Reducing runtime memory in Windows 8

Posted by bink on October 8 2011, 2:29 AM. Posted in Windows 8.

Some highlights of Windows 8 memory usage improvements:

 

Making room in Windows 8

We made hundreds of specific changes to minimize OS memory use in Windows 8. I’m going to call out a few specific areas that resulted in substantial memory savings.

Memory combining

When assessing the contents of RAM in a typical running PC, many parts of memory have the same content. The redundant copies of data across system RAM present an opportunity to reduce the memory footprint even for services and OS components.

How can this happen? Applications will sometimes allocate memory for future use and will initialize it all to the same value. The application may never actually use the memory as it may be there in anticipation of functionality that is the user never invokes. If multiple running applications are doing this at the same time, redundant copies of memory are in the system.

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).

Service changes and reductions

OS services configured to run all the time are a significant source of ambient memory use. When assessing the set of OS services during Windows 8 planning, we decided to remove a number of them (13), move a different set of services to “manual” start, and also made some of the “always running” services move to a “start on demand” model. This is where a “trigger” in the OS (like device arrival or the availability of a network address) causes the following to occur:

  1. The service starts.
  2. The service does its thing (whatever that happens to be).
  3. It hangs around for a while to make sure there isn’t anything else to do, and
  4. The service goes away.

You’ll notice that Plug and Play, Windows Update, and the the user mode driver framework service are all trigger-started in Windows 8, in contrast to Windows 7, where these services were always running.

Of course we have added a ton of new functionality (and new code) to Windows 8. Some of this new functionality is packaged in the form of new services. Of these new services, two are auto-started; all others are manual or trigger-started.

 

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