Tremont: A Wider Front End and Caches

For users that have been following our analysis of the Core microarchitecture, it has been hard not to notice that Intel’s design for that family has been, among other things, to continually go wider and wider. This means more instructions in flight, larger caches, bigger buffers, bigger TLBs, more execution ports, and support for more instructions. Going wider isn’t the only thing: the microarchitecture also has to be clever, trying to maximise utilisation, as going wider does nothing for a simple stream of commands.

With Atom, going wider is a key part of the design for Tremont, but Intel has taken a couple of fundamentally different steps to manage exactly what is going on.

A Key Uplift: Fetch and Predict

Another major jump for the Atom microarchitecture are the prefetchers and branch predictors. Intel states that rather than iterate the design from Goldmont Plus, they have transplanted a large part of the prefetchers and branch predictors from the Core design on Sunny Cove. The design isn’t identical according to Intel, due to die area and power concerns, but Intel states that the principles are similar and elements like branch prediction history tables are ‘of the same order’ as the Core design.

Intel states that there is no penalty for an L1 prediction, and that the L2 prediction penalty is smaller than previous generations. 

A Key Differentiator: Decode Engines

On the face of it, we have a 6-wide decode engine pared with a 4-wide allocation/dispatch engine. This is, on paper, very odd: normally we would expect the decode and dispatch to be equal in width, or at least be able to dispatch more than can be decoded in order to ensure that the re-order buffer doesn’t overflow. With the latest Core microarchitecture, called Sunny Cove, we have a 4-to-6 wide decode that also supports a micro-op cache, which all leads into a large reorder buffer and a 10-wide dispatch to the back-end. Tremont is, by contrast, has the opposite ratio.

Saying that this is a 6-wide decode engine is actually slightly incorrect. What Intel has done here is split the decode into dual 3-wide engines instead.

Each decode engine, when dealing with different branch predictions, can take a separate instruction stream. This allows for a higher average utilization across both of the 3-wide decode engines compared to a single 6-wide engine, but when a branch isn’t present it means that one of the decode engines can be clock gated to save power. For a single instruction stream, the Tremont design is actually only 3-wide decode, with a 4-wide dispatch.

(Technically Intel states that, through microcode, they can change the decode engines to act as a single 6-wide implementation rather than dual 3-wide engines. This won’t be configurable to the OEM, but based on demand Intel may make specific products for customers that request it.)

So just to clarify, Tremont does not have a micro-op cache. When discussing with Intel about the benefits of this dual decode engine design compared to having a micro-op cache, Intel stated that a micro-op cache can help utilize a wide-decode design better, but with a smaller per-engine decode size, they were able to see a performance uplift as well as save die area by using this dual-engine design. Intel declined to comment which one was better, but we were told that given the die size, power envelope of Atom, and the typical instruction flow of an Atom core, this design yielded the better combination of performance, power, and area.

Another improvement for Intel after the decode engines is the re-order buffer. Intel states that it can support 208 instructions, compared to 78 in Goldmont and 95 in Goldmont Plus, which is a sizeable uplift. Intel did not specify if Tremont has the ability to fuse instructions into micro-ops for the ROB (Goldmont did not), however there is a near 1:1 parity of instructions to micro-ops we were told.

Caches

Intel has also increased the size of its L1 data cache. The L1 instruction cache says at 32 KiB/core with 8-way associativity, but ever since the 22nm era, Intel has kept a 24 KiB/core L1 data cache on its Atom design. With Tremont, both the L1-I and L1-D are now a 32 KiB/core design with 8-way associativity. Intel states that its L1 data cache here has a 3-cycle latency, compared to Skylake which has a 32 KiB L1D at a 4-cycle latency, or Sunny Cove which has a 48 KiB L1D at a 5-cycle latency.

