Invariant TSC

Architecture referenceFeex

Invariant TSC dey describe time-stamp counter wey im speed dey remain di same across di architectural power and frequency state dem wey di feature cover. E no be direct measurement of current core frequency.

Wetin you need know

How e dey work

Di time-stamp counter dey add count at speed wey platform define. Invariant behavior dey separate dat speed from change dem for core running frequency. To change tick dem go elapsed time need correct frequency reference, while to compare value dem across processor dem or virtual machine dem also need synchronization and virtualization guarantee dem.

Where e dey useful

Runtime fit use correct counter take do part of im high-resolution timing setup. If person divide short workload counter difference by nominal CPU GHz value, e fit dey wrong because TSC rate and executing-core frequency na different things entirely.

How to check support and enablement

CPUID.80000007H:EDX bit 8 dey show invariant TSC under di architecture definition wey match. E beta make you use di operating system supported monotonic clock for normal timing, because e fit factor in platform and virtualization policy.

Pick clock based on wetin you dey find: elapsed time, thread CPU time, and cycles na different measurements. Di operating system clock interfaces dey carry platform policy wey raw instruction sample fit bypass.

Wetin di feature no dey guarantee

Invariant alone no mean say synchronization dey across sockets, monotonic behavior dey across VM migration, serialization dey around di instructions wey you measure, or say scheduling delays no dey. Raw counter sample no be CPU utilization percentage.

Compatibility and performance inside real life

Benchmark still need correct compiler boundaries, warmup, representative data, and make person understand migration and scheduling. To repeat measurement wey order scatter no dey make am accurate. Always report di environment and uncertainty with any result.

Practical verification checklist

Record di exact processor or platform model plus di application, OS and runtime version dem wey dey involve. Make you keep di exact word dem wey di error or requirement use.
Check di manufacturer reference for dat exact model, den inspect di real capability and enabled state inside di environment where di program dey run.
Compare every single requirement of di binary or backend wey you choose. Name wey resemble am, alias, or broad technology label no reach.
Confirm say di application select di implementation wey dem intend. Use di diagnostics wey dem document and supported fallback if requirement dem no complete.
Make only one supported change at a time, den run di same workload again. Keep di original configuration and recovery way ready.

If dem list di feature but software no fit use am

Start from di exact component wey dey fail. Hardware database dey describe capability wey dem record; di operating system or hypervisor fit show different real environment. If illegal-instruction error happen, you still need to check di exact instruction wey cause di fault and di binary build, because corrupt file or anoda instruction wey no dey supported fit give di same kind problem.

Install di application, driver or runtime version wey get support for dat exact platform, and use di vendor documented baseline or fallback build if e need am. Software update no fit create instruction wey di processor no get. Firmware and security-policy change dem suppose follow di system manufacturer procedure and recovery plan, no be just any common optimization advice.

Sources and scope of dis explanation

Intel 64 and IA-32 Architectures Software Developer's Manual

Dis reference dem establish architectural behavior and software requirement dem. Dem no dey independently verify every feature value for any linked hardware model page. For model support, use dat page manufacturer evidence and check di active system configuration. Dis article no supply benchmark, certification or independent review date.

Sources and verification

Editorial ownership: Feex. No independent review date dey recorded. How we dey do research.

Intel 64 and IA-32 Architectures Software Developer's Manual · Volume 1 and 2 describe di named instruction family dem and CPUID; Volume 3 describe state, paging, VMX and protection control dem. Architecture documentation no dey prove say particular model support am.

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