Comment 493 for bug 620074

Søren: These parameters exist to tune the system behavior. There are other parameters which control the behavior of pdflush and FS journal threads but getting these all in harmony to make the system perform well in all scenarios is not an easy task. I think the hope is that pages will be reclaimed fast enough by pdflush if its parameters are tuned as well.

But I agree that by default letting one process to dirty 60% of physical RAM before it blocks itself on IO flush, is a bad thing. Particularly, when filling RAM is many orders of magnitude faster than emptying it to disk. A couple of rogue user processes can bring the system down in a hurry.

Linux needs to account for the disparity between RAM and disk, and how that disparity has increased many folds in recent times. 2GB system is considered minimum these days. Filling 60% of it will take few microseconds even on slowest of RAM, but emptying it to disk will take many seconds if not minutes on normal drives.