[Solved] Swap Question

Question

I'm not really a n00bie, but no expert, either, and these are probably 'n00bie' Q's.
Hardware: Asus A8V-MX mobo, Athlon64 XP 3500+, .5GB DDR 333, 250 GB WD Cavier (IDE), homebuilt.
I realise I really need  1GB (at least) of real memory for the environment I run, but money is tight, and my mobo only has 2 DDR slots, so I'd have to buy a full 1 (or 2) GB and put my current memory in the closet.
Arch (Don't Panic) is fully current (x86_64).
I have 2 (relevant) partitions: /boot (40MB), and LVM  (16GB). both at the front of the drive.
LVM: / (3.5GB, 78% in use); /home (.75GB; 36%), and swap.
This build is about 2 months old. I boot to run level 3, and manually initiate X/KDE as needed, but I'm normally on the net; typically I'll have Thunderbird, a console session, and Firefox running most of the time. I'm bad about having a dozen (or more) Firefox tabs open (10 at the moment).
I frequent DailyKos.com. DKos runs AJAX-enhanced Scoop, which either trashes Konqueror, or Konqueror trashes it - it doesn't crash, but rendering in that environment simply does not work - open a page at that site with 200+ comments attached. If Konqueror rendered large DKos pages properly, I'd be using it.
DKos tends to have LARGE pages. I also have a large Yahoo 'home page' with 54 categories; I try not to leave Yahoo windows open, although the home page itself is almost always open. The nature of DailyKos is such that a lot of consecutive pages will be open, and left open, often for hours, constantly being updated by AJAX.
I initially allocated .5GB to swap. System ran OK, but bogged down especially bad when I opened a lot of browser tabs. In that environment (according to free, watching the 'buffers/cache used' item, and the 'swap used' item), both real memory and swap would 'typically' be roughly 50% in use,
I decided a few days ago to increase swap by 50% (to .75GB) to see the results. The system is running considerably smoother; it still gets bogged sometimes, but I realise with the hardware I'm running I can't expect miracles. Even with 3 or 4 DKos windows open, it's still quite quick.
Increasing swap had what hit me as an odd effect. Again, I have 10 windows open (no DKos ATM, but the effect is normally the same with them, also). free shows about 60% of real memory in use, but only 5MB of swap is in use. Given equivalent environments, I would have expected swap usage to go UP if there was more available. It appears that real memory has gone up, slightly (~50% to ~60%), while swap usage has gone WAY done (~50% to < 5%; < 1% this moment).
I'm an OLD ex-COBOL    programmer; I've done a few moderate-size (but not commercial, production-scale) projects in VB (pre-Linux) and understand the basics of OOPS, but I'm not a C/C++ programmer at all, and reading the source isn't terribly helpful. Why did swap usage decline so drastically? I'm certainly not complaining      - I'm simply confused,
Does the kernel decide on a memory management scheme at boot, based on available real memory and swapspace? Why did increasing swap DECREASE swap usage, INCREASE (more subtly) real memory usage, resulting in better performance? Given the environment I'm running, is .75GB a rational/adequate swapspace size? Any suggestions on an optimal swap size? I realise I can experiment myself; I already have a bit, obviously, and and willing to do more experimenting (and will), but some direction as to what's happening behind the scenes would be helpful.
Lastly, is 'free' (properly read) the most appropriate tool for monitoring this, or should I be looking at iostat, vmstat, something in /proc?
TIA!
Keith
Last edited by grndrush (2008-02-22 19:07:38)

Answer 1

I don't know the answer to all of your questions but I beleive this is how swap works, In order for the kernel to free up space on your Memory(ram), the kernel transfers data that is not currently being used to the Swap partition and when the data is needed again the kernel copies it back to your actual Memory. So Swap is only used when an application is in idle and no longer needs it's data to be directly in memory, then when that application is in use again the data is copied back to the Memory. The reason I beleive your swap usage is going down is because you're increasing the size, therefor the ratio's are not what they once were, you're still probably using the same amount of swap but since the size of the swap has been increased it doesn't seem like it. In a general rule a swap partition or swap file should be twice the size of the amount of actual memory you have, increasing your swap more than that really isn't going to make a change to how fast your system is since swap is only used when an application no longer needs it's data to be in ram because it's idle, so it's copied to swap until it's needed again. Hopefully this answers some of your questions, and for your last question I beleive it's a matter of opinion, each of those tools work equally well, I use 'top' myself.
Last edited by twiistedkaos (2008-02-12 19:49:36)