Intel Thread Director

One of the biggest criticisms that I’ve levelled at the feet of Intel since it started talking about its hybrid processor architecture designs has been the ability to manage threads in an intelligent way. When you have two cores of different performance and efficiency points, either the processor or the operating system has to be cognizant of what goes where to get the best result from the end-user. This requires doing additional analysis on what is going on with each thread, especially new work that has never been before.

To date, most desktop operating systems operate on the assumption that all cores and the performance of everything in the system is equal.  This changed slightly with simultaneous multithreading (SMT, or in Intel speak, HyperThreading), because now the system had double the threads, and these threads offered anywhere from zero to an extra 100% performance based on the workload. Schedulers were hacked a bit to identify primary and secondary threads on a core and schedule new work on separate cores. In mobile situations, the concept of an Energy Aware Scheduler (EAS) would look at the workload characteristics of a thread and based on the battery life/settings, try and schedule a workload where it made sense, particularly if it was a latency sensitive workload.

Mobile processors with Arm architecture designs have been tackling this topic for over a decade. Modern mobile processors now have three types of core inside – a super high performance core, regular high performance cores, and efficiency cores, normally in a 1+3+4 or 2+4+4 configuration. Each set of cores has its own optimal window for performance and power, and so it relies on the scheduler to absorb as much information as possible to determine the best way to do things.

Such an arrangement is rare in the desktop space - but now with Alder Lake, Intel has an SoC that has SMT performance cores and non-SMT efficient cores. With Alder Lake it gets a bit more complex, and the company has built a technology called Thread Director.

That’s Intel Thread Director. Not Intel Threat Detector, which is what I keep calling it all day, or Intel Threadripper, which I have also heard. Intel will use the acronym ITD or ITDT (Intel Thread Director Technology) in its marketing. Not to be confused with TDT, Intel’s Threat Detection Technology, of course.

Intel Threadripper Thread Director Technology

This new technology is a combined hardware/software solution that Intel has engineered with Microsoft focused on Windows 11. It all boils down to having the right functionality to help the operating system make decisions about where to put threads that require low latency vs threads that require high efficiency but are not time critical.

First you need a software scheduler that knows what it is doing. Intel stated that it has worked extensively with Microsoft to get what they want into Windows 11, and that Microsoft have gone above and beyond what Intel needed. This fundamental change is one reason why Windows 11 exists.

So it’s easy enough (now) to tell an operating system that different types of cores exist. Each one can have a respective performance and efficiency rating, and the operating system can migrate threads around as required. However the difference between Windows 10 and Windows 11 is how much information is available to the scheduler about what is running.

In previous versions of Windows, the scheduler had to rely on analysing the programs on its own, inferring performance requirements of a thread but with no real underlying understanding of what was happening. Windows 11 leverages new technology to understand different performance modes, instruction sets, and it also gets hints about which threads rate higher and which ones are worth demoting if a higher priority thread needs the performance.

Intel classifies the performance levels on Alder Lake in the following order:

  1. One thread per core on P-cores
  2. Only thread on E-cores
  3. SMT threads on P-cores

That means the system will load up one thread per P-core and all the E-cores before moving to the hyperthreads on the P-cores.

Intel’s Thread Director controller puts an embedded microcontroller inside the processor such that it can monitor what each thread is doing and what it needs out of its performance metrics. It will look at the ratio of loads, stores, branches, average memory access times, patterns, and types of instructions. It then provides suggested hints back to the Windows 11 OS scheduler about what the thread is doing, whether it is important or not, and it is up to the OS scheduler to combine that with other information about the system as to where that thread should go. Ultimately the OS is both topologically aware and now workload aware to a much higher degree.

Inside the microcontroller as part of Thread Director, it monitors which instructions are power hungry, such as AVX-VNNI (for machine learning) or other AVX2 commands that often draw high power, and put a big flag on those for the OS for prioritization. It also looks at other threads in the system and if a thread needs to be demoted, either due to not having enough free P-cores or for power/thermal reasons, it will give hints to the OS as to which thread is best to move. Intel states that it can profile a thread in as little as 30 microseconds, whereas a traditional OS scheduler may take 100s of milliseconds to make the same conclusion (or the wrong one).

On top of this, Intel says that Thread Director can also optimize for frequency. If a thread is limited in a way other than frequency, it can detect this and reduce frequency, voltage, and power. This will help the mobile processors, and when asked Intel stated that it can change frequency now in microseconds rather than milliseconds.

We asked Intel about where an initial thread will go before the scheduling kicks in. I was told that a thread will initially get scheduled on a P-core unless they are full, then it goes to an E-core until the scheduler determines what the thread needs, then the OS can be guided to upgrade the thread. In power limited scenarios, such as being on battery, a thread may start on the E-core anyway even if the P-cores are free.

For users looking for more information about Thread Director on a technical, I suggest reading this document and going to page 185, reading about EHFI – Enhanced Hardware Frequency Interface. It outlines the different classes of performance as part of the hardware part of Thread Director.

It’s important to understand that for the desktop processor with 8 P-cores and 8 E-cores, if there was a 16-thread workload then it will be scheduled across all 8 P-cores with 8 threads, then all 8 E-cores with the other 8 threads. This affords more performance than enabling the hyperthreads on the P-cores, and so software that compares thread-to-thread loading (such as the latest 3DMark CPU Profile test) may be testing something different compared to processors without E-cores.

