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Changes to virtual memory managment in Panther / Darwin 7.0
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Changes to virtual memory managment in Panther / Darwin 7.0

Hello,

It seems that the memory management algorithms have been modified for Panther, and I assume these changes are inherited from Darwin 7.0. I am hoping someone can provide some insight into any changes that were made.

First, the swap files in /var/vm were always 80mb in Jaguar (Darwin <= 6.8) and now the first two start at 67mb and each new file is twice the size of the previous. With 768mb of ram, it is quite easy to end up with 1gb of swap files. Ex:
67mb for #0
67mb for #1
143mb for #2
268mb for #3
536 for #4
for a total of 1.08GB

Second, Panther seems far more aggressive at moving pages from RAM to Swap files, resulting in more swap files being created. A new swap file has been created while the machine is otherwise idle.

Third, Panther applications appear to use more memory than pre-Panther. A Panther specific example, iChat uses about 200mb (physical and virtual) in Panther and 1/2 that in Jaguar.

I assume the first two items are a effort to increase performance. First, increasing the size of every successive swap file reduces the likelyhood of another being needed. Second, moving "aged" pages from ram to swap increases the performance when other pages need to moved into ram because the ram does not first need to be freed.

Can anyone provide any insight into the algorithms that are being used, as well as any wisdom regarding a realistic estimation of how large the total swap size will grow. On a G4 machine with 768mb of RAM, it easy to end up with 4 swap files after a day totally 1gb. It seems unlikely that a fifth swap file would be needed, but is it?

Also, is there anyway to tune the virtual memory algorithms, without recompiling?

Thanks in advance,

Scott McLean
1618 North 18th Street
Boise, Idaho 83702
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