Qos
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The document summarizes the mClock algorithm for providing quality of service (QoS) guarantees for storage input/output (IO) resource allocation in virtualized environments. The mClock algorithm uses a combination of weight-based and constraint-based scheduling to dynamically allocate storage IO bandwidth proportional to weights while ensuring reservations and limits are met. It assigns real-time tags to requests based on reservations, limits, and shares to schedule requests and adjusts tags when throughput varies to maintain proportional allocation and meet guarantees.
![Extracted from :
mClock: Handling Throughput Variability for
Hypervisor IO Scheduling
By Vmware
[https://labs.vmware.com/download/122/]](https://image.slidesharecdn.com/i2ohweuqthy0f9awtyj8-signature-1e22e3c02f143bdbaade1033e41083e3186e10cee46774b782105433d60d093c-poli-160517072541/85/Qos-2-320.jpg)
Qos
- 1. Quality Of Service (QOS) By: Karthik U S Sukumar Poojari
- 2. Extracted from : mClock: Handling Throughput Variability for Hypervisor IO Scheduling By Vmware [https://labs.vmware.com/download/122/]
- 3. Quality Of Service (QOS) Outline: ● Problem statement ● Related works ● mClock algorithm ● Advantages and disadvantages ● Conclusion
- 4. Problem Statement ● Virtualized hosts run many virtual machines (VMs) ● The hypervisor needs to provide each virtual machine with the illusion of owning dedicated physical resources: CPU, memory, network and storage IO. ● Existing products such as VMware ESX server hypervisor provide guarantees for CPU and memory allocation using sophisticated controls such as reservations, limits and shares. Storage IO resource allocation is much more rudimentary, limited to providing proportional shares to different VMs.
- 5. Problem Statement ● Storage IO allocation is hard because of: The variable storage workload characteristics The available throughput changes with time Must adjust allocation dynamically
- 6. Problem Statement ● Three controls: Reservation: minimum guarantee Limits: maximum allowed Shares: proportional allocation
- 8. Goal
- 9. Related Works ● Proportional Share Algorithms: WFQ, virtual-clock, SFQ, Self-clocked, WF2 Q, SFQ(D) Does not support reservations, limits and latency control ● Algorithms with support for latency sensitive applications: BVT, SMART, Lottery scheduling Does not support reservations and limits ● Reservation-based Algorithms: CPU & Memory management in ESX, Hierarchical CPU scheduling, Rialot Supports all the controls but does not handle varying service capacity
- 10. mClock Algorithm ● It is a combination of Weight Based Scheduler and Constraint Based Scheduler ● Each application has a weight wi ● Each request is assigned a tag ● Tags are spaced 1/wi apart i.e., Service allocation is proportional to wi ● Global virtual time (gvt) gets updated on every request completion ● Gvt is the minimum start tag in the system
- 11. mClock Algorithm: Symbols ● Three real time tags: Pi r : Share based tag of request r and VM vi Ri r : Reservation tag of request r from VM vi Li r : Limit tag of request r from VM vi wi : Weight of VM vi ri : Reservation of VM vi li : Maximum service allowance (Limit) for vi
- 12. mClock Algorithm ● It has 3 main components: ➢ Tag Assignment ➢ Tag Adjustment ➢ Request Scheduling
- 13. Tag Assignment /* Reservation tag */ Ri r = max { Rr−1 + 1/ri , t } /* Limit tag */ Li r = max { Lr−1 + 1/li , t } /* Shares tag */ Pi r = max { Pr−1 + 1/wi , t }
- 14. Tag Assignment VM v1 Request r Time t=10 r1 250 l1 1000 w1 1 Assume: R1 r-1 = 8 L1 r-1 = 8 P1 r-1 = 8 R1 r = max { 8+1/250 , 10 } L1 r = max { 8+1/1000 , 10 } P1 r = max { 8+1/1 , 10 }
- 15. Tag Adjustment ● Required whenever an idle VM becomes active ● The initial P tag value of a freshly active VM is set to the current time MinPtag = minimum of all P tags For each active VM vj Pj r - = minPtag - t
- 16. Tag Adjustment VM v2 Request r Time t=12 r2 100 l2 800 w2 2 MinPtag = 8 P1 r1 = 8 P1 r1 - = 8 – 12 P1 r1 = 8 P1 r2 = 10 P1 r2 - = 10 – 12 P1 r2 = 14
- 17. Request Scheduling /* Constraint based */ if ( smallest reservation tag <= current time) Schedule request with smallest reservation tag else /* Weight based - reservations are met */ Schedule request with smallest shares tag Assuming request belong to VM vk Subtract 1/rk from reservation tags of VM vk A VM is eligible if (limit tag <= current time)
- 18. •Supports all controls in a single algorithm •Handles variable & unknown capacity •Easy to implement Advantages and Disadvantages •Supports all controls in a single algorithm •Handles variable & unknown capacity •Easy to implement ● Not efficient with Distributed Architucture
- 19. Conclusion • Supports reservation, limit and shares in one place • Handles variable IO performance seen by hypervisor • Can be used for other resources such as CPU, memory & Network IO allocation as well
- 20. THANK YOU