> It's like the system discards pages of programs just because the app has been inactive for an hour or so.
From what I've read, Windows memory manager does the same thing - after a while, it swaps out unused pages, even if plenty of free memory is available.
I wonder what's the logic behind this - did the engineers assume that the speedup coming from more free memory being available for disk cache is worth the hassle of waiting for the swapped out page (when it's actually needed)?
I was under the impression(from my FreeBSD years) that pages that are not used for a while is swapped out and marked as inactive. The pages are then fast to throw away for other use, or fast to brink back to active state.
In Lion I get the impression they are just swapped out and thrown away / reused for something else despite there are no real pressure on the VM.
The logic is for things like daemons or infrequently used processes that wake up maybe once a day. They don't need to be in ram all the time needlessly. So swapping out their memory that hasn't been used for N hours isn't a bad thing.
And yes, the logic is sound, its better to use a bit of swap for an infrequent daemon and let 4-5 megs of memory be at the ready if needed than leave it in place all the time. The "speedup" is not a speedup for your use, its to allow for better memory management. Which is what the VM subsystem is there for. Second guessing it all the time just makes its job harder.
Swap use when there is free memory isn't a bad thing. This fetish people have with their OS using swap at times seems to border on the ridiculous side. My iMac at home has 16g of memory and 400g of swap used right now (8g active, lots of file cache that'll get purged). Most of the swapped files belong to things like my ruby+pry repl, a clojure repl I haven't touched for 2 days, and other random things I don't use often enough to warrant they stay in active ram. Why SHOULDN'T that memory be reclaimed and at the ready for a new program or some other request? Its just going to page it out then and likely take longer to do. The only time its "wrong" is when I start using those processes again, which takes all of 1-2 seconds.
Its a hard problem, and both OSX/Windows choose the best possible solution you can heuristically.
So why not lazy page? Swap to disk, but don't unlink the memory until it's needed by another process. If the paged app comes back to like first, it's still in memory and the paged copy on disk can be dropped.
> The logic is for things like daemons or infrequently used processes that wake up maybe once a day. They don't need to be in ram all the time needlessly. So swapping out their memory that hasn't been used for N hours isn't a bad thing.
It is, actually. I find such things unacceptable, be they on desktop or server use cases. I put as much or far more RAM in my systems than they will need, and I expect nothing to be swapped until it's actually full. Many other people do as well, which is why the Linux kernel devs finally started fixing the stupidity several years ago. Time for OS X to catch up.
> From what I've read, Windows memory manager does the same thing - after a while, it swaps out unused pages, even if plenty of free memory is available.
Windows XP does that. It was a common source of grief. I remember it being mentioned as early as 2004. Since Windows Vista the memory manager doesn't have that problem.
From what I've read, Windows memory manager does the same thing - after a while, it swaps out unused pages, even if plenty of free memory is available.
I wonder what's the logic behind this - did the engineers assume that the speedup coming from more free memory being available for disk cache is worth the hassle of waiting for the swapped out page (when it's actually needed)?