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Is Video RAM a Good Swap Device?

Posted by kdawson on from the it's-there-why-not-use-it dept.

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?"

3 of 235 comments (clear)

  1. AGP or PCI-Express by j_sp_r · · Score: 5, Informative

    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.

  2. Probably a good idea, provided you have PCIe by default+luser · · Score: 5, Informative

    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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    1. Re:Probably a good idea, provided you have PCIe by ElecCham · · Score: 5, Informative

      (ObDisclaimer: I work for Seagate.)

      On a current-model 7200RPM SATA drive, you can expect to see around 80MB/sec at the outer edge of the disk. And the rule of thumb is, you see half that at the inner edge, and three-quarters in the middle. So call it a (nearly) guaranteed 40MB/sec, and an average of 60MB/sec.

      These are not hard-and-fast numbers, but it's a pretty good estimate for a modern drive.

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