AMD Ryzen 9 3900XT Review 73

AMD Ryzen 9 3900XT Review

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The architecture is no different from the Ryzen 9 3950X or any other "Matisse," so you can click on the button below to read about it, or skip that section if you're familiar with it.

AMD Zen 2 Microarchitecture Brief

The AMD Zen 2 Microarchitecture

AMD's third-generation Ryzen processors use the "Zen 2" microarchitecture. The second-generation Ryzen chips use an enhanced first-generation "Zen" derivative called "Zen+," which has process and boost algorithm improvements eke out roughly a 4% IPC uplift. With "Zen 2," AMD's key design goal is to finally beat Intel in the IPC game. IPC, or instructions per clock, is loosely used to denote a CPU core's performance at a given clock speed. For the past 15 or so years, Intel dominated AMD at IPC, while AMD attempted to make their processors competitive by cramming in more CPU cores than Intel at any given price point for competitive multi-threaded performance. Today's software environment is increasingly multi-threaded, as are games. With "Zen 2," AMD set itself an ambitious double-digit percentage IPC uplift target to catch up or overtake Intel's latest "Coffee Lake" microarchitecture at IPC. AMD didn't stop there and even increased core counts for the platform at higher price points. The third-generation Ryzen family even includes a 16-core processor, which is a tremendous core count for the mainstream desktop platform.

Before we get into the interesting and quirky way AMD crammed 16 cores into this chip, let's talk about the "Zen 2" CPU core. After the colossal failure that was "Bulldozer," AMD set out to once again build strong and monolithic CPU cores that share nothing except L3 cache with other cores. It achieved this desired result with "Zen," which posted a mammoth 40%–50% IPC increase over "Bulldozer," catapulting AMD back into competitiveness. "Zen" core IPC sits somewhere between "Haswell" and "Skylake/Coffee Lake," which was enough for AMD as it backed the IPC increase with higher core counts compared to Intel. Over the 8th and 9th generations of Core processors that retained the same IPC as "Skylake," Intel shored up core counts to match AMD. Wanting to set up a definitive edge over Intel, AMD not only worked to increase IPC, but also core counts.

The "Zen 2" CPU core has essentially the same component layout and hierarchy as "Zen," but with major changes and broadening of key components. As with "Zen" (or most x86 CPU cores), the "Zen 2" core is made up of five key components: Fetch, Decode, Integer, Floating-point, and Load/Store. Fetch and Decode tell the CPU core what needs to be done and what data or instructions are needed; Integer and Floating-point Unit execute a mathematical model of what needs to be done depending on the data type and nature of the instruction; Load/Store are the I/O of the CPU core. At various levels, there are tiny buffers, registers that store instructions, and larger caches that cushion data transfers between various components.

AMD updated the Fetch and Decode units, which contribute to IPC, by making the CPU work "smarter." The updated Integer and FPU make the CPU work "harder," and the Load/Store unit's job is to make sure the other components aren't starved of things to do. The Fetch unit is updated with a TAGE branch predictor. Invented in 2006, TAGE is considered to be the best branch-prediction technique by the IEEE. AMD broadened the BTB (branch target buffers) at L1 and L2 by doubling the L1 entries to 512k, and L2 entries to 7,000 from 4,000. The ITA (indirect target array) has also been expanded. The design goal for updating the Fetch unit is to lower "mispredictions" (bad guesses) that wasted load/storage operations by the order of 30 percent. The 32 KB L1 instruction cache has also been improved. The Decode unit has two improvements to the Op cache: improved instruction fusion and the ability to push up to 4,000 fused instructions per clock cycle.

We now move on to the two components that contribute the most to the IPC, the Integer and Floating-point Units. The Integer unit received incremental updates in the form of a broader integer scheduler that handles 92 entries (up from 84), with four 16-entry ALU queues and one 28-entry AGU queue. The general-purpose physical register file has now been expanded to 180 entries from 168. The issue-per-cycle has been broadened to 7 from 6, which now includes 4 ALUs and 3 AGUs. The reorder buffer (ROB) has been broadened to 224 entries, up from 192. The SMT (simultaneous multi-threading) logic has been tweaked to better share the ALUs and AGUs among the logical processors. The FPU has the bulk of the innovation with "Zen 2." The load/store bandwidth of the FPU has been doubled to 256-bit, up from 128-bit on "Zen."

