Memory Page Combining

My reading of the Windows Server 2012 R2 (WS2012 R2) Performance and Tuning Guide continues and I’ve just read about a feature that I didn’t know about. Memory combining is a feature that was added in Windows 8 and Window Server 2012 (WS2012) to reduce memory consumption. There isn’t too much text on it, but I think memory combining stores a single instance of pages if:

  • The memory is pageable
  • The memory is private

Enabling page combining may reduce memory usage on servers which have a lot of private, pageable pages with identical contents. For example, servers running multiple instances of the same memory-intensive app, or a single app that works with highly repetitive data, might be good candidates to try page combining.

Bill Karagounis talked briefly about memory combining in the old Sinofsky Building Windows 8 blog (where it was easy to be lost in the frequent 10,000 word posts):

Memory combining is a technique in which Windows efficiently assesses the content of system RAM during normal activity and locates duplicate content across all system memory. Windows will then free up duplicates and keep a single copy. If the application tries to write to the memory in future, Windows will give it a private copy. All of this happens under the covers in the memory manager, with no impact on applications. This approach can liberate 10s to 100s of MBs of memory (depending on how many applications are running concurrently).

The feature therefore does not improve things for every server:

Here are some examples of server roles where page combining is unlikely to give much benefit:

  • File servers (most of the memory is consumed by file pages which are not private and therefore not combinable)
  • Microsoft SQL Servers that are configured to use AWE or large pages (most of the memory is private but non-pageable)

You can enable (memory) page combining using Enable-MMAgent and query the status using Get-MMAgent.

You’ll find that memory combining is enabled by default on Windows 8 and Windows 8.1.  That makes these OSs even more efficient for VDI workloads. It is disabled by default on servers – analyse your services to see if it will be appropriate.

There is a processor penalty for using memory combining. The feature is also not suitable for all workloads (see above).  So be careful with it.

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