On Tue, 2 Feb 2010, Olivier Galibert wrote:

On Tue, Feb 02, 2010 at 10:40:41AM -0800, Linus Torvalds wrote:

quoted

IOW, if you start off with a SEEK_END, I think it's reasonable to expect 
it to _not_ read the whole thing.

I've seen a lot of:
  int fd = open(...);
  size = lseek(fd, 0, SEEK_END);
  lseek(fd, 0, SEEK_SET);

  data = malloc(size);
  read(fd, data, size);
  close(fd);

Why not fstat?  I don't know.

Well, the above will work perfectly with or without the patch, since it 
does the read of the full size. There is no read-ahead hint necessary for 
that kind of single read behavior.

Rememebr: read-ahead is about filling the empty IO spaces _between_ reads, 
and turning many smaller reads into one bigger one. If you only have a 
single big read, read-ahead cannot help.

Also, keep in mind that read-ahead is not always a win. It can be a huge 
loss too. Which is why we have _heuristics_. They fundamentally cannot 
catch every case, but what they aim for is to do a good job on average.

			Linus

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