Intel Caches
AnandTech Tremont Goldmont+ Goldmont   Sunny Cove Skylake
Process 10+ 14 14   10+ 14++
Decode 2x3-wide 3-wide 3-wide   4-6 wide 4-5 wide
Allocate 4-wide 4-wide 3-wide   10-wide 8-wide
L1 Instruction 32 KiB/Core
8-way
32 KiB/Core
8-way
32 KiB/Core
8-way
  32 KiB/Core
8-way
32 KiB/Core
8-way
L1 Data 32 KiB/Core
8-way
24 KiB/Core
6-way
24 KiB/Core
6-way
  48 KiB/Core
12-way
32 KiB/Core
8-way
L1 Latency 3-cycle 3-cycle 3-cycle   5-cycle 4-cycle
L2 Cache 1.5-4.5 MiB
Per Module
12-18 way
1.0 MiB
Per Core
16-way
0.5-1.0 MiB
Per Core
16-way
  512 KiB
Per Core
8-way 
256 KiB
Per Core
4-way
L2 Latency 17-cycle 19-cycle 17-cycle   13-cycle 12 cycle

For the L2 cache, for most Atom cores in the past, this has essentially been a last-level cache split across all cores in a ‘module’. Depending on the generation will depend on the size of the module: for 22nm Silvermont, we saw an L2 cache of 512 KiB/core, which was increased with Goldmont up to 1.0 MB/core. With Tremont, Intel has specified that L2 will vary depending on the product, from 1.5 MiB to 4.5 MiB per module. A module for Tremont will be anything up to four cores, so we could see designs with a single core featuring 4.5 MiB of L2 cache, or a quad-core design with 1.5 MiB of L2. Within a module, all cores have access to the cache, however a core will not have access to the L2 in a different module. The L2 can be set through microcode as an inclusive or a non-inclusive cache.

Intel states that the L2 cache has an average 17-cycle latency, and the associativity will be a function of the size: 1.5 MB will be a 12-way design, while 4.5 MB will be an 18-way design. (We therefore assume that a 3.0 MB L2 will be 15-way.)

Tremont also adds support for a global L3 cache across modules. Both the L2 and L3 cache support QoS arrangements, allowing for data prioritization and memory bandwidth enforcement for specific threads or virtual machines. This is a technology that Intel introduced with Broadwell Xeons to help avoid ‘noisy neighbor’ environments in data centers.

Intel's new Atom Microarchitecture: The Tremont Core Tremont: A Wider Back End
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  • 29a - Thursday, October 24, 2019 - link

    Did Atom processors ever stop sucking? Reply
  • solidsnake1298 - Thursday, October 24, 2019 - link

    That depends on your needs. As a HTPC, starting with Apollo Lake (Goldmont) the iGPU was upgraded sufficiently that it can decode 4K HEVC. I haven't tested 4K HEVC, personally. But I have played 1080p60 HEVC without a single dropped frame. Reply
  • vladx - Friday, October 25, 2019 - link

    I have a Goldmont tablet, 4K HEVC works fine as long as the bitrate doesn't surpass the limits of its eMMC storage, in which case artefacts and stuttering is present. Maybe I should look into replacing it with a SSD if that's even possible. Reply
  • qap - Friday, October 25, 2019 - link

    Even the slowest eMMC storage can do 50MB/s sequential read. There is no way, you have 400Mbps+ HEVC video (and if that is the case, Atom is obviously not for you). The limit must be somewhere else. Most likely it supports hardware HEVC decoding up to some bitrate only and you are hitting this limit. Reply
  • vladx - Friday, October 25, 2019 - link

    400Mbps no, but I have some 100+ Mbps videos and most sit around 60 so it can definitely push the eMMC to its limits especially considering it also needs to run the OS processes at the same time. Reply
  • s.yu - Friday, October 25, 2019 - link

    A common confusion between B and b... Reply
  • eddman - Friday, October 25, 2019 - link

    Unless the storage is so crap that can't even sustain 12.5 MB/s (a.k.a 100 Mbps), it's probably the decoder itself that is unable to properly accelerate such high bit-rate videos. Reply
  • nathanddrews - Sunday, October 27, 2019 - link

    Quite a few eMMC implementations run off a USB 2.0 bus, so yes, it can bottleneck a system hard. Same thing frequently happens with networking components in devices. It will have AC/GbE, but can't reach those speeds. Reply
  • eddman - Sunday, October 27, 2019 - link

    Even a USB 2.0 eMMC should be able to sustain a 12-13 MB/s sequential read.

    It has to be the decoder. He doesn't know the difference between bit and byte and thinks 60 Mbps is too much for 50 MB/s.
    Reply
  • eek2121 - Monday, October 28, 2019 - link

    Not if the bus is shared with 2 network controllers, a bluetooth controller, etc. I haven't looked at how Atom is set up admittedly, but that is one of the major issues with SBCs. Everything hangs off the USB 2.0 bus. The USB 2.0 bus also can't really maintain true USB 2.0 speeds in quite a few cases due to hitting micro-usb power limits. Reply

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