Theseus's Design and Structure

Theseus is a safe-language OS, in which everything runs in a single address space (SAS) and single privilege level (SPL). This includes everything from low-level kernel components to higher-level OS services, drivers, libraries, and more, all the way up to user applications. Protection and isolation are provided by means of compiler- and language-ensured type safety and memory safety, as explained in a later section.

Structure of many small Cells

Theseus is implemented as a collection of many small entities called cells, a software-defined unit of modularity that acts as the core building block of Theseus. The cell concept is a term we coined to represent an individual entity of code and/or data that can be loaded into Theseus. A cell is not a thread of execution, nor is it related to Rust's std::cell types.

The Biological Cell Analogy

Cells in Theseus are inspired by and akin to biological cells in an organism, as they both have many attributes in common:

  • Cells are the basic structural unit
  • Cells are tiny parts of a greater whole, yet remain distinct despite complex interactions and hierarchies
  • Cells each have widely differing roles, but can all be viewed under a single uniform abstraction
  • Cells have an identifiable boundary (cell membrane = public interface) that explicitly regulates what enters and exits (selective permeability = naming visibility)
  • Cells can be arbitrarily "refactored" into multiple different units (meiosis/mitosis = live evolution)
  • Cells can be replaced independently after failing or dying (cell motility = fault recovery)

As such, we sometimes refer to Theseus as a cytokernel, in that it is composed of cells. This reinforces the distinction between the design of Theseus and that of other kernels, e.g., monolithic kernels, microkernels, multikernels, etc. Read more here.

Cell ≈ Crate

Currently, there is a one-to-one relationship between a cell and a Rust crate. The crate is Rust's project container that consists of source code and a dependency manifest file. The crate also serves as Rust's translation unit (elementary unit of compilation); in Theseus we configure each Rust crate to be built into a single .o object file (a relocatable ELF file).

Thus, the cell abstraction is always present in Theseus, but takes different forms as shown in the below diagram.

  • At implementation time, a cell is a crate.
  • After compile (build) time, a cell is a single .o object file.
  • At runtime, a cell 🄲 is a structure that contains the set of sections 🅂 from its crate object file, which have been dynamically loaded and linked into memory, as well as metadata about the inter-dependencies between it and others.

Theseus's cell abstraction is present across implementation, build, and runtime

In Theseus, the metadata stored for each cell is defined by the kernel/crate_metadata crate, which includes two main types:

  • LoadedCrate, which represents a single crate loaded into memory and linked against other loaded crates. The LoadedCrate owns the memory regions holding its sections, along with other metadata about sections and symbols in that crate.
  • LoadedSection, which represents an individual section within a loaded crate, as specified in its object file. A LoadedSection comprises several main items:
    • The section type, e.g., .text (an executable function), .rodata (constant data), .data/.bss (read-write data)
    • Outgoing dependencies: the list of other sections from other crates that this section depends on (and links against).
    • Incoming dependencies: the list of other sections from other crates that depend on (link against) this section.
    • References to its containing "parent" crate and location within that crate's memory region where this section is loaded.

Note that dependencies are tracked on a fine-grained, per-section basis in order to facilitate challenging OS goals like live evolution at runtime, system flexibility, fault recovery, and more. Dependencies are derived from relocation entries specified in the .rela.* sections in the ELF object file. This is much more precise than deriving dependencies from crate-level Cargo.toml manifests.

Each cell is loaded and linked into a namespace, which we refer to as a CellNamespace or CrateNamespace, which represents a true namespace of all of the publicly-visible symbols that are exposed by the cells within it. Namespaces are useful for quick dependency (symbol) resolution during dynamic linking, and also play a key role in the above system goals, especially flexibility, as they can be used to efficiently realize multiple distinct OS personalities to serve different applications with disparate needs.

A simple CrateNamespace showing three crates with sections that depend on each other

The above diagram depicts a simple set of three crates whose sections depend upon each other and are thus linked into a single namespace. The MappedPages (MP) objects are Theseus's abstraction of owned memory regions.

Comparison with other OS designs

The below figure shows the distinction between the structure of existing OS/kernel designs and Theseus.

Existing OS designs vs. Theseus

Monolithic OSes are the most common, including Linux and other Unix-like OSes and most commercial systems like Windows and macOS. In a monolithic OS, all kernel components exist and run in a single kernel address space, meaning that intra-kernel communication is fast and efficient: simply use function calls and shared memory accesses. However, monolithic OSes are less resilient to failures in the kernel, as any crash in kernel space (such as a buggy driver) can bring down the entire system. Applications must use system calls to ask the kernel to perform privileged operations on their behalf, requiring a privilege mode switch.

Microkernel OSes are less common, but still widespread in certain computing domains where reliability is key, such as embedded systems. Microkernels move as much kernel functionality as possible into separate user space "system server" processes, leaving the kernel itself very small. This improves resiliency, as each kernel entity executes in user space in its own address space; if one crashes, the rest of the system can continue execution by restarting the failed system process. However, microkernels are less efficient: all inter-entity functionality requires Inter-Process Communication (IPC), requiring costly context switches and mode switches.

Multikernel OSes offer high scalability to manycore hardware architectures by running a separate instance of a small kernel replicated across each hardware core. Depending on the underlying hardware, system service processes may also be replicated redundantly across (subsets of) cores to improve performance by reducing contention. They typically borrow standard OS interfaces and abstractions from monolithic and microkernel systems, though presenting a standard shared memory abstraction can harm performance.

Theseus OS does not base its structure on any aspect of the underlying hardware, unlike the above three system designs. Everything, including applications, system services, and core kernel components, exists and runs in a single address space and a single privilege level (in "kernel space"). The structure of Theseus is purely software-defined and based on the modularity concept of cells. Thus, communication and shared memory access is efficient because isolation and protection are ensured by the compiler. However, everything must be written in a safe language like Rust. See this section for more about Theseus's safe-language OS design.