Re: [gentoo-user] Memory manager

[Mick]

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.
-- 
Regards,

Mick