Revision as of 01:02, 25 April 2017

Skylake (SKL) is Intel's successor to Broadwell, a 14 nm process microarchitecture for mainstream desktops, servers, and mobile devices. Skylake succeeded the short-lived Broadwell which experienced severe delays. Skylake is the "Architecture" phase as part of Intel's PAO model. The microarchitecture was developed by Intel's R&D center in Haifa, Israel.

For desktop and mobile, Skylake is branded as 6th Generation Intel Core i3, Core i5. and Core i7 processors. For server class processors, Intel branded it as Xeon E3 v5, Xeon E5 v5, and Xeon E7 v5.

Codenames

Release Dates

Skylake was first demonstrated at the 2014 Intel Developer Forum in San Francisco on September 9 with the goals of launching in the second half of 2015.

Process Technology

Main article: Broadwell § Process Technology

Skylake uses the same 14 nm process used for the Broadwell microarchitecture.

Compiler support

Architecture

Overall Skylake builds upon Intel's previous microarchitecture, Broadwell, but includes a more beefed up front end, more optimized execution engine, and numerous number of smaller enhancements. Intel designed Skylake to encompass a wide range of devices and applications with a large emphasis on mobile with models ranging from as low as 4.5 W to as high as 100 W.

Key changes from Broadwell

• 8x performance/watt over Nehalem (Up from 3.5x in Haswell)
• Bus/Interface to Chipset
  • DMI 3.0 (from 2.0)
    • Skylake S and Skylake H cores, connected by 4-lane DMI 3.0
    • Skylake Y and Skylake U cores have chipset in the same package (simplified OPIO)
    • Increase in transfer rate from 5.0 GT/s to 8.0 GT/s (~3.93GB/s up from 2GB/s) per lane
    • Limits motherboard trace design to 7 inches max from (down from 8) from the CPU to chipset
• Front End
  • Larger legacy pipeline delivery (5 µOPs, up from 4)
    • Another simple decoder has been added.
  • Larger IDQ delivery (6 µOPs, up from 4)
  • 2.28x larger allocation queue (64/thread, up from 28/thread)
  • Improved branch prediction unit
• Execution Engine
  • Larger re-order buffer (224 entries, up from 192)
  • Larger scheduler (97 entries, up from 64)
    • Larger Integer Register File (180 entries, up from 160)
  • Larger store buffer (56 entries, up from 42)
• Memory
  • Support for faster DDR-2400 memory
  • L2$ was changed from 8-way to 4-way set associative
  • L3$ re-gained 512 KiB/core (See §eDRAM architectural changes for the reason)
  • A new coherent cache fabric implementation
    • doubles the throughput of the last level cache (LLC, L3$ in this case) miss handling
    • 50% improvement in bandwidth/watt
    • new eDRAM cache architecture for higher bandwidth
• TLBs
  • ITLB
    • 4 KiB page translations was changed from 4-way to 8-way associative
  • STLB
    • 4 KiB + 2 MiB page translations was changed from 6-way to 12-way associative
• Electrical
  • The fully integrated voltage regulator (FIVR) is moved back to the motherboard
    • Originally intended to be a cost-cutting measure by moving the FIVR on-die as well as making it more efficient, the move resulted in unintentionally making the FIVR the limiting factor when it came to overclocking.
• Testability
  • New support for Direct Connect Interface (DCI), a new debugging transport protocol designed to allow debugging of closed cases (e.g. laptops, embedded) by accessing things such as JTAG through any USB 3 port.

