Sitecore Data Cache Tuning Crash Course

I’ve been out of the blogging space for a while, due to a number of reasons that aren’t too relevant here . . . suffice it to say that I’ve never left the Sitecore space, just got invested in some very demanding projects.  I’m feeling the itch to write-up some more of my notes about Sitecore, and the fact is it helps me to process, retain, and better understand the subject when I shape it into a few paragraphs on this blog.

In my time at Sitecore, every couple weeks I would encounter an implementation intent on running with their cache setting Caching.DisableCacheSizeLimits configured to true.  It is a fact that the Sitecore documentation suggests this is a good idea (they say so in the Sitecore CMS Tuning Guide) for 64-bit environments with plenty of memory.  Unfortunately, this nugget of guidance can bring about a lot of problems as the “one size fits all” cache tuning logic within Sitecore leaves something to be desired.  I ran into such a situation this week, which prompted me to write this post.

I’ve seen higher CPU usage, publishing errors,  and out of memory exceptions caused by setting Caching.DisableCacheSizeLimits to true.  The cache sizes could regularly exceed the memory of the environment (out-of-memory exceptions!), or a high memory machine allowed so much memory to be set aside for the cache, that it would take a really long time to evict it . . . there is no substitute for a well-tuned Sitecore cache, tailored to the needs of each organization.  This is a case where the quick fix (letting Sitecore manage your cache automatically courtesy of the Caching.DisableCacheSizeLimits setting) can be vastly inferior to a well thought out solution.  The tortoise can beat the hare, in this case.

So, I almost always recommend setting Caching.DisableCacheSizeLimits to false and tuning the Sitecore data caches according to the CMS Tuning guide (https://sdn.sitecore.net/Reference/Sitecore%207/CMS%20Performance%20Tuning%20Guide.aspx).  Those instructions aren’t as detailed as I would like, so let me share some additional notes on how to do this:

    1. Work on a back-up of your real Sitecore databases and use a hardware profile that matches your production environment. . . running against a fraction of the real content will give you incorrect results, as will tuning a cache on a 4 GB machine when your prod servers are 32 GB!
    2. Be sure to disable the configuration setting Query.MaxItems so you’re not artificially limiting how many items the Sitecore API returns
    3. If it’s inconvenient to run a load generator, as the CMS Tuning guide suggests you should do as part of the testing, you can just run the “give me the kitchen sink” XPath query in the old XPath Builder tool.  It’s located at http://%5BServerName%5D/sitecore/shell/default.aspx?xmlcontrol=IDE.XPath.Builder –this used to be a core part of developer life with Sitecore, but now this XPath builder is a vestigial tail that doesn’t get too much use.  It’s very handy for this cache flooding exercise!  Run an XPath query like /sitecore/content//* that should flood your Sitecore caches with the entire content tree.
    4. This video is old, but technically an excellent resource for Sitecore cache tuning: http://mediacontent.sitecore.net/wmv/CacheRecordingWebinar.mp4.
      1. Among other things, the video covers what to prefetch cache, and a methodology for determining settings such as Caching.AverageItemSize using System.GC.GetTotalMemory() method calls.  It’s all about how to calculate the right values for your implementation — and I can’t recommend this video more highly.

With all that said, there is a situation when setting Caching.DisableCacheSizeLimits to true makes sense.  The most common rationale I can think of is when one encounters a production Sitecore system with no changes to Sitecore out-of-the-box in terms of data caching.  I’ve seen this situation due to an implementation team that didn’t know how important this can be for perf, for example.  In one case, I was in this situation in mid November just a few days before Thanksgiving — and the customer needed a quick caching solution that didn’t require careful analysis etc.  In this case, as a temporary measure to improve the data cash landscape for the client, I suggested setting Caching.DisableCacheSizeLimits to true until we could assist them with  a thorough cache tuning effort.  This got their system beyond the very limited data caching  configuration Sitecore comes with out of the box (probably dating back to the Win32 days!).  This wasn’t in place long term, but it was sufficient to improve their performance ahead of a busy online shopping period.

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