Admin Tip #178: How big should Windows NT pagefile be?

Hits: Failed to execute CGI : Win32 Error Code = 3

How big should you set Windows NT pagefile? Microsoft's forumla definition is size of RAM + 12MB or 2.5 X size of SAM file, whichever is larger. If you have 128MBs of RAM, that works out to 140MB. The traditional unix advice for pagefile is 2xRAM which would work out to 256MB. The maximum legal page size for Windows NT and Windows 2000 is 4095MB.

The only downside to a too large page file size is that you are consuming some of your disk space unneccessarily. Given the size of modern disks, this seems to be a trivial issue. Under Windows NT, the page file occupies a contiguous block of space in the partition. If possible split the page file between hard disks. Don't split the page file across partitions created on the same hard drive since that will decrease performance.

Anyone who has used a defrag tool like Diskeeper with a graphic disk map, sees the huge contiguous chunk of the page file in NT. Windows 2000 does not have the contiguous block requirement.

The downsides of a too small pagesize are much more serious. At a minimum, your system will experience disk thrashing where your pagefile files to maximum and NT is constantly swapping out segments of RAM to disk virtual memory. This can really impact performance. It can also result in a corrupted pagefile. Additionally, the pagesize should be at least the size of RAM+1MB in order to support core dumps. In any case, you have swapped out the fasted component in your server for the slowest. Given these factors, I recommend the unix formula.

One of Windows 2000 improvements is more control of core dumps. Windows 2000 has three sets:

Windows NT has the Complete memory dump or none. If you have a lot of RAM, I would pick the Kernel memory dump option. To set these options in Windows 2000:

For more information on dump files (.dmp files) see:

Gathering Blue Screen Information After Memory Dump nt4/w2k
Windows 2000 Memory Dump Options Overview
Use Dumpchk.exe to Check a Memory Dump File nt4/w2k
Reading Small Memory Dump Files Created by Windows 2000

But, note that but (:->, although I have never seen the unix formula to be too small, the actual pagesize might need to be bigger under some unique set of factors. The only way to be absolutely sure, one must use diskperf and monitor the pagefile usage peak object. For information to help with this task, read kb article :

How to Monitor Disk Performance with Performance Monitor Q102020

Configuring Page Files for Optimization and Recovery q197379

System Internals has released a freeware tool, PageDefrag , which will generate a fragmentation report and will defragment Windows NT 4's pagefile and registry. Windows NT is very bad about fragmentation.


For additional book recommendations on tuning, see NT Performance Tuning .