CPU changes

• Most ALU operations have 4 op/cycle 1 for 8 and 32-bit registers. 64-bit ops are still limited to 3 op/cycle. (16-bit throughput varies per op, can be 4, 3.5 or 2 op/cycle).
• MOVSX and MOVZX have 4 op/cycle throughput for 16->32 and 32->64 forms, in addition to Haswell's 8->32, 8->64 and 16->64 bit forms.
• ADC and SBB have throughput of 1 op/cycle, same as Haswell.
• Vector moves have throughput of 4 op/cycle (move elimination).
• Not only zeroing vector vpXORxx and vpSUBxx ops, but also vPCMPxxx on the same register, have throughput of 4 op/cycle.
• Vector ALU ops are often "standardized" to latency of 4. for example, vADDPS and vMULPS used to have L of 3 and 5, now both are 4.
• Fused multiply-add ops have latency of 4 and throughput of 0.5 op/cycle.
• Throughput of vADDps, vSUBps, vCMPps, vMAXps, their scalar and double analogs is increased to 2 op/cycle.
• Throughput of vPSLxx and vPSRxx with immediate (i.e. fixed vector shifts) is increased to 2 op/cycle.
• Throughput of vANDps, vANDNps, vORps, vXORps, their scalar and double analogs, vPADDx, vPSUBx is increased to 3 op/cycle.
• vDIVPD, vSQRTPD have approximately twice as good throughput: from 8 to 4 and from 28 to 12 cycles/op.
• Throughput of some MMX ALU ops (such as PAND mm1, mm2) is decreased to 2 or 1 op/cycle (users are expected to use wider SSE/AVX registers instead).

New GPU Features & Changes

• Adaptive scalable texture compression (ASTC)
• 16x multi-sample anti-aliasing (MSAA)
• Post depth test coverage mask
• Floating point atomics (min/max/cmpexch)
• Min/max texture filtering
• Multi-plane overlays

eDRAM architectural changes

In Broadwell, the eDRAM was statically attached to the LLC (last level cache, L3$), effectively stealing half a Mebibyte per core in the process, but behaving as an architectural true level 4 cache. This was fundamentally changed in Skylake. In Skylake, Intel removed the eDRAM from the LLC to its own array, re-freeing the 512 KiB (hence the 1.5 MiB/core in Broadwell and 2 MiB back in Skylake), but also removing the undesired dependency between the capacity of the eDRAM and the number of cores. Skylake's cache is effectively no longer a true level 4 cache but rather a memory side cache. This has a number of benefits such as that each and every memory access that goes through the memory controller gets looked up in the eDRAM. On a satisfied hit, the value is obtained from there. On a miss, a value gets allocated and stored in the eDRAM (subject to a number of restrictions, for example no I/O devices requests get cached on the eDRAM).

Skylake vs Broadwell eDRAM Architecture
Broadwell	Skylake

The new eDRAM changes mean it's no longer architectural - capable of caching any data (including "unreachable memory", display engines, effectively any memory transfer not bound by software restrictions) and is entirely invisible to software (one exception noted later) in terms of coherency (note that no flushing is thus necessary to maintain coherency), ordering, or other organizational details. For optimal graphics performance, the graphics driver may decide to limit certain memory accesses to only the eDRAM, only the LLC, or in both of them.

Graphics

• Improved underlying implementation of the memory QoS for higher resolution displays and the integrated image signal processor (ISP)
  • Allow for higher concurrent bandwidth
• Skylake retires VGA support, multi-monitor support for up to 3 displays via HDMI 1.4, DP 1.2, and eDP 1.3 interfaces.
• Direct X 12
• OpenCL 2.0
• OpenGL 4.4
• Up to 24 EUs GT2 (same as Haswell); 48 EUs for GT3, and up to 72 EUs on Iris Pro Graphics
  • 1,152 GFLOPS

IGP	Execution Units	GT	eDRAM	Series (Y/U/H/S)
HD Graphics	12	2+1	-	Y
HD Graphics 510	12	2+2	-	U/S
HD Graphics 515	24	2+2	-	Y
HD Graphics 520	24	4+2 2+2	-	U
HD Graphics 530	24	4+2 2+2	-	H/S
HD Graphics P530	24	4+2	-	H
Iris Graphics 540	48	2+3e	64 MiB	U
Iris Graphics 550	48	2+3e	64 MiB	U
Iris Pro Graphics 580	72	4+4e	128 MiB	H