The core now also supports a sort of AVX-256: AVX/AVX2-flagged instructions with 256-bit registers. There are many applications for this, such as physics simulation, audio-stack execution, and memory-copy performance improvement. Multiplication operation latency has been improved by 33 percent.

Lastly, we move on to the Load/Store unit with a similar round of generational enhancements. The entry-store queue is expanded to 48 entries, up from 44. The L2 TLB (translation lookaside buffer) has been expanded by 33% to 2,000 entries, and its latency improved. The 32 KB L1 Data cache has two 256-bit read paths and one 256-bit write path, with 64-byte load and 32-byte store alignment boundaries. The Load/Store bandwidth to L2 has been doubled to 32 bytes per clock.

We now move on to the cache hierarchy, which is essentially the same as "Zen." Notwithstanding the technical changes described above, the "Zen 2" core still has a 32 KB 8-way L1I cache, a 32 KB 8-way L1D cache, and a dedicated 512 KB 8-way L2 cache. AMD doubled the shared L3 cache size to 16 MB. Every CCX (quad-core compute complex) on a "Zen 2" processor now has 16 MB of shared L3 cache. The doubling in L3 cache size was necessitated not just by Intel sharing larger amounts of L3 cache among individual cores on the "Coffee Lake Refresh" silicon (16 MB shared among all 8 cores), but also because the larger L3 cache on a "Zen 2" CCX cushions data transfers with the I/O controller die.

This brings us to the interesting and quirky way AMD achieved 16 cores. The Ryzen 9 3900X and Ryzen 5 3600 processor packages are codenamed "Matisse." This is a multi-chip module (MCM) of one or two 7 nm 8-core "Zen 2" CPU chiplets and one I/O controller die built on the 12 nm process. AMD made sure only those components that tangibly benefit from the shrink to 7 nm—namely, the CPU cores—are built on the new process, while those components that don't benefit from 7 nm stay on the existing 12 nm process, on the I/O controller die. AMD carved the Ryzen 5 3600 out by using just one "Zen 2" chiplet and enabling 6 cores on it, 3 per CCX.

These components include the processor's dual-channel DDR4 memory controller, a 24-lane PCI-Express gen 4.0 root complex, and an integrated southbridge that puts out some platform connectivity directly from the AM4 socket, such as SATA 6 Gbps and USB 3.1 ports. Infinity Fabric is the interconnect that binds the three dies by providing a 100 GB/s data path between each CPU chiplet and the I/O controller. The memory clock is now practically de-coupled from the Infinity Fabric clock, which should improve memory overclocking headroom. AMD also claims to have put a lot of work into improving memory-module compatibility across brands, especially since Samsung stopped mass-production of the expensive B-die DRAM chip that favored AMD processors. The memory scaling article talks a little more about this.

Architectural Innovations Specific to Ryzen 3000XT Series

The Ryzen 3000XT family of processors are internally referred by AMD as "Matisse 2." These are almost identical to the original Ryzen 3000 "Matisse" processors based on the "Zen 2" microarchitecture, but AMD has given these processors some physical improvements. To begin with, the 8-core CCDs (compute complex dies) or "Zen 2" chiplets inside the processors are still based on TSMC N7 (the foundry's first 7 nm node), but with certain refinements. AMD claims these yields a single-digit percentage electrical improvement, which AMD used to achieve up to 200 MHz increments in maximum boost frequencies without affecting the TDP of these processors.

The TDP of Ryzen 5 3600XT remains at 95 W, just like the 3600X, while the 3800XT and 3900XT both stick with 105 W TDP. AMD's decision not to include cooling solutions with the 3800XT and 3900XT have little to do with the power or thermals of these processors and more with marketing decisions. It certainly reduces AMD's bill of materials for these chips.

AMD categorically stated that this "refined" N7 node is neither N7P nor N7+. The N7P node is the successor to N7 that sticks to DUV (deep ultraviolet) lithography, but innovates in certain other areas to eke out a power improvement. N7+, on the other hand, uses EUV (extreme ultraviolet) lithography that not only yields a much higher efficiency, but also over a 20% increase in transistor density. The node AMD is building "Matisse 2" on is still N7, but with certain refinements AMD didn't elaborate in its product brief.

