1. ANALYSIS OF TASK ASSIGNMENT POLICIES SCALABLE WEB SERVERS SYSTEMSin
SCALABLE WEB SERVERS SYSTEMS
3-Dec-18
Ravindra Sudhindra

2. To be Discussed …… Introduction Model Description Algorithms
Parameters
Evaluation methods
Results
Conclusion
3-Dec-18
Ravindra Sudhindra

3. Why Distributed Web Server
Introduction ….
Why Distributed Web Server
Scalable & Fault Tolerant than centralized servers
Exponential growth of usage
Overloaded servers
Increase usage of bandwidth
3-Dec-18
Ravindra Sudhindra

4. Introduction ….
Our Model
3-Dec-18
Ravindra Sudhindra

5. How Distr.Web Servers works ?
Introduction ….
What is a DNS ?
Domain Name Server
How Distr.Web Servers works ?
3-Dec-18
Ravindra Sudhindra

6. LG – Local Gateway used by clients to connect to the internet.
Model Description ….
LG – Local Gateway used by clients to connect to the internet.
LNS – Local Name Servers
INS – Intermediate Name Servers
TTL – Time To Live, the duration of time for which a name-to-address mapping is cached at LNSs and INSs
Obstacles for the DNS -
Address Caching limits the control of the DNS
During the TTL period, subsequent requests from a LG arrive at the same WS
If TTL ~ 0  DNS will be a bottle neck.
Zip Function – Client Distr. is a small number of large values !!!!
3-Dec-18
Ravindra Sudhindra

7. Scheduling Policies Information on LG Information on WS Load
Algorithms ….
Scheduling
Policies
Information on LG
Information on
WS Load
Combination of
(i) & (ii)
Redirection
Algorithms
3-Dec-18
Ravindra Sudhindra

8. Hidden Load Weight Algorithms ….
Nj – Name to address mapping it received from LGj
Ri,j – Number of web requests that WSi received from LGj
Hidden Load Weight ηj = Σi Ri,j / Nj
It is called the hidden load weight as this load is transparent to the DNS.
1. Using Domain info …
Two-tier Round Robin (RR2)
Class Threshold Tc = 1 / #LG
Dynamically Accumulated Load (DAL)
Bin Value - weighted load given to each WS
2. Using Load info on Web Servers …
Asynchronous Alarms
Present load information
3-Dec-18
Ravindra Sudhindra

9. Asynchronous Alarms - Not sufficient to define a scheduling policy.
Algorithms ….
Asynchronous Alarms - Not sufficient to define a scheduling policy.
Past Load Information -
Least Utilized Node (LUN) - Considers only the latest load information in deciding the assignment. (utilization is measured for a 15 seconds interval)
Present and Past Load Information -
Lowest Utilization (LU) - Same as in LUN but also considers the last one hour as the interval for the utilization estimate.
Lowest among Past and Present Utilizations (LPPU) -
The DNS selects the WS with the lowest weighted pputi
are the utilization samples of WS evaluated in the past five intervals of 15 seconds each and G =
3-Dec-18
Ravindra Sudhindra

10. Sequence of Previous Assignments
Algorithms ….
Sequence of Previous Assignments
LPPU Modified and LUN Modified - Here the policy look for the first and second minimum among the WSs utilization samples. If the first minimum is the same WS of the previous assignment, the other WS is chosen.
Algorithms combining Domain and Server Information
Single Threshold (Thr1)
Double Threshold
Temporal Threshold
DAL-set bin to top (DAL-ST)
DAL set to actual number of requests (DAL-AN)
Minimum Residual Load (MRL)
3-Dec-18
Ravindra Sudhindra

11. Redirection Algorithms for Load Sharing
Alternative architecture that integrates the DNS dispatching mechanism with a redirection technique carried out by the WS through the redirection mechanism.
Centralized Synchronous Redirection
Distributed Asynchronous Redirection
Centralized Synchronous Redirection
The decision is centralized at the DNS. Every t seconds each Web server sends some status info. to the DNS.
DNS builds Assignment Table
DNS serves the address resolution requests by using this table.
3-Dec-18
Ravindra Sudhindra

