Virtual Memory and Disk cache on an ASIP Server

By: Richard Glaser - Revised: 2006-06-07 devin

Turning off VM and reducing the Disk caches helps in many ways:
  1. Open Transport is SLOW when VM is on. Guesstimate around 10% slower. Just terrible.
  2. The Web/File/FTP server (not sure of print/mail) does not use the disk cache, so setting it lower just frees more memory for the server to use in its own internal file caching. A file already in the cache can be read or written at pretty much the speed limit of your network (assuming you donÕt have really slow CPU). A file that is not in the cache transfers at a much slower speed which is very dependent on the speed of your hard disks, but is almost always slower than reading it straight from our internal RAM cache. Adding a ton more RAM to your file server is always going to be helpful since it means more files will fit in the server caches which increases your chance that the file you want is already in the cache and ready to go.
  3. Of course with VM off, you never have to worry about getting page faults and having to wait while some code is being swapped in for you.
  4. And of course, with lots more RAM, you can worry less about running out of RAM.
  5. Open Transport will also pick up some speed when you have more RAM. OT will continue to allocate more internal buffers up to a certain % of total RAM in the CPU. With more buffers, OT will run faster.
This is all assuming that you have fast enough hardware (both CPU and network) to make it worthwhile. For example, if your server is dealing with LocalTalk, then no matter how fast your server is or how much RAM it has, the performance bottleneck will always be LocalTalk.