To say it simply, the prefetcher is in charge of loading data into memory before it is even required by the program. Sounds like magic ? Well, not really : whenever you look at a film for example, it's pretty obvious that the movie file will be loaded sequentially. So after the program has requested data from position 1M to position 2M, it's quite logical to expect the next read operation to be from 2M to 3M.
By prefetching HDD data into memory, the OS makes it (almost) instantly available to the program, smoothing video playback, or whatever the program wants to do with it.
Obviously, simplest prefetching would be to load the entire file into memory. In this case, we call it a "cache". But this is not really practical. A film is several GB large for example. So a simpler version would be to simply load the next KB of data, just in case they would be required by the program later.
In practice, this works relatively well. There are nonetheless some subtle differences between systems, one of which i just learned this week, studying XP behavior.
I was puzzled by the fact that newest v0.4 version of Zhuff was reported sometimes slower on some systems running XP, while the underlying compression algorithm was proven to be much faster. The only other identified change was a move from 512KB chunks to 1MB chunks.
This proved enough to put too much load on XP prefetcher. While such a change produced no effect on my Windows Seven test system. Hence my surprised.
I worked my way through a new I/O code, which would chunk code into smaller parts, down to 256KB. Below that point, there is a small compression penalty. Nothing too large though...
It required a bit more than words, decoupling buffer from read/write operations. Anyway, the results were better than expected. To give an example, using a RAM disk drive under Windows XP, one of my test file went down from 5.20s (v0.4) to 4.50s (v0.5). Quite a large improvement for just an I/O change.By contrast, Windows Seven did not resulted into any change : timing remained steady at 4.75s, whatever the chunk size.
Finally, you can get your hand on the result, a new version of Zhuff, that you can download here.
Improvements will be mostly visible using a very fast hard disk, such as an SSD, or a RAM drive.