Revision as of 01:26, 26 January 2017

Gen9 LP (Generation 9 Low Power) is the microarchitecture for Intel's graphics processing unit utilized by Skylake-based microprocessors. Gen9 LP is the successor to Gen8 LP used by Broadwell. The Gen9 microarchitecture is designed separately by Intel and then integrated onto the same Skylake SoC die.

Codenames

Various models support different Graphics Tiers (GT) which provides different levels of performance. Some models also support an additional eDRAM side cache.

Models

Hardware Accelerated Video

Process Technology

Main article: Broadwell § Process Technology

Gen9 LP are part of the Skylake SoC die which uses the same 14 nm process used for the Broadwell microarchitecture.

Architecture

Gen9 LP presents a large departure from the Gen8 LP and previous architectures.

Key changes from Gen8 LP

• Architecture is drastically different
  • Gen9 LP is composed of 3 truely independent major components: Display block, Unslice, and the Slice.
• Unslice
  • Now sits on its own power gating/clock domain
    • Capable of running at higher speeds if the situation allows (irrespective of slice clock)
    • Can allow for pure fixed media alone
  • Higher throughput
  • Tessellator AutoStrip
  • Fixed function video encoder in the Quick Sync engine
  • codec (decode&encode) support for HEVC, VP8, MJPEG
  • RAW imaging capabilities
• Slice
  • L3 Cache
    • Increased to 768 KiB/slice (up from 576 KiB/slice)
    • Request queue size was increased
• Subslice
  • Adaptive scalable texture compression (ASTC)
  • 16x multi-sample anti-aliasing (MSAA)
  • Post depth test coverage mask
  • Multi-plane overlays
  • Texture samplers now natively support an NV12 YUV
  • Preemption of execution is now supported at the thread level
  • Round robin scheduling of threads within an execution unit.
  • new native support for the 32-bit float atomics operations of min, max, and compare/exchange.
  • 16-bit floating point capability is improved with native support for denormals and gradual underflow
• L4$
  • The eDRAM is now a side cache instead of an L4$ like it was in Gen8 LP. (See Skylake §eDRAM architectural changes for the reason)
  • Side-cache eDRAM was moved into the system agent adjacent to the display controller

Block Diagram

Entire SoC Overview

Gen9 LP

This block is for the most common setup, which is GT2 with 24 execution units.

Individual Core

See Skylake#Individual_Core.

Display

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

Unslice

The Unslice is one of Gen9's major components and is responsible for the fixed-function geometry capabilities, fixed-function media capabilities, and it provides the interface to the memory fabric. One of the big changes in Gen9 is that the Unslice now sits on its own power/clock domain. This change allows the Unslice to operate at its own speed provided higher on-demand performance when desired. This change has a number of other benefits such as being able to turn off the slices (one or more) when they're not used in cases where pure fixed-function media is used. Additionally, the Unslice is now capable of running at a higher clock while the slice can run at a slower clock when the scenario demands it (such as in cases where higher fixed-function geometry or memory demands occur).

The Command Stream (CS) unit manages the the flow of execution for the FF Pipeline (3D Pipeline) and the Media pipelines. The CS unit performs the switching between pipelines and forwarding command streams to the different stages. Data in the pipeline are passed to the next unit using a messaging network. Messages can be passed directly through registers or by using the URB. The Command Stream also manages the allocation of the URB and supports the Constant URB Entry (CURB) function. The Unified Return Buffer (URB) is globally shared and is explicitly addressed. The pipeline's fixed-function blocks have both read and write access to the URB, additionally the shader cores have write access to the URB.

The media general-purpose pipeline consists of two fixed-function units: Video Front End (VFE) and the Thread Spawner (TS). The VFE unit handles the interfacing with the Command Streamer, writes thread payload data into the Unified Return Buffer, as well as prepares threads to be dispatched through TS unit. The VFE unit also contains the hardware Variable Length Decode (VLD) engine for MPEG-2 video decode. The TS unit is primarily responsible for interfacing with the Thread Dispatcher (TD) unit which is responsible for spawning new root-node parent threads originated from VFE unit and for spawning child threads (either leaf-node child threads or branch-node parent thread).

3D Pipeline Stages

Slice

Slices are a cluster of subslices. For most configurations in Gen9, 3 subslices are aggregated into 1 slice to form a total of 24 execution units (depending on the model, some low end models do have less). The slice incorporates the thread dispatch routine, level 3 cache (L3$), a highly banked shared local memory structure, fixed function logic for atomics and barriers, and a number of fixed-function units for various media capabilities. The Global Thread Dispatcher (GTD) is responsible for load balancing thread distribution across the entire device. The global thread dispatcher works in concert with local thread dispatchers in each subslice.

Scalability

Gen9 can scale from 1 to 3 slices producing SKUs ranging from 12 to 72 execution units (note that the 12 EUs are formed from half a slice effectively).

GT1 (ULP)

GT1 is the most compact configuration offering two benefits: reduced cost and reduced power. GT1 is made of 1 slice containing 2 subslices with 6 EUs/subslice for a total of 12 EUs. With the scale-down, GT1 changes the ratio to 6:1 EU:sampler ratio. Note that this does retains the same ratio of 12 texels/clock and 8 pixels/clock at the backend. This configuration is better suited for some of the low power worlkload (e.g. ASTC-LDR+HDR, ETC1/2 compression). Note that software stack remains unchanged compared to the larger models.

GT1.5

GT1.5, offers 3 subslices of 6 EUs each for a total of 18 EUs.

GT2

GT2 is the standard configuration consisting of 1 slice with 3 subslices and 8 EU/subslice for a total of 24 EUs.

GT3

GT3 consists of 2 slices with 3 subslices in each and 8 EU/subslice for a total of 48 EUs.

Halo (GT4)

Codename Halo (GT4) is the most complex configuration offering the highest execution units count. Halo incorporates 3 slices with 3 subslices/slice and 8 EU/subslice for a total of 72 EUs.