In article <Pine.NEB.4.33.0109290945400.336-100000@wojtek.3miasto.net>,
Wojciech Puchar <wojtek@wojtek.3miasto.net> wrote:
>i've changed values in that struct (in uvm_fault.c) to larger values like
>this. looks like some programs loads faster but difference is smaller
>than i expected.


>but what exactly means nback value??

>static struct uvm_advice uvmadvice[] = {
>	{ MADV_NORMAL, 32, 31 },
>	{ MADV_RANDOM, 0, 0 },
>	{ MADV_SEQUENTIAL, 32, 31},
>};

>#define UVM_MAXRANGE 64 /* must be max() of nback+nforw+1 */



when you page-fault on a page, uvm can look for resident pages adjoining
the one being faulted on and map them in at the same time, maybe saving
you a couple of page faults.

the numbers you quote control how far from the faulting page uvm_fault
is willing to look.    nback is the number of pages previous to the
faulting page to look.

e.g.  if you fault ("x") on page #2 and nforw = 4 and nback = 2 then
      it will look for 2 "back" pages (0, 1) and 4 "forward"  pages
      (3 to 6).


  b  b  x  f  f  f  f
  0  1  2  3  4  5  6


for sequential access, it doesn't make sense to search for nback pages.


chuck