On the question of Linux, Intel only went as far to say that Windows 11 was the priority, and they’re working upstreaming a variety of features in the Linux kernel but it will take time. An Intel spokesperson said more details closer to product launch, however these things will take a while, perhaps months and years, to get to a state that could be feature-parity equivalent with Windows 11.

One of the biggest questions users will ask is about the difference in performance or battery between Windows 10 and Windows 11. Windows 10 does not get Thread Director, but relies on a more basic version of Intel’s Hardware Guided Scheduling (HGS). In our conversations with Intel, they were cagy to put any exact performance differential metrics between the two, however based on understanding of the technology, we should expect to see better frequency efficiency in Windows 11. Intel stated that even though the new technology in Windows 11 will mean threads will move more often than in Windows 10, potentially adding latency, in their testing it wasn’t in any way human perceivable. Ultimately because the Win11 configuration can also optimize for power and efficiency, especially in mobile, Intel puts the win on Windows 11.

The only question is if Windows 11 will launch in time for Alder Lake.

Alder Lake: Intel 12th Gen Core Golden Cove Microarchitecture (P-Core) Examined
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  • TristanSDX - Thursday, August 19, 2021 - link

    "decreasing the manufacturing cost for Alder Lake, by using all the defect chips and reserving the good ones for Sapphire Rapids."
    Alder Lake and Shapire Rapids are two totally diffrerent chips
  • mode_13h - Thursday, August 19, 2021 - link

    > Designed as its third generation of vector instructions

    Depends on how you're counting. First is definitely MMX. That was extended in a few subsequent CPUs, but they didn't call those extensions MMX2 or anything. MMX was strictly integer, however, and total vector width was 64 bits. MMX had the annoying feature of reusing the FPU registers, which complicated mixing it with x87 code and basically requiring a state reset, when going from MMX -> x87 code.

    Then, SSE came along and added single-precision floating-point. It also added a distinct set of vector registers, which were 128 bits. Finally, it included scalar single-precision arithmetic operations, beginning the era of x87's obsolescence.

    SSE2 followed with double-precision and integer operations, making MMX obsolete and further replacing x87 functionality.

    SSE3, the wondefully-named SSSE3, and a couple rounds of SSE4 came along, but all were basically just rounds of various additions to flesh out what SSE/SSE2 introduced.

    Then, AVX was introduced as something of a replacement for SSE. AVX registers are 256 bits. Like SSE, AVX was initially just including single-precision floating-point support. And like SSE2, AVX2 added double-precision and integer operations.

    Then, Xeon Phi (2nd gen) and Skylake-SP introduced the first variations on AVX-512 support. You can see what a mess AVX-512 is, here:

    Anyway, AVX-512 should be considered Intel's FOURTH family of vector computing instructions, in x86. I think the first time they dabbled with vector instructions was in the venerable i860 - a very cool, but also fairly problematic step in the history of computing.

    > (AVX is 128-bit, AVX2 is 256-bit, AVX512 is 512-bit),

    No, not at all. The register width for AVX and AVX2 is 256 bits, as I explained above.

    However, even that is a slight simplification. AVX introduced some refinements in vector programming, such as a more compiler-friendly 3-operand format. Therefore, it was meant to subsume SSE usage, and included support for 128-bit operations. Similarly, AVX-512 introduced further refinements and the capability to use it on 128-bit and 256-bit operands.

    For more, see:
  • mode_13h - Thursday, August 19, 2021 - link

    One more correction:

    > Some workloads can be vectorised – multiple bits of consecutive data all require
    > the same operation, so you can pack them into a single register and perform it
    > all at once with a single instruction.

    Intel's vector instruction extensions aren't strictly SIMD. They include horizontal operations that you don't see in classical SIMD processors or most GPUs.
  • mode_13h - Thursday, August 19, 2021 - link

    > One could argue that if the AVX-512 unit was removed from the desktop
    > cores that they would be a lot smaller

    That's what I thought, but the area overhead it added to a Skylake-SP core was estimated at a mere 11%.

    Of course, we can't yet know how much of Golden Cove it occupies, but still probably somewhere in that ballpark.
  • mode_13h - Thursday, August 19, 2021 - link

    > Intel isn’t even supporting AVX-512 with a dual-issue

    Perhaps because AVX-512 doubled the number and size of vector registers. So, just the vector register file alone would grow 4x in size.
  • Schmide - Thursday, August 19, 2021 - link

    64bit packed doubles are in avx as are some 64bit ints. AVX2 filled in a lot of gaps such as full vector operands and reorders. So as much as AVX2 finished off the 32 and 64bit ints (epi) functions. There was already a fair amount in avx.
  • Schmide - Thursday, August 19, 2021 - link

    not to be misleading. There were really no usable int functions in avx other than load and store.
  • maroon1 - Thursday, August 19, 2021 - link

    Gracemont beats skylake ???? Really ? I'm reading the article correctly

    So these small cores are actually very powerful !!
  • vegemeister - Thursday, August 19, 2021 - link

    The hypothetical 8% increase in peak performance seems like wishful thinking to me. The chart looks like "graphic design" marketing wank, not plotted data. I would only go by the printed numbers. That is, at an operating point that matches Skylake peak performance, Gracemont cores use less than 60% of Skylake's power, and if you ran Skylake at that same power, it would have less than 60% of Gracemont's performance.
  • mode_13h - Thursday, August 19, 2021 - link

    > I would only go by the printed numbers.

    Okay, so are those numbers you used hypothetical, or where did you see 60%?

    Also, there's no fundamental reason why the ISO-power and ISO-performance deltas should match.

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