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The scheduler was xv6's round-robin: run everything RUNNABLE in slot order and yield on every timer tick. Slot order is not fairness, a process alone on a CPU paid a full context switch a hundred times a second to be handed straight back to itself, and there was no way to say that one thing mattered more than another. Every process now carries vruntime, the CPU time it has used divided by its weight, and the scheduler runs the runnable process with the smallest. Fairness and interactivity both fall out of that one rule; nice is a weight, 1.25x per level as UNIX has always had it. A slice is 20 ms shared among the runnable processes, never below 2 ms, and the timer tick preempts only once it is spent. A process waking from a long sleep is brought up to min_vruntime less one period, so it is scheduled promptly without being owed everything it slept through; fork inherits vruntime, so forking does not reset a debt. getpriority/setpriority are real, with Linux's offset of 20 on the return; /proc/<pid>/schedstat and the ctxt_switches lines of status report what the scheduler did. docs/architecture/scheduler.md has the reasoning, including why the run queue is still a scan of ptable. This was written in an earlier session and left uncommitted. Finished here: a signal waking a sleeper goes through the same vruntime floor as wakeup() does, and sched_yield() yields -- it returned 0 doing nothing, harmless while every tick was a yield and a spin of a whole slice once ticks stopped being one. Checked under QEMU on 1 and 4 CPUs: a nice 10 loop beside a nice 0 one gets about a tenth of a shared CPU, and a command typed meanwhile answers in 26 ms. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4
120 lines
6.9 KiB
Markdown
120 lines
6.9 KiB
Markdown
# The system, layer by layer
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What SchOS is today. Everything in this directory is built and running; what is
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merely intended lives in [../plan/](../plan/).
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---
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## The names, and what each one covers
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`uname -s` reports `SchIFT`, the way every Unix reports its kernel rather than
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its distribution.
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| Layer | Name | What it is |
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|---|---|---|
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| System | **SchOS** | The distribution — kernel, userland, tooling |
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| Kernel | **SchIFT** | Multi-core x86-64 kernel with a modular VFS |
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| Filesystem | **SchFS** | Transactional, copy-on-write, the root filesystem |
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| API | **SchAPI** | The native programming interface — *designed, not built* |
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| Compositor | **Schcom** | Owns the display; v0 runs |
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Six virtual terminals sit under all of it -- `/dev/tty1`..`tty6`, AltGr+F1..F6
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or `chvt N` --
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each with its own screen, line discipline and foreground process group. See
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[terminals.md](terminals.md).
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---
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## The stack, bottom to top
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```
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┌─────────────────────────────────────────────────┐
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│ toybox kilo mksh zsh musl first-party cmd/ │ userland
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├─────────────────────────────────────────────────┤
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│ vgpud — a driver that is an ordinary process │
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├─────────────────────────────────────────────────┤
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│ schinit — PID 1, job manifests, supervision │
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├─────────────────────────────────────────────────┤
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│ syscall table — Linux numbering, one .inc file │ ← the ABI seam
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╞═════════════════════════════════════════════════╡
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│ VFS │ sockets │ signals │ mmap │ poll │ procfs │
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├──────┴─────────┴─────────┴──────┴──────┴────────┤
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│ SchFS │ lwIP │ vt/gfx │ buffer cache │
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├─────────┴────────┴──────────┴───────────────────┤
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│ device model — class vtables, bus binding │ ← the driver seam
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├─────────────────────────────────────────────────┤
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│ /dev/pci — MMIO, DMA and IRQs to a process │ ← the eviction seam
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├─────────────────────────────────────────────────┤
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│ drivers: xHCI e1000 virtio-gpu VBE IDE PS/2│
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├─────────────────────────────────────────────────┤
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│ scheduler │ paging │ traps │ IRQ │ SMP │ DMA │ SchIFT core
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└─────────────────────────────────────────────────┘
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▲
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UEFI loader (boot/uefi)
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```
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Three seams matter. Above the **syscall table**, software is ordinary and
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portable — musl and toybox are unmodified. Below the **device model**, a driver
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never names its consumer and a consumer never names a driver. And at
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**`/dev/pci`**, a driver stops having to be in the kernel at all: a process is
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handed a device's registers, DMA memory and interrupts, and the in-kernel driver
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that held it releases it cleanly. `vgpud` is the virtio-gpu driver living out
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there today. See [../plan/hybrid.md](../plan/hybrid.md).
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---
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## Where it came from
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SchOS began as [xv6](https://github.com/mit-pdos/xv6-public) and is now mostly
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something else. What survives from xv6 is the *shape* of the process table, the
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sleep/wakeup discipline, and the buffer cache. What has been replaced: the
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scheduler and trap path (rewritten for x86-64 and for more than one processor),
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the memory manager, the entire filesystem, the syscall layer, and every driver.
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That inheritance is worth knowing when reading the code, because the parts that
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still look like xv6 follow xv6's conventions and the parts that do not, do not.
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---
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## The documents
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Read in this order if you are new:
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| [boot.md](boot.md) | Firmware to the first userspace process. The shortest path through the whole system. |
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| [kernel.md](kernel.md) | Memory, processes, scheduling, traps, signals, SMP. |
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| [syscalls.md](syscalls.md) | The one table everything is generated from, and how a musl binary runs unmodified. |
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| [scheduler.md](scheduler.md) | Fair share by virtual runtime, slices, and `nice`. |
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| [filesystem.md](filesystem.md) | The VFS, the buffer cache, mounts, procfs. |
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| [schfs.md](schfs.md) | The copy-on-write root filesystem. |
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| [fat.md](fat.md) | FAT12/16/32 with long names: USB sticks, SD cards, the ESP. |
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| [devices.md](devices.md) | Buses, classes, binding, and the vtables consumers talk to. |
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| [usb.md](usb.md) | xHCI, the USB core, and the three class drivers. |
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| [graphics.md](graphics.md) | Framebuffer, text rendering, the console, and PSOGM. |
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| [terminals.md](terminals.md) | The six virtual terminals, and the chord that switches them. |
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| [networking.md](networking.md) | NIC drivers, lwIP, and both socket families. |
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| [audio.md](audio.md) | `/dev/dsp`, AudioEngine, and the AC'97 driver under it. |
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| [init.md](init.md) | `schinit`, PID 1 as a role, and the job manifests. |
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| [debugging.md](debugging.md) | Symbols, a log that outlives the kernel, and asking the kernel a question. |
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| [ldls.md](ldls.md) | Running somebody else's drivers: a Linux `.ko`, in a process, reaching hardware through `/dev/pci`. |
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---
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## Three rules that hold across all of it
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**The kernel and the host tools compile the same SchFS source.** `mkschfs`
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cannot disagree with the kernel about the on-disk format, because there is only
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one implementation. Format bugs are found on the host, where a test takes a
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second instead of a boot.
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**Syscalls are declared in exactly one place.** `kernel/sys/table.inc`.
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The constants, the dispatch table and the Linux translation map are all
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generated from it, so a syscall cannot be half-registered.
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**A list you can read beats a list assembled by side effects.** Drivers are
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written out by hand in `kernel/drivers/registry.c` rather than collected by a
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linker section, and platform devices are registered in one function rather than
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inside each driver's init. The tradeoff is deliberate: self-registration is
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nicer to write, and it means the list of what is in your kernel is not written
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down anywhere you can read.
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