AMD chiarisce molti aspetti del comportamento delle CPU Ryzen

Trascorse circa 2 settimane dal debutto dei primi processori della famiglia Ryzen (a questo indirizzo la nostra analisi), con le versioni Ryzen 7 1800X, Ryzen 7 1700X e Ryzen 7 1700, AMD ha scelto di rendere disponibili alcune informazioni dettagliate sui propri nuovi processori con dettagli sulle prestazioni con tecnologia SMT abilitata e informazioni sulle temperature di esercizio.

Sin dal debutto di questi processori sul mercato sono emerse varie analisi che hanno cercato di dare risposta ad alcuni specifici comportamenti di queste CPU. In particolare le prestazioni con giochi 3D alle risoluzioni non troppo elevate, inferiori a quelle delle CPU Intel della famiglia Core basate su architettura Kaby Lake per quanto molto più elevate rispetto alle proposte AMD di precedente generazione della famiglia FX.

Queste le informazioni fornite pubblicamente da AMD, e che meritano a nostro avviso un plauso per la chiarezza con la quale sono state esposte:

• Thread scheduling: AMD sostiene che lo scheduler di Windows 10 si comporta in modo corretto nella gestione delle risorse di elaborazione che le CPU AMD Ryzen 7 mettono a disposizione. Il sistema operativo gestisce in modo corretto i core fisici e quelli logici abilitati attraverso tecnologia SMT, senza che questo porti ad una penalizzazione prestazionale;
• disabilitare la tecnologia SMT (Simultaneous Multi-Threading): il comportamento con i giochi disabilitando l'SMT non permette mediamente di ottenere, secondo AMD, incrementi delle prestazioni che siano tali da giustificare questa scelta. L'azienda quindi consiglia di mantenere abilitata tale impostazione per ottenere le migliori prestazioni complessive;
• differenze di prestazioni tra Windows 7 e Windows 10 utilizzando processori AMD Ryzen non sono giustificate dalla differente gestione dello scheduling dei processori tra i due sistemi operativi, quanto dalla differente architettura alla base dei due OS di Microsoft;
• temperatura della CPU: alcuni software riportano valori di temperatura della CPU superiori a quelli effettivi del processore, con i modelli Ryzen 7 1800X e Ryzen 7 1700X, in quanto AMD ha implementato un offset di 20 gradi centigradi tra il valore di temperatura riportato dal sistema e quello effettivo della CPU, rilevato come True Junction Temperature cioè temperatura del punto di collegamento tra core della CPU e heatspreader del processore. Questa scelta è legata alla volontà di AMD di armonizzare la policy di intervento della ventola di raffreddamento della CPU tra le varie versioni di CPU Ryzen. Con i modelli Ryzen 7 1800X e Ryzen 7 1700X, quindi, la temperatura effettiva della CPU è di 20 gradi inferiore rispetto a quella rilevata dai software attualmente disponibili;
• Power plan: AMD continua a consigliare di importare la modalità High Performance nel profilo di risparmio energetico di Windows 10 in quanto questo permette di disabilitare la modalità Core Parking, così da rendere i core immediatamente disponibili quando richiesti dallo scheduler del sistema operativo, oltre a permettere di gestire cambiamenti della frequenza di clock con intervalli di 1 ms, valore non raggiungibile con altre modalità di risparmio energetico.

AMD chiarisce molti punti legati all'analisi e al comportamento delle CPU Ryzen, confermando quanto era parso essere da subito chiaro escludendo le tipiche teorie di complotti che possono emergere online. Non ci sono problemi di compatibilità tra quese CPU e l'architettura del sistema operativo Windows 10 di Microsoft. Il comportamento prestazionale con giochi alle risoluzioni più basse non è legato all'OS o al comportamento della tecnologia SMT, che non è pertanto da disabilitare.

Di seguito abbiamo riportato l'intero post del blog di AMD, accessibile online a questo indirizzo.

It’s been about two weeks since we launched the new AMD Ryzen™ processor, and I’m just thrilled to see all the excitement and chatter surrounding our new chip. Seems like not a day goes by when I’m not being tweeted by someone doing a new build, often for the first time in many years. Reports from media and users have also been good:

• “This CPU gives you something that we needed for a long time, which is a CPU that gives you a well-rounded experience.” –JayzTwoCents
• Competitive performance at 1080p, with Tech Spot saying the “affordable Ryzen 7 1700” is an “awesome option” and a “safer bet long term.”
• ExtremeTech showed strong performance for high-end GPUs like the GeForce GTX 1080 Ti, especially for gamers that understand how much value AMD Ryzen™ brings to the table
• Many users are noting that the 8-core design of AMD Ryzen™ 7 processors enables “noticeably SMOOTHER” performance compared to their old platforms.

While these findings have been great to read, we are just getting started! The AMD Ryzen™ processor and AM4 Platform both have room to grow, and we wanted to take a few minutes to address some of the questions and comments being discussed across the web.

