 512 small pages � Reduces TLB misses, makes them cheaper
� Significant win for virtualization � x86 nested paging hardware: Intel EPT, AMD RVI � Two-dimensional page walk, quadratic cost � Large pages reduce number of levels
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Ballooning and Large Pages � ESX hypervisor large-page management � Start with large-page mappings � Fragment on overcommit, re-coalesce
� Primitive guest OS large-page support � Often pinned in memory, so can’t balloon! � Windows can’t swap, Linux swaps some
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Future Research Directions: Mechanisms
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� Coping with larger page granularity � Severe dedup impact, HICAMP (ASPLOS ’12)
� Coarsened visibility
� Extreme design points, PrivateCore vCage
� Meta-mechanisms � Cost-benefit, choose most appropriate � E.g. dedup, balloon, compress, swap
� End-to-end QoS controls
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4. Intuitive Policies Specifications, microeconomics, automation
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The limits of your language are the limits of your world. — Ludwig Wittgenstein
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Expressing Policies
� Resource Level � Provided by modern virtualization systems � Physical resource allocation: GHz, GB, Gbps
� Application Level � Metrics more meaningful to user � Response times, transaction rates, …
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Resource-Level Policies
� Basic VM controls � Reservations, Limits � Shares
� Resource pools � Manage sets of VMs � Hierarchical � Cloud service providers
Org
Dev Test
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2:1 Policy
200.Org 100.Org
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Practical App-Level Policies
� Real world? � Formal QoS/SLAs/SLOs surprisingly rare � Admins running virtualized datacenters
� Expressing utility functions even harder
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I never had a policy; I have just tried to do my very best each and every day. — Abraham Lincoln
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Microeconomic Techniques
� Market-based resource allocation � Price equilibrates supply and demand � Distributed solution to conflicting goals � “Invisible hand” improves social welfare
� Much of real world works this way � Plenty of interesting analogies � Rent, taxes, arbitrage, …
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Spawn: Early Computational Economy
� Xerox PARC, late 80s
� Distributed auction
� Jobs bid for time slices � Hosts maximize profit � Sealed bid, second price
� Complex dynamics � Simple bidding strategy � Proportional control � Oscillations, chaos
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Computational Economies Today
� Why not more common? � Better alternatives for simple policies � Auction overheads, stability concerns
� Public cloud pricing � VM resources rented for real money � Multi-tenancy requires sophisticated policies � Trends: finer-grain, market-based pricing
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Bidding Strategies
� Determining what resources are worth � Utility as function of performance � Performance as function of allocation
� Getting a good price � Mechanical bid adjustment algorithm � Game theory
� Need to automate, build into apps � Apps aware of own performance tradeoffs � Dynamic stability, volatility
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A More Direct Alternative?
� “Unhappy” button � Primitive, single-bit feedback � Squeaky wheel gets the grease
� Empathic Systems Project (Northwestern) � Incorporate direct user feedback � User-driven scheduling of interactive VMs
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Future Research Directions: Policies
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� Raising abstraction level � Single resource à multiple resources � Physical allocation à application goals � Many deep challenges
� Intuitive ways to specify � Application-level vocabulary? � Market-based prices? � Empathic systems?
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Research Directions Toward More Autonomic Systems
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We can only see a short distance ahead, but we can see plenty there that needs to be done.
— Alan Turing
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� Intuitive policies � KISS, app-level, empathic, market-based
� Effective mechanisms � End-to-end QoS, coarse control, meta
� Practical modeling � Multi-resource, big data, MRC dynamics
� Accurate measurement � Distortion, hardware access, distributed
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Vision for Future: RMaaS
� Resource Management as a Service
� Offload decisions to “RM provider” � Remote monitoring and control � Leverage “big data” across customers
� Hybrid automation � Transparently escalate to human experts � Crowdsourcing possibilities
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