Load Balancing Scheduling Methods Explained

Submitted by davidquaid on Thu, 06/06/2013 - 12:19

There are several load balancing methods provided by the LoadMaster, which are known as  "Scheduling Methods" or “algorithms”:

Round Robin

With this method incoming requests are distributed sequentially across the server farm (cluster),  i.e. the available servers. If this method is selected, all the servers assigned to a Virtual Service should have the similar  resource capacity and host identical applications. Choose round robin if all servers have the  same or similar performance and are running the same load. Subject to this precondition, the  round robin system is a simple and effective method of distribution. However, if the servers have different capacities, the use of the round robin system can mean  that a less powerful server receives the next inquiry even though it has not yet been able to  process the current one. This could cause a weaker server to become overloaded.  

Weighted Round Robin

This method balances out the weakness of the simple round robin: Incoming requests are  distributed across the cluster in a sequential manner, while taking account of a static  “weighting” that can be pre-assigned per server. The administrator simply defines the capacities of the servers available by weighting the servers.  The most efficient server A, for example, is given the weighting 100, whilst a much less powerful  server B is weighted at 50. This means that Server A would always receive two consecutive  requests before Server B receives its first one, and so on.

Least Connection

Both round robin methods do not take into account that the system does not recognize how  many connections are maintained over a given time. It could therefore happen that Server B is  overloaded, although it receives fewer connections than Server A, because the users of this  server maintain their connections longer. This means that the connections, and thus the load for  the server, accumulate. This potential problem can be avoided with the "least connections" method: Requests are  distributed on the basis of the connections that every server is currently maintaining. The server  in the cluster with the least number of active connections automatically receives the next  request. Basically, the same principle applies here as for the simple round robin: The servers  related to a Virtual Service should ideally have the similar resource capacities. Please note that in configurations with low traffic rates, the traffic will not balance out and the  first server will be preferred. This is because if all the servers are equal, then the first server is  preferred. Until the traffic reaches a level where the first server continually has active traffic, the  first server will always be selected.Copyright © 2002 - 2013 KEMP Technologies, Inc. All Rights Reserved. Page 13 / 47 LM5305-FIPS Product Overview

Least Connection Slow Start Time 

For both the Least Connection and the Weighted Least Connection scheduling methods, when a  Real Server is initially brought online, a period of time can be configured where the number of  connections is initially restricted and gradually increased. This provides a ‘ramp-up time’ for a  Real Server to ensure that it is not overloaded by a flood of connections upon startup.  This value is configured in the L7 configuration screen.

Weighted Least Connection

If the servers have different resource capacities the “weighted least connection” method is more  applicable: The number of active connections combined with the various weights defined by the  administrator generally provides a very balanced utilization of the servers, as it employs the  advantages of both worlds. This is, in general, a very fair distribution method, as it uses the ratio of the number of  connections and the weight of a server. The server in the cluster with the lowest ratio  automatically receives the next request. Please note that the caveat for Least Connections regarding low traffic rates applies here as well.

Agent Based Adaptive Balancing

In addition to the methods above the LoadMaster contains an adaptive logic, which checks the  state of the servers at regular intervals and independently of the configured weighting. For the extremely powerful “agent based adaptive balancing” method the LoadMaster  periodically checks the system load on all the servers in the farm: Each server machine should  provide a file that contains a numeric value in the range between 0 and 99 representing the  actual load on this server (0 = idle, 99 = overload,101 = failed,102 = administratively disabled).  The LoadMaster retrieves this file by an HTTP GET operation. It is the server’s job to provide the  actual load in the ASCII file. There are no prerequisites, though, how the servers evaluate this  information. Two different strategies are applied, depending on the overall load of the server farm: During normal operation the scheduling algorithm calculates a weighting ratio out of the  collected load values and distributes the connections according to it. So if excessive overloading  of a server occurs, the weighting is readjusted transparently by the system. As with the weighted  round robin, incorrect distribution can then be countered by assigning different weights to the  servers available. During a period of very low traffic, however, the load values as reported by the servers will not  build a representative sample. A load distribution based on these values would result in  uncontrolled, oscillating directives. Therefore in such a situation it is more reasonable, to  calculate the load distribution based on the static weight ratio. The LoadMaster switches to the  weighted round robin method automatically when the load on all servers falls below a limit  defined by the administrator. If the load rises above the limit the LoadMaster switches back to  the adaptive method.Copyright © 2002 - 2013 KEMP Technologies, Inc. All Rights Reserved. Page 14 / 47 LM5305-FIPS Product Overview For further information regarding Adaptive Balancing, please refer to the API for Agent Based  Adaptive Balancing, Technical Note document.

Fixed Weighted

The highest weight Real Server is only used when other Real Server(s) are given lower weight  values. However, if highest weight server falls, the Real Server with the next highest priority  number will be available to serve clients. The weight for each Real Server should be assigned  based on the priority among Real Server(s).

Weighted Response

Time The traffic is scheduled using the weighted round robin method. The weights used by the  weighted round robin method are calculated using the response times from healthcheck  requests. Each healthcheck request is timed to see how long it takes to respond. Please note that is  assumed that the speed of the healthcheck depends on how slow the machine is which may not  always be the case. The total response times of all the real servers on the Virtual Service are added together, from  which the weight of the individual Real Server is calculated. The weights are recalculated approximately every fifteen seconds.

Source IP Hash

A hash of the source IP is generated and used to find the correct real server. This means that the  real server is always the same from the same host. With this scheduling method, you don't need any source IP persistence.  N.B. This MAY cause real server imbalance

So there you have it if you want to know more about Load Balancing or scheduling methods then feel free to get in touch.

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