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Is Video RAM a Good Swap Device?
sean4u writes "I use a 'lucky' (inexplicably still working) headless desktop PC to serve pages for a low-volume e-commerce site. I came across a gentoo-wiki.com page and this linuxnews.pl page that suggested the interesting possibility of using the Video RAM of the built-in video adapter as a swap device or RAM disk. The instructions worked a treat, but I'm curious as to how good a substitute this can be for swap space on disk. In my (amateurish) test, hdparm -t tells me the Video RAM block device is 3 times slower than the aging disk I currently use. If you've used this technique, what performance do you get? Is the poor performance report from hdparm a feature of the hardware, or the Memory Technology Device driver? What do you use to measure swap performance?"
This doesn't sound like the most stable thing to do especially if your running a server on the same computer. It sounds good on paper but implementing it is a whole different game. From my years in IT never try anything like this on production servers, thats what test servers are for.
Are you sure the system has video RAM? Doesn't built-in video generally share the system RAM?
Video RAM is designed for performance, not for stability. If a bit flips in your video RAM, a pixel is going to be bad or a texture will be slightly different. You're not going to notice.
A bit flip in your swap space (or main RAM), now that is something you really don't want to happen....
Heck, I remember RAM expansion cards for ISA slots. I'm sure this is faster, though I didn't get any meaningful boost when I tried this once. Nevertheless, if you're running headless system, it's better IMHO if you get some use of the display hardware, rather than no use. Even if it's a little slow. You shouldn't rely on swap as a memory expansion anyway, it's just a way to gracefully degrade performance when you hit the limit.
I think it's also nice to have swap on a different physical device/bus from your main hard drive. Maybe the swap isn't any faster, but at least it isn't slowing down any other hard drive usage.
Escher was the first MC and Giger invented the HR department.
Problem is, most people who think the whole "swapping is bad" thing are windows users, an OS that still has a tendency to swap out the most interesting and useful things.
:)
Swap is great for a server or workstation, once set on a single task it needs never do anything else till shut down, but for a windows PC that could at any time have anything run on it (not to mention a sub-standard disk cache system) having parts swapped out to make room for a disk cache that doesn't do a whole awful lot is less than optimal.
This is of course the point where you point out that converting all your junk to windows vista and training all your staff to use the new office 2007 "ribbon" is about the same cost as training them to use linux and OOo, the latter being a lot cheaper too
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Neither of the technologies he listed were PCI. VESA came out in the late 80s/early 90s, as did EISA. to the best of my knowledge EISA was never used on video cards unless it was highly specialized. they went from the VESA local bus, to PCI, to AGP and its various speeds, to PCI-E x16.
I think one of the points of confusion here seems to be that most people don't realize that while something is built into a motherboard it doesn't have some magical interface that makes the bits fly differently than if it was in a slot. I think that is what is attempted to be said by the multiple posts this comment has generated
As a rock-in-roll Physicist once said, No matter where you go, there you are.
Which is, of course, a completely useless and disingenuous answer to a person who already has the graphics memory sitting around, and wants to know if it is better than a hard drive.
You seem to be advocating wasting perfectly good VRAM in favor of buying more system RAM. If the VRAM is essentially free (ie. comes with the system no matter what), there is no good reason not to try to put it to good use.
Also, your "No" is completely unqualified. You offer no details of how VRAM performs worse as swap space than hard drives, let alone actual benchmarks or citations. (And I have the feeling that most graphics memory would be significantly better than your average IDE hard drive for swapping.)
Mod parent overrated.
I hope so, because that's where I like to keep my swap partition. Actually, that's not necessarily optimal. If you were reading a file on the inner edge and get a page fault, the disk will have to do a full seek all the way to the other side to be able to get the page. You're better off putting the page file halfway between the inner and outer edges to lower the average length of your seeks for page faults. Of course, that depends on how much thrashing you're experiencing and how much file access is mixed in with that, so YMMV
IIRC, NTFS has some of its main data structures in the middle of the partition for that reason.