Swap size limitations / default rules

Question

I've been playing with software raid to speed up my hd performance.
I currently have 4 x 160 gig hd's (2 x sata & 2 x pata).
I got my 4'th hd today, so I did a clean install (got too much spare time) and for some reason decided to set my swap to 1gig on each drive witch should be 4gig total (just to be sure).
However, after getting my system up and running, checking with gnome-system-monitor & top, I see only 2gig of swap space is in use.
I did a quick search here on the forum & google, and it seems that by default swap is 2 x ram, I got 1gig ram so it add's up.
I checked :
[root@Cray loke]# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid5] [multipath] [raid6] [raid10]
md1 : active raid0 sdb2[0] sda2[1] hdb2[2] hda2[3]
      3919360 blocks 64k chunks
md2 : active raid0 sdb3[0] sda3[1] hdb3[2] hda3[3]
      15229184 blocks 64k chunks
md3 : active raid0 sdb4[0] sda4[1] hdb4[2] hda4[3]
      605577792 blocks 64k chunks
md0 : active raid1 sdb1[0] sda1[1] hdb1[2] hda1[3]
      96256 blocks [4/4] [UUUU]
unused devices: <none>
And did a speed test comparing /dev/md/1 (swap)  & /dev/md2 (/)
[root@Cray loke]# hdparm -t /dev/md/1
/dev/md/1:
Timing buffered disk reads:  366 MB in  3.01 seconds = 121.61 MB/sec
[root@Cray loke]# hdparm -t /dev/md/2
/dev/md/2:
Timing buffered disk reads:  348 MB in  3.02 seconds = 115.40 MB/sec
Looking at the speed, the swap file is using 4 drives but only 2gig of the 4 available, I doubt I ever going to  use all the swap space, but since its already there and I'm not gonna add more hd's anytime soon, why not have it all available .
So my question is, where do I alter / force the swap size to go byond 2gig or 2 x ram?
Thanks.

Answer 1

kth5 wrote:i think that in this case the oh so great 32bit addressspace limitation comes into effect.
after this the total can only be 4gig of memory which is presented by swap and physical memory together. so if you have 2gigs physical memory in your machine that's it. no way to have another 4gig swap. anyway, as you only have one it might be the md setup or swapfs limit after all, then i have no idea.
you may want to look into highmem support etc. in the kernel, but i actually doubt that it'll help much.
I too would say that this could be the result of a 32bit cpu and addressing range. I have done some research on a similar subject, how to get 4GB of physical RAM working under various windows OS-s, and found out that most motherboards don't even have the address space for reaching those 4 GB. In other words, you may have 4x1GB sticks in your mobo and it can even "officialy" support 4GB ram, but you would get 3,3 or 3,5  if you're lucky, because the rest of the address space is used for addressing various I/O devices, like usb, firewire, ata/sata, pci bus, and so on. Things get more complicated from there on, since  better motherboards can have the last GB moved up into "5th" GB address space so you don't "loose" it.
And the reason for me writing this is to also suggest enabling highmem support in the kernel.