12. The redirected entities can be – Entire Domains (DR)
Algorithms ….
The redirected entities can be –
Entire Domains (DR)
Individual Domains (CR)
Both (CDR)
Since the HTTP redirection works in an individual basis, domain redirection means that all the clients of the same domain are subject to the same redirection decision
Domain Redirection Scheme
DNS estimates the domain hit rate for each connected domain, and on this basis it orders the domains from the most to the least popular.
DNS orders the servers from the least to the most loaded.
DNS builds the assignment table through the domain hit rate information
Determines the potential load assigned to each server through a bin, which contains the sum of the hit rates of the domains assigned to that server by the Assignment table.
The server bin is updated after each assignment.
The assignment is done through greedy approach.
3-Dec-18
Ravindra Sudhindra

13. DNS calculates the Average Bin level of all the Web Servers.
Algorithms ….
Client Redirection
The assignment table is used only in the first level assignment carried out by the DNS when it receives an address resolution requests.
The second level assignment carried out by the Web servers is instead based on the Server Percentage list to indicate the percentage of client requests that needs to be redirected.
DNS calculates the Average Bin level of all the Web Servers.
Broadcasts it to all the Web Servers.
The Web Server will generate a random number between 0 and 1.
If the number generated is greater than the average bin number of the Web server then the request is redirected.
If the Server percentage is zero then request wont be redirected.
From point (vi) there can be three possibilities for the choice of the server that has to receive these redirected request.
3-Dec-18
Ravindra Sudhindra

14. Domain and Client Redirection
Algorithms ….
CR_RR Policy - All requests are reassigned to the server in a Round Robin manner for the servers having the percentage equal to 0%.
CR_PRR Policy – Client redirection is done in a probabilistic round-robin way, where the probability is based on the available server capacity using the latest load information
CR_LL Policy – In this the server with the least load will be assigned the request.
Domain and Client Redirection
Combination of the first and the second method.
Alarm Message
Any of the above mentioned synchronous algorithms can be combined with a feedback alarm mechanism.
3-Dec-18
Ravindra Sudhindra

15. Asynchronous Redirection schemes -
Algorithms ….
Asynchronous Redirection schemes -
The feedback mechanism talked about in the previous topic can be used to activate the redirection process itself.
The web cluster remains a typical DNS-dispatcher based systems where the DNS carries out the first level assignment through a RR or RR2 scheme.
This DNS assignment process is integrated with a second level (re)assignment mechanism triggered by any overloaded server.
DNS maintains the so called ‘Available Server list’ which is the list of severs that are not overloaded at that moment.
This is list is transmitted in reply to the server alarm
The overloaded will redirect the client request using this list.
3-Dec-18
Ravindra Sudhindra

16. Category Parameter Values (default) Parameters ….
Web-Server System
Number of Servers
4-9(7)
Average Utilization
(0.6667) LG
Number
10-100(20)
Client Distribution
Zipf
Geometric (p = 0.3)
TTL (Seconds)
0-360 (240)
Client
(1500)
Web page requests
Exponential (mean 20)
Hits per page
Uniform (5-15)
Hit Service time (milliseconds)
Exponential (mean 4.5)
Inter-request time (seconds)
Exponential (mean 15)
A good index - measure the load level and overloaded instances is the utilization of the Web servers.
3-Dec-18
Ravindra Sudhindra

17. Evaluation ….
Performance Stability – is the main concern for the distributed web server system that is subject to non-uniform bursts of arrivals of which only a small fraction is controlled by the DNS.
The impact of the DNS algorithms on avoiding the overload at any servers is more important than equalizing the system workload
Utilization of the Web Servers indicates the Load Level / Overloaded Situations.
If the maximum utilization at an instant is low, it means that no server is overloaded at that time.
By tracking the period of time the maximum utilization is above or below a certain threshold, we can get an indication of how well the distributed system is running.
Determine the percentage of time for which the at least one of the servers is critically loaded, which is given by the cumulative frequency.
3-Dec-18
Ravindra Sudhindra

18. Results….
The goal is to measure how effectively the DNS scheduling algorithms, that control only a very small percentage of the address resolution requests, can minimize overload situations of a distributed Web Server systems.
3-Dec-18
Ravindra Sudhindra

