The Page File

How tired is this topic? I don't want to be "reheating boring leftovers," as Ned puts it, but maybe this post will help finally put it to bed for anyone still wondering.

Ever since I became an "NT geek" a little over 15 years ago, there has always seemingly been so much mystery and intrigue shrouding "the page file," also known as the "swap file."  Windows does not really do "swapping" anymore, so I will only refer to it as a page file from here on out. Still to this day, in 2011, people commonly ask me about what size their page file should be. "Does my page file need to be this big?  What happens if I shrink it?  Can I put it on another logical drive? Can I disable it?"  And naturally, the Web can be a cesspool of misinformation which only serves to further confuse people and cause them to rely on "rules of thumb" for which there's no true reasoning or sense behind them. To add to this mess, the exact ways in which Windows has used the page file have changed slightly over the years as Windows has evolved and the average amount of RAM in our machines has increased. (*cough* Superfetch *cough*)

I'm focusing on Windows Vista and later here. (Win7, Server 2008, R2, etc.)

Just follow the "Advanced"s to get there.

 

 

First I want to clear something up: Forget any "rules" you have ever heard about how the page file should be 1.5x or 2.0x or 3.14159x the amount of RAM you have.  Any such formula is basically useless and doesn't scale to modern systems with different amounts of RAM. Get your vestigial 20th century old wives tales out of my Windows.

Alright, sorry about that. I'm just really tired of hearing those rules of thumb about page file sizing. The only part about this that sort of embarrasses me is that Windows itself still uses a formula like this if you choose to let Windows manage your page file for you. Older versions of Windows use this formula to choose the page file size for you:

System Memory Minimum Page File Maximum Page File
< 1GB 1.5 * RAM 3 * RAM
> = 1GB 1 * RAM 3 * RAM

Windows 7 and 2008 R2 set the paging file size to the amount of RAM + 300MB.

Furthermore, the page file is dynamic by default and can expand on demand. And for the most part that still works just fine for the average home PC user.  But if you have a server with 192GB of RAM, Windows will by default create for you a nice, fat 192GB page file.  Our "rule of thumb" now looks absurd.

No, you do not need (or want) a 192GB page file gobbling up space on your nice 15k SAS drives.  The only reason you might ever want such a huge page file is only if you're interested in generating full crash dumps (full memory dumps) when the machine crashes. (You need a page file that is the size of RAM plus a couple hundred megabytes for this.) In later versions of Windows you can use a dedicated crash dump file to store memory dumps, further decreasing the need for a huge page file.  Also, you'll still get minidumps that can contain useful information about system crashes even if your page file is too small to support a full memory dump.

Other types of crash dumps are explained here.

The truth is the more RAM you have, the less page file you need - with a couple of stipulations.

The answer to "how big should the page file be?" is "just big enough so that you don't run out of commitable memory." The amount of memory that your system can commit is equal to your RAM + page file. You can put your system under a heavy workload and use Performance Monitor to see how much RAM you're using. You can also use Process Explorer by Mark Russinovich and watch the peak memory value. Just make sure that you have enough RAM + page file that you can support that peak memory usage at all times, or else your page file will be forced to expand if it can, and if your page file can't expand then your application might hang, or crash, or any number of systemic weirdnesses will occur.

Unfortunately, Superfetch complicates this test because it goes around in the background, pre-loading things into memory from disk all the time, so it'll always make it seem like you're running low on memory. In reality, a lot of that is Superfetch putting your memory to work, and it's actually really good at reducing disk seeks on PCs.  But Superfetch is turned off by default if Windows was installed on an SSD (because seek times are near 0 on an SSD anyway,) and it's also disabled in Windows Server. 

Also keep in mind that certain applications such as MSSQL and LSASS have their own memory management systems that can operate outside the purview of the NT Memory Management system, which can lead to things like those applications hogging up more than their fair share of memory.

 

Process Explorer by Mark Russinovich

On the other hand, don't disable the page file completely, even if you have 192GB of RAM.  (You should be able to move it off of the system drive with no ill-effects though, unless you're dealing with a very poorly written application.) Windows can run perfectly fine with no page file if you have plenty of RAM. However, some applications may not. Some third-party applications, and even some MSFT applications like AD DS and SQL may be simply assuming that there is a page file, and if there isn't one, it may result in unpredictable results. (Read: Hard to troubleshoot headaches.)

On the other other hand, keep in mind that having a huge pagefile (like 192GB) will affect system performance if your system is actually having to use that page file a lot.  Just having a large file on your disk won't affect performance by virtue of it just being there, but if your system is having to manipulate that file by pushing memory pages to it all the time it will. (And if you find yourself having to push memory pages to a huge file on disk very often, you obviously needed more RAM a long time ago.)

Lastly - yes, there are some "page file optimization" techniques that still apply, such as striping the page file across multiple spindles, setting a static size to eliminate file system fragmentation, etc. However, with RAM being as inexpensive as it is these days, your main concern should be minimizing having to touch the page file at all anyway.

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