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Remove graph_lock from the _schema_at hot path

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评估

难度
5/5
预计耗时
一周以上
新手友好度
35/100
Issue 类型
重构
描述清晰度
需要澄清
活跃度
活跃
技术栈
julia
领域
performance

调研方向

Start in src/runtime.jl at _schema_at, then inspect _build_schema_at and the accesses to spec.subschemas and spec.graph_lock. Compare the precomputation and immutable-cache options described in the issue, including their interaction with generated descriptors and concurrent decoders. Done means removing the lock from the lookup hot path without unsafe concurrent Dict reads.

由索引模型根据 Issue 内容生成。

描述

Follow-up from #126, raised in review.

_schema_at (src/runtime.jl) now caches subschema views in spec.subschemas, guarded by spec.graph_lock:

cached = lock(() -> get(spec.subschemas, key, nothing), spec.graph_lock)
cached === nothing || return cached
schema = _build_schema_at(spec, descriptor, direction)
lock(() -> (spec.subschemas[key] = schema), spec.graph_lock)

Every lookup takes the lock once, and the first lookup of a key takes it twice. Before #126 this path was lock-free once the graph existed, but it rebuilt the view on every call. Uncontended, the lock costs about 200 ns per call. Under multi-threaded decoding it serializes lookups.

The lock is required: subschemas is a plain Dict written by concurrent decoders, so the read cannot simply drop it.

Options:

  • Precompute views for all known descriptors when the graph is built. Descriptors are per-call-site constants in generated code, so this needs codegen to register them.
  • Keep an immutable cache and swap it atomically on miss. Reads become lock-free, and a miss copies the cache.
主要语言
Julia
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派生
16
平均合并
1 天 51 分钟
30 天内合并 PR
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