How do I tune the MOSS Object Cache for performance and economy?

Tuning the size of the MOSS Object Cache is done via the Max Cache Size settings on the Site Settings > Object Cache Settings page. It is important to recognize that this maximum cache size setting is a limit and not a static value. For example; Just because the maximum cache size setting for the Object Cache is configured to 100MB does not mean that it will consume 100MB of memory at startup. It simply means that if the cache were to exceed 100MB it will be compacted to reduce its memory consumption to a level below that maximum value.

For purposes of this discussion we will categorize cache compaction rates into three categories, unacceptable, acceptable and optimal. A cache compaction rate of more than 6 per hour should be considered unacceptable. A rate of between 2 and 6 compactions per hour is acceptable and between 0 and 1 cache compactions per hour is optimal. You should only target the optimal level of cache compactions if you have sufficient amount physical memory installed on your servers to achieve this goal and still have sufficient free physical memory remaining on the server to support other critical system operations.

You should regularly monitor for cache flushes because they are extremely expensive in terms of cache performance impact. A cache flush results in the ejection of all cache contents. Cache flushes are triggered by the creation, deletion or moving of a web. They can be monitored via the "SharePoint Publishing Cache/Total number of cache flushes" counter in Performance Monitor. If you are seeing cache flushes throughout the production day you should reconsider how you manage webs during those hours.

Based on these considerations we have developed the following guidance for how to tune the Object Cache size for acceptable performance using the "SharePoint Publishing Cache / Total number of cache compactions" performance counter.

We feel this level of cache performance will meet an economical customers performance needs without unduly sacrificing their limited memory resources. The steps to achieve this are fairly straightforward and must be applied in an iterative fashion until the desired level of performance is achieved. The recommended steps are:

1. Start with the default cache settings of 100MB on the site collection.

2. Capture at least 8 hours worth of performance data from the WFEs while the system is under a typical load using the "SharePoint Publishing Cache/Total number of cache compactions" counter. Since we are interested in tracking compactions per hour it is acceptable to capture this data at 1 minute intervals.

3. After analysis of the data, if we are exceeding the threshold for acceptable cache compactions we will need to add an additional 50MB to the Maximum Cache Size value and run the test again.

4. We should continue this process until such time as we have achieved an acceptable cache compaction rate.