AMD B550 and X570 Chipsets

With premium AMD X570 chipset-based motherboards starting at $150, it's less likely that someone would pair the third-generation Ryzen 3 with it. Choosing a cheaper B450 motherboard would mean giving up on killer features such as PCIe gen 4.0. AMD hence launched the new B550 mid-range chipset. The B550 chipset lets you have PCI-Express gen 4.0 connectivity from the "Matisse" processor, while limiting general purpose PCIe downstream connectivity to gen 3.0.

On a typical B550 chipset motherboard, the main PCI-Express x16 slot will be gen 4.0 if paired with a third-generation Ryzen "Matisse" processor, as would one of the board's M.2 NVMe slots that's wired to the processor. All other PCIe or M.2 slots which are wired to the B550 chipset will be gen 3.0. This way, future-proofing of the platform for next-generation graphics cards and SSDs remains intact. The B550 chipset provides up to six SATA 6 Gbps ports with AHCI and RAID capability, up to two 10 Gbps USB 3.1 gen 2 ports (in addition to four such ports put out by the "Matisse" processor), two additional USB 3.1 gen 1 ports, and six USB 2.0 ports. The platform's HDA and LPCIO buses are located on the processor.

A word on compatibility. The B550 chipset only supports third-generation Ryzen "Matisse" processors as of this writing, and AMD confirmed support for next-generation processors based on the "Zen 3" architecture. You cannot pair a B550 motherboard with older Ryzen 2000/1000 processors or even the 3200G or 3400G APUs based on the older "Zen+" microarchitecture. There will be clear labeling on B550 chipset motherboard boxes to this effect.

What we like most about the B550 is its low TDP, which lets motherboard designers make do with passive heatsinks; unlike X570, which requires active fan heatsinks.

AMD delivered on its promise of third-generation Ryzen "Matisse" processors being backwards compatible with older Socket AM4 motherboards, going all the way back to the AMD 300-series chipset, with a simple BIOS update. To make the most out of Ryzen "Matisse"—namely, PCI-Express gen 4.0 connectivity and increased CPU/memory overclocking headroom, you're expected to use one of the latest motherboards that use the AMD X570 chipset. The X570 is an entirely different chip from X470 and X370. The older chipsets were supplied by ASMedia, and were rather slim in their downstream connectivity.

The X470 only puts out 8 PCIe gen 2.0 downstream lanes, for example. The X570 modernizes all I/O by putting out up to 16 PCIe gen 4.0 downstream lanes. This enables additional M.2 PCIe gen 4 slots on your motherboards for the latest SSDs featuring PCIe gen 4 support and creates room for many new bandwidth-hungry onboard devices, such as 10 GbE adapters, next-generation Thunderbolt, 802.11ax controllers, etc. Along with the "Matisse" SoC, the X570 also puts out a number of 10 Gbps USB 3.1 gen 2 ports. Motherboards based on X570 also implement modern network connectivity options, such as 2.5 GbE and 802.11ax WLAN.

Given there are highly capable motherboards based on the B550 chipset with serious VRM solutions and high-end connectivity, it's good enough for any Ryzen 3000XT series processor, including the 3900XT. The B550 chipset also offers multi-GPU support. Your choice between the B550 and X570 should hence boil down to whether you plan to have more than one M.2 NVMe SSD that can take advantage of PCI-Express gen 4.0, or an NVMe RAID setup of 2–3 PCIe gen 4.0-capable M.2 SSDs. Serious overclockers should still consider the X570 since the most beastly VRM setups are still found on boards such as the MSI MEG X570 GODLIKE, ASUS ROG Crosshair VII Formula, and GIGABYTE X570 AORUS Xtreme.

AMD StoreMI 2.0 Technology

AMD today is also debuting the second generation StoreMI technology, a value addition to its Socket AM4, TR4, and sTRX4 platforms. StoreMI is a free software for AMD users which allows you to build volumes that span across multiple storage devices, such as SSDs and HDDs. Depending on the "heat" (frequency of access) of the data, the software decides what data to store in the fastest media. Unlike the original StoreMI technology that debuted with AMD's 400-series chipset, StoreMI 2.0 is an in-house development by AMD and improves on an important front—the software doesn't physically move data between various storage devices. Rather, depending on available space and heat, it copies data from the slower media to the faster one, and points the OS to the copy that's on the faster media. This way, there's no scope for data loss. AMD also redesigned the user interface.
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