Using video card memory as system memory

[hard candy]

I had a 2GB DDR kit as system memory, one of the modules just became bad. I am returning it to Patriot Memory but since it was sold as a 2 GB kit, I have to return both modules. So I figure for about 10 days I will be using a 512 MB module that I have as a spare. Not much headroom since I have the fancy graphics running and do some video processing from time to time.
So I was thinking, if the video card is not using all it's memory, would I be able to use part of that memory as system memory? Would it be faster than a swap file. It is a Nvidia 7900GT using the PCIe slot, it has 256 MB RAM. Asus motherboard with a Nforce4 chipset.
Found one article:
Kernel Trap
Has anyone heard of doing this? I am thinking that maybe the video RAM could be used as faster swap resource.
Really just curious, I'll probably end up just waiting, since it is only 10 days but, like the cat, the curiousity is strong.

[bwkaz]

It may be possible, but I wouldn't count on it necessarily being faster than the hard drive.

Actually, thinking about it a bit more, it likely is, but there's at least one performance-related issue with it. There are several different blits that graphics cards can do. The first, and fastest, is vid->vid (video memory to video memory). This is what the card's designed for, but it wouldn't be used much if the card was just used for storage. The second, and middle-speed, is sys->vid (system memory to video memory). This needs to be relatively fast so that the initial loading of texture data, etc. into the video card doesn't take forever, but it doesn't have to be blazing fast unless you don't have enough video memory. It would be used if the card was used for storage (if it was a swap device, for instance, then you would only be writing to it when swapping stuff out), but not *that* often.

The last, and slowest, is vid->sys (video memory to system memory). This almost never happens under the "normal" usage of a video card -- knowing what texture data is loaded is very rare. (Of course it does happen when someone's reading the framebuffer, though.) This would be used every time something swaps back in; at least as often as the swap-out frequency.

With a disk, the swap-out and swap-in times are about equal. With a video card, they'd be very much different. I suspect it could be made to work, I'm just not sure how well it'd act.

If you do get it working, let us know.

[rocketpcguy]

It may be possible, but I wouldn't count on it necessarily being faster than the hard drive.

what??!? a harddrive is hardly 60MB/s, while PCI express can handle 4GB per second, and add that to hard_candy's 256MB DDR3, and you have one fast storage area. of course it cant use up all of 4GB/s, and it'd have different swap in/out times, but itd be whooping faster than any disk based hard drive!

[bwkaz]

But just because the PCIe or AGP bus can go that fast does not mean that vid->sys blits can. (Or sys->vid, for that matter...)

They cannot. They are MUCH, MUCH slower than vid->vid (which does not even require PCIe to be in the picture) or sys->vid (which does).

Now, this does not mean that video card swapping would be slower than a disk (I don't know, and in fact I believe I said something about that later in the post) -- just that the performance difference between writing to the card and reading from the card would make it ... strange. (Yes, I probably should have edited out the first few sentences in that post.)

[dr_te_z]

An old link, maybe still userfull: http://hedera.linuxnews.pl/_news/200...long/1445.html