19. Algorithms using Domain Information -
Results….
Algorithms using Domain Information -
Distribution for RR Scheduling Policy
3-Dec-18
Ravindra Sudhindra

20. Comparison of 3 Algo’s using Domain information
Results….
Comparison of 3 Algo’s using Domain information
3-Dec-18
Ravindra Sudhindra

21. Algorithms combining server load information
Results….
Algorithms combining server load information
3-Dec-18
Ravindra Sudhindra

22. Summary … Worst Poor Good Better Best
Results….
Summary …
Worst
Poor
Good
Better
Best LU
Random
LPPU-Naive
LUN
Naïve
DAL -
LPPU-Modified
LUN-Modified
Thr1
The threshold policy using the state information provides the most substantial performance improvement. This motivates the research for policies that combine client and server information!!!
3-Dec-18
Ravindra Sudhindra

23. Algorithms combining domain and server load information
Results….
Algorithms combining domain and server load information
This section shows that all the algorithms with any alarm feedback from WS to the DNS have better performance than their non feedback counterparts.
3-Dec-18
Ravindra Sudhindra

24. Results….
Although no policy guarantees that the Web Server system is never overloaded, the graphs indicate that, for the proposed policies these overloaded server occur very rarely in more than one server.
3-Dec-18
Ravindra Sudhindra

25. Results….
MRL-Thr1 and RR2-Thr1 can be considered as the best policies and DAL-Thr1 stands 3rd.
3-Dec-18
Ravindra Sudhindra

26. Sensitivity of DNS Scheduling Algorithm
Results….
Sensitivity of DNS Scheduling Algorithm
Sensitivity of TTL
3-Dec-18
Ravindra Sudhindra

27. Results….
For RR2-Thr1 and MRL-Thr1 the probability that no server is overloaded is around 90% for more than 40 domains.
3-Dec-18
Ravindra Sudhindra

28. Results….
The probability that no server is overloaded remains over 0.8 for more than 40 domains, which is significantly better than the 0.68 value achieved for the default value.
Another interesting aspect is the sensitivity of the scheduling policy for a singleton domain.
3-Dec-18
Ravindra Sudhindra

29. Again DAL-Thr1 and MRL-Thr1 performs the best.
Results….
Performance improves as the client distribution resembles the uniform distribution and the skews reduce.
Again DAL-Thr1 and MRL-Thr1 performs the best.
3-Dec-18
Ravindra Sudhindra

30. Results of redirection Algo ….
3-Dec-18
Ravindra Sudhindra

31. Sensitivity to the frequency of Assignment table updating -
Results….
Sensitivity to the frequency of Assignment table updating -
In fig 2 we compare the sensitivity to the assignment table update interval using the probability that no server is overloaded (exceeding 95% utilization) as the performance metrics.
3-Dec-18
Ravindra Sudhindra

32. Asynchronous redirection performance
Results….
Asynchronous redirection performance
3-Dec-18
Ravindra Sudhindra

33. Results….
Sensitivity…
3-Dec-18
Ravindra Sudhindra

34. Classic Algorithms such as RR or least loaded server are not adequate.
Conclusion….
Classic Algorithms such as RR or least loaded server are not adequate.
On the other hand detailed load information also does not help.
Extensive simulation results have shown that the strategies that take into account the domain of the requests and alarms from the servers.
The most promising algorithms use asynchronous alarms combined with the estimation of the hidden load weight.
RR2-Thr1 is the simplest to implement.
MRL-Thr1 and RR2-Thr1 stand out the best algorithm with DAL-Thr1 closely behind.
3-Dec-18
Ravindra Sudhindra

35. Conclusion….
The scheduling policy combined with redirection mechanism are effective.
Asynchronous Algorithms combined with redirection performs the best even in case of a highly skewed load.
The intra-cluster communication overhead is typically higher than that introduced in the asynchronous policy
Comparing the 2 different categories it can be concluded that when a simple best performing algorithm using domain and server information combined with the asynchronous redirection policy gives the best results.
3-Dec-18
Ravindra Sudhindra

36. Thank You
3-Dec-18
Ravindra Sudhindra