New instructions

Main article: See §Added instructions for the complete list

Skylake introduced a number of new instructions:

• SGX - Software Guard Extensions
• MPX -Memory Protection Extensions
• AVX-512 - Advanced Vector Extensions 512 (Only on high-end Xeon models (SKX))

"Speed Shift" (new power management)

Ever since the introduction of the modern power management unit on a microprocessor, it was effectively the role of the operating system to determine the desired operating frequency and voltage (i.e. a p-state) for the current workload. When the CPU utilization peaked, it was the role of the operating system to bump up the frequency to help cope with it. The issue has always been the limitation of the operating system. One such major limitation is the granularity of the operating system response time - usually in the 10s of milliseconds (anything lower than that would likely be too intensive and would not yield better result). A second major issue is that the operating system doesn't have an instantaneous observation of the microarchitectural behavior of the workload.

Intel introduced Speed Shift with Skylake, a new methodology for quickly alternating core frequencies in response to power loads. Intel introduced a new unit called Package Control Unit (PCU) which is effectively a full fledged microcontroller (containing power management logic and firmware) that collects and tracks many internal SoC statistics as well as external power telemetry (e.g. Psys and iMon). PCU is also capable of interfacing with the OS, BIOS, and DPTF. Speed Shift improves the performance of frequency shifting by off-loading the control from the operating system to the PCU.

Speed Shift effectively eliminates the need for the OS to manages the P-states - though it does have the final say (unless special exceptions occur such as thermal throttling). Intel calls this "autonomous P-state", allowing Speed Shift to kick in in a matter of just ~1 millisecond (whereas the operating system-based p-states control can be as slow as 30 ms). Speed Shift effectively reduces hitting peak frequency in around ~30 ms from over 100 ms (OS-based implementation as before). While Speed Shift is capable of full range shift by default, the operating system can set the minimum QoS, maximum frequency and power/performance hints when desired. The final result should be higher performance and specially higher responsiveness at power constrained form factors.

Power of System (Psys)

Psys (Power of System) is a way for the PCU to monitor the performance and the total platform power provided to the chip. The chip uses a number of autonomous algorithms (one for "Low Range" and one for "High Range"). The Low Range algorithm frequency is lowered to conserve energy. Algorithm is capable of overriding the low P state - a state calculated ever millisecond based on the active workload and system characteristics. The High Range algorithm deals with elevating frequency for the benefit of increase performance (at the cost of increase energy/inefficiency). The exact ratio of ΔPower/ΔPerformance ≤ αPreference can be finely controlled via the OS and user preferences.

Other Power Optimization

Skylake includes a number of additional power optimization changes:

• AVX2 is now power gated - prior to Skylake, AVX2 was not power gated which meant it was susceptible to leakage. Starting with Skylake, those instruction are full power gated and turn off when not used.
• Many older/legacy underused resources have been downscaled.
• Various scenario-based power optimizations were done, including:
  • Idle power is reduced further
  • C1 state power reduction (improved dynamic capacitance C_dyn)

Overall Skylake enjoys better performance/Watt per core for 8x performance/watt over Nehalem.

Block Diagram

Entire SoC Overview

Individual Core

Gen9

See Gen9#Gen9.

Memory Hierarchy

Other than a few organizational changes (e.g. L2$ went from 8-way to 4-way set associative), the overall memory structure is identical to Broadwell/Haswell.

