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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?"
Is your adapter an AGP or PCI-Express card? Because PCI-Express has fast lanes both ways, and AGP is not so fast in writing back. That could explain a part of the performance problems.
I'm assuming your ancient system uses an AGP interface for graphics, which has a very fast download rate, but very poor upload. The maximum performance of AGP uploading data from the card memory to the rest of the machine is pretty slow (less than 100MB/sec, IIRC), and it will vary depending on the implementation. This is probably the reason you got such slow benches.
PCIe will likely give you performance more in-line with main memory (most implementations now are hitting 1-2 GB/s).
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But there is a fundamental problem: vidRAM is optimized for writes from main RAM. Not reads. In many cases, reading vidram is extremely slow because the raster generator is busy reading it. Writes are buffered. Reads cannot be.
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.
Err... Which hdparm timings are you looking at?
One of the biggest advantages of using VRAM for disks is the nearly 0 seek latency.
As a result even if the card is slower than disk on read you are still likely to have an overall performance gain.
In addition to that there is a number of architectural vagaries to consider. AGP is asymmetric. Reading is considerably slower than writing (can't find anywhere by how much. Damn...).
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Are you sure the system has video RAM? Doesn't built-in video generally share the system RAM?
I know headless means that the system doesn't have a screen but I still get this idea of a box strapped to a horse, chasing down Ichabod Crane.
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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....
Not quite so.
If it is really old it may be running one one of the early Intel Pentium Triton chipsets. The TX will not cache any memory above 64 and the HX needs to be reconfigured to cache above 64. Even after reconfiguration it will just about work for 512MB. There are other similar vagaries related to most old hardware. Ali depending on release and version tanks at 384 or 768 and so on. Even chipsets as recent as Intel 815e while capable of 2G were deliberately bastardised to support only 512MB in order not to undercut the inexistent market for high-end Rambus/i810 based workstations.
So there are quite a few cases where it is more cost effective to use an old and long past its hayday high end video card as a swap device. All the way up to around 2001-2002. From there onwards nearly everything supported sane memory sizes so it is pointless.
Baker's Law: Misery no longer loves company. Nowadays it insists on it
http://www.sigsegv.cx/
Now if you want truly blazing speed, you can track down some of that dual-ported static RAM that came in 40-pin DIPs. Full random access on both ports would let you serve dynamic web pages while you run customer transactions, all with zero wait states on the ISA bus!
Bzzt - wrong.
Even high-end systems use swap space, because it allows for swapping out parts of memory that isn't called, freeing up that memory for things like disk cache, which does have a positive effect.
Doing "free" on a system here, I see that there's 886492 kB of free memory, of which 879896 kB is used for disk cache. 72892 kB is swapped to disk, and if there were no swap, the disk cache would have been that much smaller. Even if I had umpteen gigabytes of RAM free, that still would be 70 MB of extra cache by using a swap partition. That's a Good Thing.
What's a Bad Thing is when swap is used because you run low on memory -- then you get trashing and a seriously slow system. But on a healthy system with enough free memory, where the kernel can swap out pages not because it has to, but because it makes sense, using swap is a Good Thing.
This sort of brings up another issue he might be happening. A lot of on board video cards use system memory to function properly. If the swap space is actually in system memory, the extra transfer overhear of going up the bus to the controller that then send it back to the memory that is actually sending the stuff to the video ram.. Well, you see what I mean. The extra few steps could be enough latency per read write operation to slow the thing down compared to a direct access method that would be present with an IDE connection as well as video memory built on to the video car itself.
I think the differences might be as noticeable as turning DMA (direct memory access) on and off. And yes, you can see a big bit of difference. It was actually worth me buying new drives just to have DMA access when it first started becoming available. I remember earlier versions of windows 98 and (95 I think), that wouldn't turn it on by default. After making sure the drives supported it and enabling it, people would almost think they had a new computer. There was that much of a difference in performance.
Uh... no. I've heard that argument before, but I don't buy it. An ideal system is one that doesn't page anything out to disk. In fact, I make it a point to always have enough memory that my pageout count does not increase during normal use. As soon as you page out anything, you're taking a performance hit. Period. Paging out data to disk in order to make room for disk cache is almost never a good idea, as the changes of needing to later access a data page page in an application are typically far greater than the chances of reusing a randomly read block on disk.
A disk cache (a limited amount of readahead notwithstanding) is only useful for data that is used more than once, which makes it a highly transient data store. Storing most data in cache longer than a few minutes usually doesn't buy you anything in terms of performance because if data isn't reused fairly soon after initial use, odds are it won't ever be.
By contrast, data explicitly loaded into RAM by an application (assuming the app is reasonably well written) is in memory for a reason, and if the data were transient, the app would have repeatedly reused a single chunk of temporary storage instead of keeping the data around. The odds that any data won't ever be used again should be vanishingly small unless an app is written poorly. For example, in a word processor, the majority of memory pages associated with a file will probably get touched when you save changes to disk even if you never actually scroll to the end of the file. Yes, there are ways to avoid that by manually organizing your data structures in memory, but it usually doesn't make sense to optimize memory organization that heavily.
In any case, regardless of memory organization, it is safe to assume that the vast majority of application data pages (not the actual executable code pages) will be reused at some point in the future. As such, paging out any of this data will require that the data be paged back in at some point, causing a noticeable stall for the user. This is slightly less significant for background daemons, but still true.
Thus, cache is a great example of the principe of diminishing returns. Doubling cache does not necessarily double the benefits. Once cache gets to a certain point, doubling it no longer significantly increases the number of additional hits in the cache. Every increase beyond that point will likely hurt performance by increasing the management overhead without actually increasing the number of successful hits.
Indeed, the only thing that makes sense to not keep in core is infrequently used code text, but that can be thrown away without ever paging it out; it can always be paged back in from the original executable if needed. Even then, an optimal system should not throw out anything except in small physical memory configurations. If you don't have enough RAM, increasing the inherently small disk cache by a small amount actually will result in a significant increase in hits and throwing out infrequently used code pages won't result in a significant performance penalty by comparison. In a large memory configuration, increasing the amount of disk cache won't have much benefit at all, and throwing out those pages probably has a much greater chance of resulting in a performance hit than throwing out a previously read random block on disk; if a page has been used once, the odds are better that it will be used again.
Note: this all assumes that your OS is smart enough to only load in application pages as they are used rather than loading the entire app in at launch. If it isn't, then you have bigger problems, of course.... :-)
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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.