swap on ZRAM, zswap, and Rust was: Better interactivity in low-memory situations

Hi, This is yet another follow-up for this thread: https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.o... Basics: "zswap" compresses swap and uses a defined memory pool as a cache, with spill over (still compressed) going into a conventional swap partition. The memory pool doesn't appear as a separate block device. A conventional swap partition on a drive is required. https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/Doc... "swap on ZRAM" A ZRAM device appears as a block device, and is effectively a compressed RAM disk. It's common for this to be the exclusive swap device, of course it is volatile so in that configuration your system can't hibernate. But it's also possible to use swap priority in fstab to cause the ZRAM device to be used with higher priority, and a conventional swap partition on a drive with a lower priority. https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/Doc... What they do: Either strategy can help avoid swap thrashing, by moderating the transition from exclusively RAM based work, to heavy swapping on disk. In my testing, the most aggressive memory starved workloads still result in an unresponsive system. Neither are a complete solution, they really seem to just be moderators that kick the can down the road. But I do think it's an improvement especially in the incidental swap use case, where transition from memory to swap isn't noticeable. Which is better? I don't know. Seriously, that's what all of my testing as come down to. A user won't likely notice the difference. Both dynamically allocate memory to their "memory pools" on demand. But otherwise, they really are two very different implementations. Regardless, Fedora Workstation and probably even Fedora Server, should use one of them by default out of the box. IoT folks are already using swap on ZRAM by default, in lieu of a disk based swap partition. And Anaconda folks are doing the same for low memory devices when the installer is launched. I've been using zswap on Fedora Workstation edition on my laptop, and Fedora Server on an Intel NUC, for maybe two years (earlier this summer I switched both of them swap on ZRAM to compare). How are they different? There are several "swap on ZRAM" implementations. The zram package in Fedora right now is what IoT folks are using which installs a systemd service unit to setup the ZRAM block device, mkswap on it, and then swapon, during system startup. Simple. The ideal scenario is to get everyone on the same page, and so far it looks like systemd's zram-generator, built in Rust, meets all the requirements. That needs to be confirmed, but also right now there's a small problem, it's not working. So we kinda need a someone familiar with Rust and systemd to take this on, if we want to use the same thing everywhere. https://github.com/systemd/zram-generator/issues/4 Whereas zswap is setup by using boot parameters, which we could have the installer set, contingent on a conventional swap partition being created. zswap.enabled=1 zswap.compressor=lz4 zswap.max_pool_percent=20 zswap.zpool=zbud Zswap upstream tells me they're close to dropping the experimental status, hopefully by the end of the summer. It might be a bit longer before they're as confident with zpool type z3fold. Hackfest anyone? -- Chris Murphy