Configuring Eviction Policy

There is a storage limit, maximum size is the capacity to which a cache can add objects in it. If cache reaches its maximum size and eviction is turned on, the specified ratio of items will be evicted to make space for new objects. On the other hand, if cache reaches its maximum storage limit and eviction is turned off, items are neither evicted nor added to the cache any further.

An eviction scheme is used to select the objects that need to be removed from cache. A particular eviction policy can be selected at the creation time but once it is set it cannot be altered in the running cache. User can change eviction policy after creating a cache only if it is not running. Three types of Eviction Policies are provided by NCache.

Least Frequently Used
Least Recently Used
Priority Based Eviction

Least Frequently Used

In this scheme a counter that counts the access made to an object is used. At the time of eviction objects with lowest counter number are evicted first. Here too a random selection is made between two objects with same counter.

Least Recently Used

This eviction scheme associates a timestamp with all objects. This timestamp is updated each time the object is accessed. When evicting, objects with the oldest timestamp are picked first. In case of two objects with the same time stamp, a random selection is made.

Priority Based Eviction

This is the default eviction policy and has the same mechanism as LFU with the addition of a relative cost that is associated with an object at the time of its addition to cache. Objects with lower cost are removed before objects with higher cost. Client application can choose from a list of priorities that is predefined (Low, Below Normal, Normal, Above Normal, High and Not Removable). In case a priority is not chosen, it is set by default as Normal.

These eviction policies for a cache can be specified in one of the following two ways.

Using NCache Manager

Click on the cache name in Cache Explorer to open cache settings.
Go to the Policies tab.
Uncheck the Turn off Eviction check box.

Select the Eviction Policy from drop down list.

If you have selected the Priority based eviction, then Default Priority can also be changed in the drop down list.

Change the Eviction Percentage if required. Eviction percentage is the percentage of items to be removed on each eviction.

Right click the cache in Cache Explorer and select Apply Configuration option.

Through config.ncconf:

You can select the Eviction Policy for a cache through Config.ncconf as follows.

<cache-config name="mypartitionedcache" inproc="False" config-id="1" last-modified="6/20/2010 3:50:34 PM">

<eviction-policy enabled="True" default-priority="normal" policy="priority" eviction-ratio="0.05"/>

</cache-config>

Eviction-ratio=0.05 is equal to 5% of total cache size. Eviction is not recommended in case of sessions.

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