Mick wrote: > On Sunday, 20 October 2019 16:03:42 BST Dale wrote: >> Here's the >> thing about using swap on my rig, once it does, the system gets >> extremely slow. Even switching desktops can take a minute or longer. >> Other than trying to get to what is eating up memory and killing it, the >> system is virtually useless. Even my video stops playing. > Swapping can bring the system to its knees, but only under certain operating > scenarios. This is how I understand it works: > > Say you're browsing and keep opening tabs. The browser application will > preemptively allocate memory for more tabs, in case you carry on opening even > more tabs. Then you open yet another big application in terms of memory > usage > and start running it. The kernel will reallocate some of the browser memory > not currently utilised to the other application and keep things working > smoothly. With more applications/tabs being opened you will eventually run > out of RAM and the kernel will swap some of the memory pages to disk. The > swapping is meant to be selective, i.e. things you haven't used in a while > will be taken out of RAM and saved onto your disk. > > Under the above scenario you may notice a momentary latency on your desktop > as > data is swapped onto the disk, but afterwards the desktop should be > responsive > once more - unless more swapping is again demanded by your actions. If you > try to access an application which has had parts of its memory allocation > swapped out to disk you will notice a delay in its reactions. > > Now, in a gentoo scenario, say a mammoth compile like Chromium, with a large > count of jobs specified for it, you could end up swapping part or all of one > or more jobs into memory, only to swap it out again in order to process it. > The compile keeps swapping in and out a job at a time in order to carry on > compiling. The disk thrashing is now continuous and indeed interacting with > your desktop will be painful - potentially waiting for minutes at a time > before an application responds. The way out of this bottleneck is to either > increase your RAM, or minimise the use of memory by reducing the job count in > MAKEOPTS. Shutting down desktop applications and login out of any desktop > sessions to release RAM will also help. > > On a laptop with 4G RAM compiling Chromium is quite challenging when even a > single gcc job could grow to 3G or more. Swapping and a disk I/O bottleneck > becomes unavoidable and moving the compile of binaries to a bigger PC becomes > a rather wise solution. > > Another occasion when swapping can cause havoc is when you have a memory leak > due to some buggy application and all your RAM followed by swap is chewed up > until an OOM ensues. > > For these reasons I always set up swap on my gentoo systems.
What you describe in your first scenario, that is when it is so slow and virtually unresponsive. I don't generally run out of memory when compiling since I close other stuff, like Firefox, to free up memory, even in the 16GB days. What I run into is a tab, or tabs, in Firefox that are chewing through memory like a hungry junk yard dog on a crook. When Firefox does that, it is slow, extremely slow. When I get to where I can see what is going on, it is still slow to even close or kill that process. If I get to a Konsole, I usually run htop and kill it from there. Thing is, it may take a minute even for htop to start and show the problem. That's a fairly small program but it can take a minute or two to even load and show its screen. I have two sites in particular that does this. They run for days with no problem and I may not even be using that tab or that site. Then it's like it gets mad and starts using more and more memory. I have caught it in time to just refresh the tab and it go back to normal. Once it starts using swap tho, it's very slow. I wish Firefox had a way to fix that out of control memory usage. It may not be Firefox itself doing it but it would seem it can see something isn't right and put a stop to it. It's sad when a Linux desktop has 32GBs of memory eat up like that, usually by one program at that. Still, no swap at all would result in a crash or reset. It's better than nothing but I wish the root cause could be fixed. Dale :-) :-)