• Cache
  • L1I Cache:
    • 32 KiB 8-way set associative
      • 64 B line size
      • shared by the two threads, per core
  • L1D Cache:
    • 32 KiB 8-way set associative
    • 64 B line size
    • shared by the two threads, per core
    • 4 cycles for fastest load-to-use (simple pointer accesses)
      • 5 cycles for complex addresses
    • 64 Bytes/cycle load bandwidth
    • 32 Bytes/cycle store bandwidth
    • Write-back policy
  • L2 Cache:
    • unified, 256 KiB 4-way set associative
    • 12 cycles for fastest load-to-use
    • 64B/cycle bandwidth to L1$
    • Write-back policy
  • L3 Cache/LLC:
    • Up to 2 MiB Per core, shared across all cores
    • Up to 16-way set associative
    • Write-back policy
    • Per each core:
      • Read: 32 B/cycle (@ ring clock)
      • Write: 32 B/cycle (@ ring clock)
    • 42 cycles for fastest load-to-use
  • Side Cache:
    • 128 MiB eDRAM
    • Per package
    • Only on the Iris Pro GPUs
    • Read: 32B/cycle (@ eDRAM clock)
    • Write: 32B/cycle (@ EDRAM clock)
  • System DRAM:
    • 2 Channels
    • 8B/cycle/channel (@ memory clock)
    • 42 cycles + 51 ns latency

Skylake TLB consists of dedicated level one TLB for instruction cache and another one for data cache. Additionally there is a unified second level TLB.

• TLBs:
  • ITLB
    • 4 KiB page translations:
      • 128 entries; 8-way set associative
      • dynamic partition; divided between the two threads
    • 2 MiB / 4 MiB page translations:
      • 8 entries; fully associative
      • Duplicated for each thread
  • DTLB
    • 4 KiB page translations:
      • 64 entries; 4-way set associative
      • fixed partition; divided between the two threads
    • 2 MiB / 4 MiB page translations:
      • 32 entries; 4-way set associative
      • fixed partition
    • 1G page translations:
      • 4 entries; 4-way set associative
      • fixed partition
  • STLB
    • 4 KiB + 2 MiB page translations:
      • 1536 entries; 12-way set associative
      • fixed partition
    • 1 GiB page translations:
      • 16 entries; 4-way set associative
      • fixed partition

Overview

Skylake inherits much of the Core design philosophy which was enhanced significantly over the past number of architectures. Skylake, like its predecessor Broadwell, is also a dual-threaded and complex out-of-order pipeline. Skylake which builds on Broadwell incorporates large number of enhancements that has improved performance and efficiency in order to cover a large spectrum of devices from ultra-low power to high-performance computing. Additionally, a large number of improvements were done to the integrated graphics and multimedia capabilities as well as a new set of security technologies were introduced.

Design goals & new goals

Skylake tries to address 4 major design goals: Scalability, Performance, Power, and Media & Graphics. Skylake started out as a "traditional client product" when initial design goals and development started back in 2010. At that time, Skylake was expected to cover products ranging from thin-and-lights all the way up to desktops. This translated to roughly 3x TDP scale and 2x form factor between the smallest and biggest models. With the introduction of Ultralight, Skylake design goals were extended to a new class of smaller form factors (down to 15 W). Skylake design goals were later further extended to more mobility segments and even smaller form factors. The final end result is a microarchitecture that now spans 20-fold TDP scale (from 3.5 W all the way to 80 W+) and up to 4-fold form factor between the lowest power model and the highest performance model. Intel claims that Skylake also succeeded in reducing power by 40-60% during important workloads such as video, graphics, and idle power which especially affect models where battery life is absolutely critical.

It's interesting to note that Skylake's end result managed to overlap and to some degree exceed Intel's own ultra-low power (ULP) series of microarchitectures (i.e. Silvermont and Goldmont).

