Linux (3.4) SSD swap partition usage causes extreme latency – how to eliminate

If performance is good - you're not having to wait while switching between apps, with the disk thrashing and the activity LED solid on - then there isn't going to be a lot of wear to worry about either.

You could allocate a 2G space for swap at install time, and later tune the partition/swap size down e.g with GNOME Disks. Personally I would try 512M or 1G to start with. I assume you're not thinking about hibernation (which would usually want 2G+ swap).

Then I could see how much swap I used, and if I hit problems with out-of-memory or slowdowns with full swap. I confess I'm not sure how I'd distinguish thrashing the swap (too much active apps, enabled by too much swap) v.s. the cache (not enough swap to page out inactive app memory). In both cases you'd see the disk light on and pretty full swap. Maybe vmstat 1 and watch the swap column, if I could test a problem case reliably.

I would be cautious about 2GB swap on an eMMC (lower performance than expensive SSDs). Linux desktop memory management has a somewhat checkered history, and the swapping won't have been designed for eMMC.

If you use all that swap you may find the machine's become literally unusable anyway. Running out of memory can be better because you get the machine back without rebooting. Though it may kill programs you wanted & expected to survive.

My GNOME shell with a few apps has about 1G "used" (swapable) plus 1G+ "cached" (read-only data/code, can just be re-read from filesystem).

If you had punted 1G to swap, how long would it take to swap all that back, given your eMMC? (GNOME Disks can benchmark partitions for you :). E.g. 10 seconds to read it? Then if you're swapping the previous working set out at the same time, the (slower) writes on the eMMC are going to take even longer. Doesn't sound great to me.

One small reason for swap is tmpfs. My /run accumulates a meg or so, I doubt all needs to be in ram.

Statically configured swap space (the type that pretty much every distribution uses) is configured in /etc/fstab just like filesystems are.

A typical entry looks something like:

UUID=21618415-7989-46aa-8e49-881efa488132    none    swap     sw      0  0

You may also see either discard or nofail specified in the flags field (the fourth field). Every such line corresponds to one swap area (it doesn't have to be a partition, you can have swap files, or even entire swap disks).

In some really specific cases you might instead have dynamically configured swap space, although this is rather rare because it can cause problematic behavior relating to memory management. In this case, the configuration is handled entirely by a userspace component that creates and enables swap files as needed at run time.

As far as how many you need, that's a complicated question to answer, but the number of different Linux distributions you plan to run has zero impact on this unless you want to be able to run one distribution while you have another in hibernation (and you probably don't want to do this, as it's a really easy way to screw up your system).

When you go to run the installer for almost any major distribution (including Fedora, OpenSUSE, Linux Mint, Debian, and Ubuntu), it will detect any existing swap partitions on the system, and add those to the configuration for the distribution you're installing (except possibly if you select manual partitioning), and in most cases this will result in the system being configured in a sensible manner.

Even aside from that, I would personally suggest avoiding having multiple swap partitions unless you're talking about a server system with lots of disks, and even then you really need to know what you're doing to get set up so that it performs well.