Fifth of five; stacked on #38049. The split gave every namespace a file, but
only for the half an operator types. This is the other half.
## The parallel quotients are declared, not written out
`attn_tp_size` and its five siblings were sixty lines of near-identical
properties in the runtime context, a file away from the leaves they are
quotients of, so reading `parallel.py` told you what you could set and nothing
about what that decides.
They are declared in `Parallel` now, in the same class as those leaves. They
carry no annotation, so they are not dataclass fields and
`collect_input_fields` never puts them on the record -- the same mechanism that
already keeps `_NS_PATH` off it. That is the right exclusion: a quotient has no
operator input to preserve, and the record is what crosses a process boundary,
where a stamped width is one an elastic scale-up will not refresh.
## A quotient is a value in the bag, like every other derived one
`_derived_width` answered from a stamp or, failing that, a live process group.
The group read could never disagree with the stamp:
- `initialize_model_parallel` stamps all six as its last statement,
unconditionally;
- an elastic scale-up restamps `attn_dp_size` through
`update_dp_attention_post_scale` -- the comment claiming it does *not* was
wrong;
- no hardware backend builds groups of its own;
- `multimodal_gen`, which has its own `initialize_model_parallel` and does not
stamp, never reads a quotient.
So a built group was always already stamped, and the group read goes -- and with
it the last reason for a quotient to be resolved on every read.
Every input to `derive_parallel_widths` is a record field. `dcp_enabled` is
`decode_context_parallel_size > 1`, not a fact about a built group; it was
spelled `_DCP is not None`, which is a longer way to say the same thing. So the
six are fixed once the configuration is fixed -- the same test every other
`Derived(fn=...)` in this PR passes. They are declared the same way and computed
the same way: once, at publish, into ordinary bag leaves.
What remains is override -> stamp -> published leaf. The stamp stays above the
leaf because an elastic scale-up restamps `attn_dp_size`; the override stays on
top because that is how a test names a width.
## One answer for the config-derived predicates
`enable_mamba_extra_buffer` and its lazy variant, `is_ep_joiner`,
`is_ep_scale_joiner`, `is_startup_weight_load_overlap`: each existed as a
`ServerArgs` member for the resolution pipeline and, for most of them, again as
a `runtime_context` function for readers after publish. Three places to keep
saying the same thing.
A `Derived(fn=...)` is a pure function of the published configuration, so
`publish` computes it once and stores it as an ordinary bag leaf -- a plain
attribute load, which is what a read inside compiled model code needs. The
function is handed the whole resolved config rather than the bag it lands in,
because a derivation is free to span namespaces and the mamba one does: it
reads `memory.disable_radix_cache` alongside its own `exec.mamba` strategy,
which is why it could never have been a method on either bag.
The pre-publish helpers stay -- resolution needs the predicate before there is
a bag to read -- and three readers keep them, because they run before their own
process publishes: `initialize_dp_attention`, which the weight-cache daemon
calls while building its groups thirty lines before its `publish`, and
`PortArgs.init_new`, a factory handed the record that already reads eighteen
other fields off it.
## Notes for a reviewer
**Overriding a leaf does not move its quotient.** `override(tp_size=2)` leaves
`attn_tp_size` where the published config put it, because nothing is recomputed
on read. A test states a topology by publishing a config -- which is what a
real process does -- or by naming the width it wants, `override(attn_tp_size=2)`.
Six tests say it that way now. This is the price of having one answer computed
once, and it is the same price every other derived value in the config already
carries.
A caller that reads a quotient without publishing or overriding now gets an
explicit error naming the field, instead of a default that an uninitialised
group happened to supply. One fixture was in that state --
`TestMlaWriteDoorsUnderDcp` built a bare pool and asked whether DCP was on --
and it publishes a config now, which is what the process it stands in for
does.
Eighteen sites read these predicates without calling them. That is correct --
they are properties -- but it is worth saying they were checked, because a
census that assumes otherwise reports eighteen always-true conditions.
