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TTscp::GetApproximate takes the processor id from the rseq fast path
(GetCurrentCpuId) and the instant from a non-serializing rdtsc, instead of the
single serializing rdtscp of TTscp::Get.
TPerCpuGauge::Update switches to it: the per-shard timestamp only orders writes
across shards to pick the freshest value, so the lower precision is fine. Update
is virtual and now YT_PREVENT_TLS_CACHING -- the fiber-TLS boundary the inlined
rseq read needs.
#### Benchmark
sas2-2769 (glibc 2.31 + tcmalloc, rseq fast path), median of 5:
| primitive | time |
|---|---|
| TTscp::Get() (rdtscp) | 14.1 ns |
| TTscp::GetApproximate() (rseq + rdtsc) | 10.6 ns (-25%) |
commit_hash:b277b6551accd6d0b879f8ffb168bcbe8d9fbb74
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Self-contained current-CPU-id reader backed by Linux **rseq** (restartable
sequences), with **no third-party dependency** (no librseq):
* The rseq ABI is hand-defined; the calling thread is registered lazily via the
rseq syscall.
* Fast path is a single inlined, **branch-free** thread-local read. The offset
always points at a readable `cpu_id` -- the glibc-owned area when glibc registers
rseq (>= 2.35, via the weak `__rseq_offset`/`__rseq_size`), otherwise our own
area -- so an unregistered thread reads `-1` and routes to the slow path.
* Falls back to `sched_getcpu()` (Linux) or `0` (darwin/windows). Works on glibc
**and musl** alike (librseq does not build on musl).
Fiber-TLS contract: the inlined read must be reached only via a non-inlinable,
fiber-switch-free frame (a virtual call or `YT_PREVENT_TLS_CACHING`).
#### Benchmark -- cost of one cpu-id read
| source | time / call |
|---|---|
| `GetCurrentCpuId()` (rseq) | **0.34 ns** |
| `sched_getcpu()` (vDSO) | 3.5 ns |
| `rdtscp` (what `TTscp::Get()` does) | 23 ns |
This is an alternative to the librseq-based review/13886037 -- same speed, but no
contrib dependency and it also covers musl. The unit test pins to each allowed CPU
and asserts the reported id matches.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
commit_hash:09d282c2f48755836b1cd68cedbffc3c6a662eed
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