Give a container the swap it asks for - #2182
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A container whose workload exceeds its memory limit meets the out of memory killer, with nothing between the two. The runtime it sits on can hand the guest a swap area to reclaim to instead, but nothing here asks for one. Take a size on the command line, keep it on the container's resources beside the memory limit it plays against, and make the area from it at bootstrap: a raw block file in the container's own bundle, alongside the root filesystem, which the guest agent writes a swap header to and enables. The size counts swap alone rather than the memory and swap total the runtime spec carries, so it reads against `--memory` rather than including it. A container that asks for none gets none, which is what one expected to stay within its memory wants. The area holds nothing that outlives the container, so it is made afresh at each bootstrap and the host is told not to synchronize it.
The guest reaches the swap area as a block device, which the kernel takes as a single extent without consulting the host's layout, so the hole-free requirement that binds a swap file inside the guest does not bind the host file backing the device. A sparse backing costs the host only the pages the guest has actually swapped out and gives them back on discard. https://github.com/torvalds/linux/blob/master/mm/swapfile.c
Swap was the only container resource a user could ask for but not configure a default for: --swap existed with nothing behind it, while --cpus and --memory both fall back to their [container] keys. Add [container] swap, defaulting to zero so a container still gets no area unless one is asked for, and resolve it the way the other two resolve.
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Motivation and Context
Closes #2181.
A container whose workload exceeds its memory limit meets the out of memory killer, with nothing between the two. The runtime it sits on can hand the guest a swap area to reclaim to instead, but nothing here asks for one.
--swaptakes a size on the command line, kept on the container's resources beside the memory limit it plays against, and the area is made from it at bootstrap: a raw block file in the container's own bundle, alongside the root filesystem, which the guest agent writes a swap header to and enables. The size counts swap alone rather than the memory and swap total the runtime spec carries, so it reads against--memoryrather than including it. A container that asks for none gets none, which is what one expected to stay within its memory wants. The area holds nothing that outlives the container, so it is made afresh at each bootstrap and the host is told not to synchronize it.The backing file is sparse. The guest reaches the swap area as a block device, which the kernel takes as a single extent without consulting the host's layout, so the hole-free requirement that binds a swap file inside the guest does not bind the host file backing the device: https://github.com/torvalds/linux/blob/master/mm/swapfile.c . Sparse backing costs the host only the pages the guest has actually swapped out and gives them back on discard.
Swap was also the only container resource a user could ask for but not configure a default for, so
[container] swapsits besidecpusandmemoryand resolves the way they do, defaulting to zero.Dependency and Merge Order
The guest half is in Containerization: apple/containerization#878, discussed in apple/containerization#877. That must land, and this repository's Containerization dependency be advanced past it, before this can build. This branch targets stock
apple/container:mainand its diff is the CLI, configuration, and runtime-service halves alone.Testing
Unit coverage for the configuration resolution and the size arithmetic. Verified end to end against a Containerization checkout carrying apple/containerization#878: a container given
--memory 128m --swap 256msurvives a workload that the same container without swap is killed for, and the bundle's swap file gives blocks back as the guest stops using them.Integration suite: 397 passed. Unit suite: 772 passed.
make fmt,make checkclean.