In defence of swap: common misconceptions
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tl;dr:
Having swap is a reasonably important part of a well functioning system. Without it, sane memory management becomes harder to achieve. Swap is not generally about getting emergency memory, it's about making memory reclamation egalitarian and efficient. In fact, using it as "emergency memory" is generally actively harmful. Disabling swap does not prevent disk I/O from becoming a problem under memory contention. Instead, it simply shifts the disk I/O thrashing from anonymous pages to file pages. Not only may this be less efficient, as we have a smaller pool of pages to select from for reclaim, but it may also contribute to getting into this high contention state in the first place. The swapper on kernels before 4.0 has a lot of pitfalls, and has contributed to a lot of people's negative perceptions of swap due to its overeagerness to swap out pages. On kernels >4.0, the situation is significantly better. On SSDs, swapping out anonymous pages and reclaiming file pages are essentially equivalent in terms of performance and latency. On older spinning disks, swap reads are slower due to random reads, so a lower vm.swappiness setting makes sense there (read on for more about vm.swappiness ). Disabling swap doesn't prevent pathological behaviour at near-OOM, although it's true that having swap may prolong it. Whether the global OOM killer is invoked with or without swap, or was invoked sooner or later, the result is the same: you are left with a system in an unpredictable state. Having no swap doesn't avoid this. You can achieve better swap behaviour under memory pressure and prevent thrashing by utilising memory.low and friends in cgroup v2.
As part of my work improving kernel memory management and cgroup v2, I've been talking to a lot of engineers about attitudes towards memory, especially around application behaviour under pressure and operating system heuristics used under the hood for memory management.
A repeated topic in these discussions has been swap. Swap is a hotly contested and poorly understood topic, even by those who have been working with Linux for many years. Many see it as useless or actively harmful: a relic of a time where memory was scarce, and disks were a necessary evil to provide much-needed space for paging. This is a statement that I still see being batted around with relative frequency in recent years, and I've had many discussions with colleagues, friends, and industry peers to help them understand why swap is still a useful concept on modern computers with significantly more physical memory available than in the past.
There's also a lot of misunderstanding about the purpose of swap – many people just see it as a kind of "slow extra memory" for use in emergencies, but don't understand how it can contribute during normal load to the healthy operation of an operating system as a whole.
Many of us have heard most of the usual tropes about memory: "Linux uses too much memory", "swap should be double your physical memory size", and the like. While these are either trivial to dispel, or discussion around them has become more nuanced in recent years, the myth of "useless" swap is much more grounded in heuristics and arcana rather than something that can be explained by simple analogy, and requires somewhat more understanding of memory management to reason about.
This post is mostly aimed at those who administer Linux systems and are interested in hearing the counterpoints to running with undersized/no swap or running with vm.swappiness set to 0.
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