Thread Scheduling

We have investigated reports alleging incorrect thread scheduling on the AMD Ryzen™ processor. Based on our findings, AMD believes that the Windows® 10 thread scheduler is operating properly for “Zen,” and we do not presently believe there is an issue with the scheduler adversely utilizing the logical and physical configurations of the architecture.

As an extension of this investigation, we have also reviewed topology logs generated by the Sysinternals Coreinfo utility. We have determined that an outdated version of the application was responsible for originating the incorrect topology data that has been widely reported in the media. Coreinfo v3.31 (or later) will produce the correct results.

Finally, we have reviewed the limited available evidence concerning performance deltas between Windows® 7 and Windows® 10 on the AMD Ryzen™ CPU. We do not believe there is an issue with scheduling differences between the two versions of Windows.  Any differences in performance can be more likely attributed to software architecture differences between these OSes.

Going forward, our analysis highlights that there are many applications that already make good use of the cores and threads in Ryzen, and there are other applications that can better utilize the topology and capabilities of our new CPU with some targeted optimizations. These opportunities are already being actively worked via the AMD Ryzen™ dev kit program that has sampled 300+ systems worldwide.

Above all, we would like to thank the community for their efforts to understand the Ryzen processor and reporting their findings. The software/hardware relationship is a complex one, with additional layers of nuance when preexisting software is exposed to an all-new architecture. We are already finding many small changes that can improve the Ryzen performance in certain applications, and we are optimistic that these will result in beneficial optimizations for current and future applications.

Temperature Reporting

The primary temperature reporting sensor of the AMD Ryzen™ processor is a sensor called “T Control,” or tCTL for short. The tCTL sensor is derived from the junction (Tj) temperature—the interface point between the die and heatspreader—but it may be offset on certain CPU models so that all models on the AM4 Platform have the same maximum tCTL value. This approach ensures that all AMD Ryzen™ processors have a consistent fan policy.

Specifically, the AMD Ryzen™ 7 1700X and 1800X carry a +20°C offset between the tCTL° (reported) temperature and the actual Tj° temperature. In the short term, users of the AMD Ryzen™ 1700X and 1800X can simply subtract 20°C to determine the true junction temperature of their processor. No arithmetic is required for the Ryzen 7 1700. Long term, we expect temperature monitoring software to better understand our tCTL offsets to report the junction temperature automatically.

The table below serves as an example of how the tCTL sensor can be interpreted in a hypothetical scenario where a Ryzen processor is operating at 38°C.

Product Name	True Junction Temp (Example)	tCTL Offset for Fan Policy	Temp Reported by tCTL
AMD Ryzen™ 7 1800X	38°C	20°C	58°C
AMD Ryzen™ 7 1700X	38°C	20°C	58°C
AMD Ryzen™ 7 1700	38°C	0°C	38°C

Power Plans

Users may have heard that AMD recommends the High Performance power plan within Windows® 10 for the best performance on Ryzen, and indeed we do. We recommend this plan for two key reasons:

1. Core Parking OFF: Idle CPU cores are instantaneously available for thread scheduling. In contrast, the Balanced plan aggressively places idle CPU cores into low power states. This can cause additional latency when un-parking cores to accommodate varying loads.
2. Fast frequency change: The AMD Ryzen™ processor can alter its voltage and frequency states in the 1ms intervals natively supported by the “Zen” architecture. In contrast, the Balanced plan may take longer for voltage and frequency (V/f) changes due to software participation in power state changes.

In the near term, we recommend that games and other high-performance applications are complemented by the High Performance plan. By the first week of April, AMD intends to provide an update for AMD Ryzen™ processors that optimizes the power policy parameters of the Balanced plan to favor performance more consistent with the typical usage models of a desktop PC.

Simultaneous Multi-threading (SMT)

Finally, we have investigated reports of instances where SMT is producing reduced performance in a handful of games. Based on our characterization of game workloads, it is our expectation that gaming applications should generally see a neutral/positive benefit from SMT. We see this neutral/positive behavior in a wide range of titles, including: Arma® 3, Battlefield™ 1, Mafia™ III, Watch Dogs™ 2, Sid Meier’s Civilization® VI, For Honor™, Hitman™, Mirror’s Edge™ Catalyst and The Division™. Independent 3^rd-party analyses have corroborated these findings.

For the remaining outliers, AMD again sees multiple opportunities within the codebases of specific applications to improve how this software addresses the “Zen” architecture. We have already identified some simple changes that can improve a game’s understanding of the "Zen" core/cache topology, and we intend to provide a status update to the community when they are ready.

Wrap-up

Overall, we are thrilled with the outpouring of support we’ve seen from AMD fans new and old. We love seeing your new builds, your benchmarks, your excitement, and your deep dives into the nuts and bolts of Ryzen. You are helping us make Ryzen™ even better by the day.  You should expect to hear from us regularly through this blog to answer new questions and give you updates on new improvements in the Ryzen ecosystem.