What are the pdflush processes?

Answer:

When you use the top command, you might notice 2 or more pdflush processes are running actively and wondering what do they actually do.

In fact, when the Linux system want to write data to disk, e.g. write to a file, a special in-memory copies will hold the data blocks belonging to that files which is called the "page cache". For performance reasons, data is usually not always written out (sync) to disk and is temporarily stored in page cache (called "dirty pages").

The jobs of pdflush threads are used to write data out from dirty page caches to disk.

Reference: http://lwn.net/Articles/326552/