We have a webapp (ASP.NET MVC 4) running on IIS 7.0. Recently, it's started crashing, logging
OutOfMemoryException as the cause. We also get a load of "unhealthy worker process" notifications, as well as "failed to initialise" and "failed to create AppDomain". In testing, we've noticed that IIS spawns between three and six worker processes for every request, even simple GET requests for relatively lightweight pages. This obviously results in a jump in memory usage - often between 100 and 200MB. Unsurprisingly, it's not long before it runs out of memory, slowing to a halt and eventually crashing.
This server has been running this app for some time, and while there have been some changes to it lately, they're mostly small bugfixes or cosmetic tweaks, and this issue persisted when I replaced it with an older backup.
What could be causing this, and how do I resolve it?