No paging, so why more RAM?

ericmurphy · #1 February 1st, 2005, 06:41 PM

I've got two Power Macs: a 500 MHz G4 with 1.25 Gig of RAM, and a dual 2 GHz G5 with an even gig of RAM. If I run the same number and type of applications and processes (e.g., mail, address book, safari, quicken, photoshop, illustrator, terminal running top, activity manager) I notice that the G4 (which actually has more RAM) seems to use significantly more physical memory than the G5 does, with about the same amount of paging in the same amount of uptime. Right now the G4 has about 70 processes running, 1.22 gig used, 36 meg free. Meanwhile, the G5 has 80 processes running, 740 meg used, 280 meg free. Is the system architecture of the G5 maybe more efficient with RAM? Both machines have a total of 1 Meg of L2 cache, so that doesn't have anything to do with it.

Anyway, the real point is, for the mix of applications I currently use, I wonder if additional RAM for the G5 would be money well-spent. I know OS X tends to grab as much memory as available, so the fact that I have more than 250 Meg free leads me to believe that until I start editing video or rendering 5-minute animations, maybe I should hold off on more RAM.

Thoughts?

cfleck · #2 February 1st, 2005, 08:06 PM

which apps are using the extra mem?

ericmurphy · #3 February 1st, 2005, 08:09 PM

Both machines are running basically the same group of apps, so I don't think it's a matter of which applications are running. I have the feeling that it might be a matter of different system architecture and bandwidth.

And in the final analysis, it doesn't matter. What the question really boils down to is, with 25% of physical memory unused under typical workload, will I see a performance benefit from additional RAM (e.g., 1 Gig increased to 3 Gig)?

cfleck · #4 February 1st, 2005, 08:12 PM

i think you misundersood my question. if you are running the exact same apps on both machines but the memory usage is different i would expect that some of those apps are using more memory on one machine than the other to create that imbalance. which apps are doing this?

and no, you won't see a performance boost if you never run out of ram to begin with.

Lycander · #5 February 1st, 2005, 09:01 PM

Hardware does not know/care, will not care, about your RAM. It's completely up to the OS (the kernel more specifically). It's not like the G5 CPU itself will say "oh I'm gonna reserve X amount of RAM for this app, and Y amount for that app." Nope, don't worry yourself over that at least.

The kernel has its own logic for figuring out what to do with RAM, think of it as artificial intelligence if you want. So on 2 identical hardware systems (for the sake of a control) you can boot it up and run the same apps in the same order do everything the same, there's no guarentee they both will use (or not use for that matter) the same amount of RAM.

scruffy · #6 February 1st, 2005, 09:03 PM

Possibly some apps are compiled with G5 optimizations that end up using less RAM when they run on a G5. A recent version of Photoshop seems like a likely candiate. Check the activity monitor for the VM size of the apps you're running.

cfleck · #7 February 1st, 2005, 09:05 PM

actually won't it be the same on setups where you don't approach the end of available memory? that is, so long as there no threat of running out of memory a system X will give the same maximum amount of memory requested to safari as will system Y in the same circumstances.

i was under the impression that things only got interesting if they had to.

ElDiabloConCaca · #8 February 1st, 2005, 11:39 PM

I think we first need to address the misunderstanding about RAM under OS X in the first place. Just because "top" or the system reports that there's "XXX amount of RAM free" doesn't mean that's all the unused RAM that's available.

It's going to be a combination of "free" memory and "inactive" RAM that gives an indicator as to how much RAM is available for the systems to use -- and even these numbers will change constantly.