Product Development Vectors:

• Form factor reduction - Emphasis was placed on reduction of form factor which includes both actual die size and package size.
• Platform minimization - Effort was spent on reducing the overall platform size include reduction of system board size, components, and power.
• Better life scenario power reduction - (mobile segment) The reduction of power during critical workloads such as video playback, video conferencing, and various other multimedia applications where the CPU itself is mostly idle.
• IA performance - Improvements to both power and performance of the CPU core
• IGP performance - Improvement to both power and performance of the GPU
• New Security technology - Better protection against hardware and software attacks

SoC design

The Skylake system on a chip consists of a five major components: CPU core, LLC, Ring interconnect, System agent, and the integrated graphics. The image shown on the right, presented by Intel at the Intel Developer Forum in 2015, represents a hypothetical model incorporating all available features Skylake has to offer (i.e. superset of features). Skylake features an improved core (see § Pipeline) with higher performance per watt and higher performance per clock. The number of cores depends on the model, but mainstream mobile models are typically dual-core while mainstream desktop models are typically quad-core with dual-core desktop models still offered for value models (e.g. Celeron). Accompanying the cores is the LCC (last level cache or L3$ as seen from the CPU prospective). On mainstream parts the LLC consists of 2 MiB for each core with lower amounts for value models. Connecting the cores together is the ring interconnect. The ring extends to the GPU and the system agent as well. Intel further optimized the ring in Skylake for low-power and higher bandwidth.

Accompanying the cores is the Gen9 integrated graphics unit which comes in a number of different tiers ranging from just 12 execution units (used in the ultra-low power models) all the way the GT4 (Gen9 § Pipeline) with 72 execution units boasting a peak performance of up to 2,534.4 GFLOPS (HF) / 1,267.2 GFLOPS (SP) on the highest-end workstation model. The two highest-tier models are also accompanied by dedicated eDRAM ranging from 64 GiB to 120 GiB in capacity. The eDRAM is packaged along with the SoC in the same package.

On the other side is the System Agent (SA) which houses the various functionality that's not directly related to the cores or graphics. Skylake features an upgraded integrated memory controller (IMC) with most mainstream models supporting faster memory and dual-channel DDR4. The SA in Skylake also includes the Display Controller which now supports higher resolution displays with up to three displays for all mainstream models.

The SA also incorporates up to 20 lanes of PCIe with lesser amounts depending on the model. Of the 20 lanes, x16 PCIe lanes are offered for an external dedicated graphics hookup with the other four lanes reserved for communication with the southbridge chipset over Intel's new proprietary DMI 3.0 bus. The upgrade from DMI 2.0 (used in previous architectures) to 3.0 increased the bandwidth by 60% (8.0 GT/s from 5). For some models where form factor is exceptionally critical, such as those used for ultralight device, the chipset is packaged along with the SoC utilizing an on-package-interconnect (OPI) instead.

The last component of the System Agent and an entirely new addition in Skylake is the Image Processing Unit (IPU) which incorporates an image signal processor (ISP) on-die. The IPU is only available on mobile models and was added in order to improve and streamline (i.e. form factor and consistent set of features and quality) the implementation and performance of tablets and 2-in-1s. Previously this would require the assistance of an external component and the implementations varied by designer.

Pipeline

Front-end

Skylake had its front-end bandwidth of µOPs deliver to the execution engine increase. The µOPs Cache now delivers 6 µOPs per clock (previously the cache only delivered 4 µOPs/clock), likewise the decoders now deliver 5 µOPs/clock (previously they were capable of only 4µOPs/clock). The branch predictor has also been improved. The branch predictor now has reduced penalty (i.e. lower latency) for wrong direct jump target prediction. Additionally, the instruction fetch unit is capable of looking much deeper into the stream of bytes. Finally, the allocation queue which interfaces between the front-end (in-order) and the execution engine (out-of-order) itself has been more than doubled to accommodate 64/thread (from 28/thread in Broadwell).

Execution engine

Like the front-end, the execution engine's ReOrder buffer has been increased to 224 entries (from 192 in Broadwell) in order to extract more instruction-level parallelism. Likewise the scheduler itself was increased considerably to 97 entries (from 64 in Broadwell). The integer register file was also slightly increased from 160 entries to 180.

