The new method allows access using standard memory semantics instead of as a block device, drastically improving performance.
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CRAM was conceptualized by Gregory Price and his team at Meta. The central realization that seems to have spurred the development of CRAM is that the largest portion of the performance hit from compressed memory isn't the compression; that's tiny. No, the largest hit apparently stems mostly from the fault itself and the swap behavior. So the thinking seems to have been "what if we just do zram, but completely in memory instead of as swap?"
That's a gross simplification of an enormously complex project , but what CRAM seems to be doing is making use of mechanisms Linux already has to enable radically higher-performance compressed memory, particularly on reads. It uses a private NUMA node (essentially, a ghost CPU) instead of pretending to be a block device, and this lets Linux continue using all of its memory semantics, including migration and ballooning, to manage CRAM.
A critical part is the "Chicken Bit," which tells Linux that it has to stop trying to use CRAM while it is busy managing allocations. See, compressed memory seems straightforward until you start actually thinking about it. Compressibility of data varies tremendously based on what you're compressing, all the way from big piles of zeroes (perfectly compressible) up to already-compressed data (non-compressible). Given that, how do you know how much "logical" RAM you have when some of it is compressed? And how do you know when you're going to run out?
CRAM hasn't actually solved that problem yet, it seems; the slides I'm working from seem to establish this as an unsolved problem and an area of ongoing research. But the Chicken Bit is one way it can at least stop cascading failures (colorfully called a "poison storm" in the presentation) from happening when writes outpace CRAM's ability to allocate them.
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Key considerations
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Reference reading
- https://www.tomshardware.com/software/linux/SPONSORED_LINK_URL
- https://www.tomshardware.com/software/linux/new-linux-tech-compresses-memory-in-ram-as-ram-for-452x-speedup-new-cram-method-offers-giant-boost-to-compressed-memory-reads#main
- https://www.tomshardware.com/membership
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