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Okay, why does `swapoff` exist on Linux? It's completely useless as far as I can tell. It's been running for 24h to swap in like 1.5GB of used swap or so, on a largely idle server with >32GB free RAM. How can it be *this* hilariously inefficient?
This is what it's done over the past 24h. It got rid of most of it quickly, and then the last 100MB or so are taking forever to swap in. WTF? Over the last 50 minutes it seems it hasn't managed to swap in a single page, but it's doing I/O. What is this, an O(nᴸᴼᴸ) algorithm?pic.twitter.com/RArORjswRX
It's managed to swapoff, er, about 100KiB since that tweet (~10h)? So it's not *stuck*, but you could ask someone trained in telegraphy to transmit the swapspace contents in morse code manually and it would be faster.
Disk stats say it's *writing* 6MB/s of stuff, at 1400 IOPS or so (plausible for spinning rust on battery backed RAID card), 80% I/O usage, paging "out" 1400 pages per second. What? Why is it paging stuff *out* with >32GB of RAM free?
Maybe if your Linux is running on a microsd memory don’t want to have unnecessary disk writess. Don’t you think?
Pretty much the entire swap infrastructure on Linux is laughably bad. Like, what, are they reading a page at a time and waiting for it to finish getting read before paging in something else?
It's far worse than that. It would be far faster if it could just read in the pages one at a time, which is basically what happens if they're all in the swap cache. If the page isn't in swap cache, it has to figure out where it's being used to handle it.https://twitter.com/DanielMicay/status/1147380984448507905 …
It iterates through the swapped pages one by one and in the worst case it has to walk through the page tables for every process to look for the uses of the page. It would often be far more efficient to walk through the page tables a single time, but it doesn't know how to do it.
There's a swap cache to track the pages in swap efficiently, but if they're not cached, dealing with them is incredibly expensive. The way it works is especially horrific with a modern SSD or even more so with in-memory swap like zram. You're really far better off just rebooting.
I’ve used swapoff many times. It’s usually worked for me.
I’ve had cases where it can’t finish with like 32kB left. For some reason just couldn’t find it or something. Had to reboot.
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