Here's a cache behavior I've touched on in passing several times butI feel like covering explicitly now. It's what I've taken to call thecache eviction death spiral, where cache evictions start you down a pathto grinding death.

Cache evictions of useful data have an obvious problem; they slow downthe program when you have to pull the needed data back into cache. Butin a contended cache under pressure from multiple sources, they have asecond effect as well; because your program

One of the important general difference between read IO and write IO isthat read IO is generally synchronous (in its normal form), and not justbecause the operating system interfaces are usually synchronous. Read IOis synchronous in general because the program is almost always going toimmediately look at and do something with the data that it's read, andit can't go on until it's done this. It's a relatively rare program thatdoesn't look at the data but instead either discards