>How is it even remotely possible that turning off swap restores memory >shared between processes? Is the Linux kernel going from process to process >comparing pages of memory as they re-enter RAM? "Oh, those two look >identical, they'll get shared?"
This is a good point. I really have no clue how the kernel deals with swapping/sharing, so I can only speculate. I could imagine that it's possible for it to do this, if the pages are marked properly, they could be restored. But, I'll admit, it seems unlikely. ...and, I had this thought before. Maybe this apparent loss of shared memory is an illusion. It appears to make the amount of memory that the httpds use grow very high, but perhaps it is a kind of shared-swap, and thus the calculation I'm using to determine overall memory usage would need to also factor out swap. ...in which case, there's no problem at all. But, I do see an albeit qualitative performance increase and CPU load lowering when I get the httpds to stay shared (and unswapped). So, I think it does matter. Though, if you think about it, it sort of makes sense. Some portion of the shared part of the httpd is also not being used much, so it gets swapped out to disk. But, if those pages really aren't being used, then there shouldn't be a performance hit. If they are being used, then they'd get swapped back in. ...which sort of disproves my qualitative reasoning that swap/unshared is bad. my head hurts, maybe I should join a kernel mailing list and see is someone there can help me (and if I can understand them). -bill