The scheduler had its ports rearranged to better balance various instructions. For example, divide and sqrt instructions latency and throughput were improved. The latency and throughput of floating point ADD, MUL, and FMA were made uniformed at 4 cycles with a throughput of 2 ops/clock. Likewise the latency of AES instructions were significantly reduced from 7 cycles down to 4.

Execution Units

Memory subsystem

Skylake has had its store buffer enlarged to 56 entries (up from 42 in Broadwell). Special care was taken to reduce the penalty for page-split loads: previously scenarios involving page-split loads were thought to be rarer than they actually are. This was addressed in Skylake with page-split loads are now made equal to other splits loads. Expect page split load penalty down to 5 cycles from 100 cycles in Broadwell. The average latency to forward a load to store has also been improved and stores that miss in the L1$ generate L2$ requests to the next level cache much earlier in Sklake than before.

The bandwidth from L2$ to L3$ has been improved and write bandwidth from L2$ to L3$ has also been increased from 4 cycles/line to 2 cycles/line.

Clock domains

Skylake is divided into a number of clock domains, each controlling the clock frequency of their respective unit in the processor. All clock domains are some multiple of the [virtual] bus clock (BCLK).

• BCLK - Bus Clock - The system bus interface frequency (once upon a time referred to the actual FSB speed, it now serves as only a base clock reference for all other clock domains). The bus clock is 100 MHz.
• Core Clock - The frequency at which the core and the L1/L2 caches operate at. (Frequency depends on the model and is represented as a multiple of BCLK).
• Ring Clock - The frequency at which the ring interconnect and LLC operate at. Data from/to the individual cores are read/written into the L3 at a rate of 32B/cycle operating at Ring Clock frequency.
• IGP Clock - The frequency at which the integrated graphics (Gen9 GPU) operates at. Data from/to the GPU are read/written into the LLC at a rate of 64B/cycle operating at this frequency as well.
• eDRAM Clock - The frequency at which the embedded DRAM operates at (only available for certain models). Data is read/written from/to the LLC at a rate of 32B/cycle operating at this frequency as well.
• MemClk - Memory Clock - The frequency at which the system DRAM operates at. DRAM data is transfered at a rate of 8B/cycle operating at MemClk frequency.

Die

Skylake desktop and mobile come and 2 and 4 cores. Each variant has its own die. One of the most noticeable changes on die is the amount of die space allocated to the GPU. The major components of the die is:

• System Agent
• CPU Core
• Ring bus interconnect
• Memory Controller

System Agent

The System Agent (SA) contains the Image Processing Unit (IPU), the Display Engine (DE), and the I/O bus. Note that the mainstream desktop (i.e., quad-core die) does not have an IPU (The memory controller actually occupies a portion of where it would otherwise be).

Dual-Core Die

Quad-Core Die

Integrated Graphics

The integrated graphics takes up the largest portion of the die. The normal dual-core and quad-core dies come with 24 EU Gen9.5 GPU (with 12 units disabled on the low end models).

Dual-core

Die shot of the dual-core Skylake processors. Those are found in mobile models, and entry-level/budget processors:

• 14 nm process
• 11 metal layers

Quad-core

Die shot of the quad-core Skyllake processors. Those are found in almost all mainstream desktop processors.

• 14 nm process
• 11 metal layers

Added instructions

SGX - Software Guard Extensions

MPX - Memory Protection Extensions

AVX-512 - Advanced Vector Extensions 512; These instructions can only be found on selected high-end Xeon models (codename SKX)

Cores

This section is empty; you can help add the missing info by editing this page.

All Skylake Chips

Documents

• 6th Gen Intel® Core™ Mobile Processor Family: Platform Brief
• 6th Gen Intel® Core™ processor family and Intel® Xeon® processors Factsheet
• 6th Gen Intel® Core™ Processors: Y-series, U-series, and H-series Product Brief
• 6th Gen Intel® Core™ vPro™ Processor Family Product Brief
• 6th Generation Intel® Core™ Desktop Processors i7-6700K and i5-6600K Product Brief

See also

• AMD Zen