## The skill that documents this subsystem is updated with it
`.claude/rules/modify-component-must-read.md` points at
`.claude/skills/sglang-runtime-context/SKILL.md` before anyone touches these
files, so a stale sentence there is a wrong instruction rather than a stale
note. Four of its load-bearing statements stopped being true across this series
and are corrected here: `NS(...)` is no longer how a field states its namespace
(the declaring class is); the DCP degrade rule is gone, because the quotients
are not live reads; `mamba_extra_buffer_enabled()` and the other predicate
functions it named as the shape to copy no longer exist; and the
namespace-coverage ratchet is described in terms of the marker. The docstring of
`test_server_args_namespaces.py` said the same thing and is fixed too.
The consequence a test author actually trips over is stated there as well:
overriding a leaf no longer moves its quotient, so a topology is stated by
publishing a config or by naming the width.
## Verification
A full registered-unit sweep (648 files) against this stack's merge-base:
19 failures on both sides, the same 19 -- AMD `gfx950`, `modelopt`,
`cuda_vmm`, `weight_checker` and friends, none of them config. The narrower 139-file config sweep used earlier in this series
does not contain the files this change reaches -- `test_kv_index_translator`
never names `get_parallel()`, it constructs an object that does -- which is why
the baseline differential over everything is what is quoted here.
The previous batch counted `self.server_args.X` and called the runner surface
done. It was not: the same read spelled through a local alias --
`server_args = model_runner.server_args` (or `sa = kvc.server_args`, `args = ...`)
followed by `server_args.leaf` -- is the same process-global read wearing a
local name, and the AST census counts **57 of them** across eleven files that
the grep never saw. Census per function, following the alias.
52 were leaves and go to their bag (`spec` 11, `schedule` 9, `memory` 7,
`exec.graph` 5, `exec.moe` 5, `parallel` 4, `disagg` 4, `model` 3,
`exec.mamba` 2, `exec.overlap` 2). Five were not leaves:
three derived members on the eager runner --
`max_speculative_num_draft_tokens` and `enable_mamba_extra_buffer` already had
accessors, and `max_prefill_buffer_tokens` gets one (all its inputs are `schedule`
leaves plus the configured PP size, so it derives from the bags and follows a
post-publish override; `TestDerivedPredicatesAgreeAcrossTiers` pins it against
the member over a 48-case matrix) -- plus `get_attention_backends()`, which the
same commit routes through `attention_backends()`, and a dict that merely shares
the name (`server_args_dict.items`). That dict is the one read left behind.
`build_attention_backends` also stops resolving the pair from the record: it
runs after publish, so it asks `attention_backends()` like every other consumer.
The draft override on the runner still wins first.
`dispatch_event_loop`'s three PP checks read the *configured* PP size, not the
live topology: the MLX runner stub never initializes torch.distributed, so the
live property asserts before the MLX event loop can start (a Codex catch). The
configured leaf answers the same value wherever the live groups exist.
`flashinfer_gdn_prefill_default`'s guard is the one read here that asks what the
*operator* named rather than what the config resolved to, and the bag leaf now
answers exactly that: the per-runner auto-default is stamped on the runner and
deliberately never recorded process-wide, so nothing writes that leaf after
launch and reading it back cannot mistake another runner's default for a flag.
Three test doubles injected a `SimpleNamespace`/`MagicMock` record for exactly
these reads and now publish instead (pool configurator, cache registry, GDN
prefill policy) -- the fixture publishes what the case configures and hands the
published instance to the whole-object contracts that still take one.
The functions this sweep partially converted stop mixing sources (review
catches): the flash-attention constructor's remaining seed reads
(`speculative_eagle_topk`, `speculative_algorithm`, both deterministic gates)
read their bags next to the leaves already converted;
`_should_disable_scheduler_metadata_precompute` reads the parallel config
leaves itself instead of taking the record (its alias binding was the last
use); and the autotune gates (`disable_flashinfer_autotune`, deterministic,
`flashinfer_autotune_skip_ops`) join the moe leaves the same function already
reads from the bags. The pool-configurator fixture drops a parameter nothing
published or read.
