View Full Version : virtual memory

03-23-2001, 05:18 PM
I have always been a little puzzled by the virtual memory scheme. I have assumed that the usefulness of it was somewhat tied to the speed of the drive that was being used.

I installed a LVD drive and a Miles2 card so I thought that would help the situation.(I have been going with system ram until I installed Photoshop 6 and read that Adobe recommends enabling virtual ram) I have a PPC 7300 w/ a Sonnett G3 400/304 ram and 8.6.

At a studio I work in part time I configured the G4 400 running OS 9 to use virtual ram. System Ram is 576m and total ram available is 800 (with v ram). I barely can tell the difference system wide and Photoshop is unquestionably better. On the 7300 at home, the whole thing turns into a dog. Is this just a fact of life with the older hardware and an improvement in the OS and hardware in the new machines?


Greg W

03-25-2001, 02:13 AM
Virtual memory is a complicated little technology, designed for multi-user systems to enable them to cope with peak demands on their limited RAM (read: UNIX). It is supposed to be used in concert with a truly multithreaded operating system, which does an analogous thing with processor resources. The basic way it works is by dividing real RAM into segments, or pages, which the OS then writes to a designated section of a disk whenever it judges that the section of RAM is not likely to be used soon and it needs the real RAM for new or changing data.

Since the disk drive speed (whether a hard drive or any other non-RAM type of media) is about a thousand times slower than recording to RAM, the speed of the disk drive does affect virtual memory speed; using virtual memory is vastly slower than having more real RAM. Differences between hard drive speeds does affect VM speed too, but not that much.

Also relevant is that fact that the Mac OS's virtual memory scheme is not very technically robust compared to that of a UNIX or Linux system. The Mac fails to implement it properly and that's why apps like Photoshop have until recently gone to great lengths to avoid it, even using their own virtual memory schemes so as to avoid the instability and poor speed of the OS VM.

So, to answer your questions,
1) your G4 with 500+ MB real RAM is vastly faster for Photoshop than a G3 with 300+ MB, due to the 200 MB RAM difference and the G4's Altivec engine, which Photoshop takes full advantage of.

2) Virtual memory is better implemented in OS 9 than 8.6. Implementation has a lot to do with it.

3) Photoshop has always recommended turning system VM off, and you might want to try it that way with 8.6 and the G3. Check the docs closely to make sure they were talking about OS VM, and not the internal VM in Photoshop (called scratch disks in the prefs).

03-27-2001, 10:37 PM
Thanks for the reply. It was quite informative. I learned something which was the whole point. I guess its time to buy some ram and start saving for the OSX boxes that Apple has in the pipeline for the summer. I'll bet those will be sweet.

Greg W