Archive for July, 2014

Tabular and NUMA

July 30, 2014

Recently a great white paper Entitled “An Analysis Services Case Study: Using Tabular Models in a Large-scale Commercial Solution” was released which provides a lot of information that would be helpful when on is faced with evaluating the various Microsoft BI tools (SSAS Tabular, SSAS Multi-dimensional, and ColumnStore Indexes).

I will not reproduce the whole white paper on my BLOG but below are some quotes from the white paper that I found particularly interesting:

ColumnStore Indexes:

“Unpredictable Query speed. In SQL 2012, specific query plans can really take advantage of columnstores (which are an absolute need for the volume of data of inContact). However, as soon as the query became complex (for example, mixing SUM with DISTINCT COUNT in the same query) SQL Server stopped using the columnstore and reverted to row mode, leading to poor performance. It looked nearly impossible to produce a general-purpose query engine that gave enough freedom to users in expressing their query while retaining efficiency.”

While I too had problems with SQL Server 2012 Non-Clustered ColumnStore Indexes and the query optimizer picking the much slower “row mode” instead of “batch mode”, after using SQL Server 2014 Clustered ColumnStore Indexes for almost a year, I have not seen any major issues related to “batch mode”. As a matter of fact, I have even observed about a 15% – 30% performance improvement of Clustered ColumnStore Indexes over the non-clustered variety.

The section that by far was the most surprising to was the section entitled “The complexity and ubiquity of NUMA ” as it reminded me of something that I ran across a few years ago when I was testing the then brand new SSAS Tabular with NUMA servers.

AS the article clearly states:

“Most of the servers available on the market with enough expansion to handle terabytes of RAM are based on the Non Uniform Memory Access (NUMA) architecture. This is a big issue because Tabular is not NUMA aware.”

In other words any enterprise hardware will make use of NUMA nodes and so putting a none NUMA app on this kind of hardware would be the wrong approach. What then follows in the article is a good explanation of what NUMA is and why it is important. The final two paragraphs really hit home for me why SSAS Tabular, without a significant work around, is not enterprise worthy:

“As you can see, programmers need to optimize the software to run efficiently on NUMA nodes. You can’t simply use any software on NUMA and expect good performance. The problems of memory access speeds are compounded when the software relies heavily on RAM.

Unfortunately, not only is Tabular not NUMA aware, it also uses RAM as primary storage. Thus, the marriage between Tabular and NUMA is a not an easy one.”

While this is a bit of an understatement, it is now easy to find out if your server has multiple NUMA nodes by simply opening up Task Manager to the CPU tab. If your server has multiple NUMA nodes then you will see CPU usage for each node. See example below of an 8 NUMA node server:

numa nodes

 

The solution to the Tabular NUMA node problem is (assuming that it is possible) to break the data up into separate distributions according to the number of NUMA nodes on the server and then create an equal number of VMs, each with an instance of SSAS Tabular. See below:

“· Use Hyper-V VMs to set node affinity. Tabular might not be NUMA aware, but it runs great if you install a tabular instance on VM that is affinitized to a particular node. This approach has an added benefit; you don’t need to adjust memory configuration settings.

· Distinct count performs better in a Tabular implementation. This is a well-known fact about Tabular, but if you are new to the technology, it’s worth remembering. This is especially true if the alternative you are evaluating is SSAS Multidimensional.”

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