The Samsung Galaxy S10+ Snapdragon & Exynos Review: Almost Perfect, Yet So Flawed

The Snapdragon 855 SoC - A Recap

Although the Galaxy S10 is the first Snapdragon 855 device we’re reviewing, Qualcomm’s new chipset shouldn’t come with any major surprises. We had the opportunity to conduct an extensive and in-depth performance preview back in January at CES, which answered a lot of our initial questions about Qualcomm's new flagship SoC. Indeed the Snapdragon 855 largely met our expectations: The new CPU on the new process node  performs very similarly to the other 7nm + A76 design we've already seen – the Kirin 980 chipset from HiSilicon – with only minor differences on the CPU complex.

Where Qualcomm strongly differs from the competition is in in the auxiliary accelerator blocks such as GPU, DSP and the new tensor units. Let’s go over the specifications again:

The Snapdragon 855 is Qualcomm’s first SoC powered by Arm’s new Cortex-A76 CPU core, which we also saw in the Kirin 980. Qualcomm still makes use of Arm’s “Built on Cortex Technology” license, where it requests changes to the CPU IP to be delivered by Arm. The end product ends up marketed as a Kryo CPU – in the case of the Snapdragon 855 the new “Kryo 485” CPU.

In past iterations it’s not always been clear exactly what changes Qualcomm had made to the CPU cores, so it was a surprising and much welcomed change to have the company actually provide concrete examples in the case of the new Snapdragon 855 CPU cores: The two big disclosed changes are an increase of the core’s reorder buffer from 128 entries to a higher, unspecified amount, as well as tuning the prefetchers to better work with floating point workloads.

The one thing that makes the Snapdragon 855 unusual though is the new physical CPU configuration. Both the Kirin 980 as well as the Snapdragon 855 both contain four Cortex A76 cores, however the two companies implement these in two completely different ways.

While HiSilicon had opted for a 2+2 core configuration where one pair clocks up to 2.6GHz and the other only reaches 1.92GHz, Qualcomm opts to go with a 1+3 setup. Under Qualcomm's setup one core is clocked up to 2.84GHz, and meanwhile the other three cores reach up to 2.42GHz. While at first glance this makes sense, things get confusing when accounting for the fact that the Snapdragon still only has a single voltage plane for all four CPU cores, whereas the Kirin’s CPU pairs both have their dedicated rails.

Qualcomm has explained that this was a deliberate choice which took into account the actual benefits, as well as (most importantly) the costs of the platform. Having an additional voltage rail means your PMIC needs an additional buck converter and you need to have additional inductors and capacitors on the motherboard, a cost not only in terms of actual component costs but also in terms of valuable PCB space.

What this means is that the power difference between the two CPU groups is much less than one would expect, but most interestingly it will be a difference that is solely dictated by the different physical implementations of the two cores.

In later sections we’ll address the efficiency difference between the two groups of cores, and one thing that was surprising is that the “middle” cores weren’t that much more efficient than the “prime” core. I extracted the power curves out of the scheduler, as dictated by Qualcomm, and this reveals a bit more information and clarification.

What we see is that the middle “Gold” cores’ power curve shape is shifted down towards lower power, meaning it starts growing at an exponential rate earlier than what we see on the “Prime” core. This would explain why at peak performance, the efficiency difference between the two cores is minor. When we look at the middle frequency points in particular though, we see what this power difference is more notable and actually at its greatest point does represent up to 20% lower power on the mid cores compares to the prime core.

Qualcomm also differentiates the large CPU cores by their cache configuration: The Prime core gets 512KB of L2 while the middle cores make due with 256KB. The Cortex A55’s have the usual 128KB and Qualcomm clocks them conservatively at 1.78GHz.

Finally, the DSU’s L3 cache comes in at 2MB. A big question I had is exactly how fast Qualcomm had clocked the cache at, and the answer is 1.6GHz. This represents a slight increase over the 1478MHz of the L3 cache found in the Snapdragon 845.

Other large architectural changes in the Snapdragon 855 are the new Adreno 640 GPU. Here Qualcomm supposedly has increased the execution units by 50% - yet only advertise a 20% boost in performance. The explanation here lies in the clock frequency of the new GPU. The Adreno 640 in the Snapdragon 855 runs at only 585MHz, markedly slower than the 710MHz of the Adreno 630. I suspect that Qualcomm saw some of the increasing power usage of the higher clock frequencies and decided it’s better to go wider and slower. Indeed, we’ll see that the Snapdragon 855 has managed to reduce power usage in 3D workloads ever so slightly compared to the Snapdragon 845 – something which should definitely help thermals and sustained performance.

Finally, the new Hexagon 690 DSP block has seen its biggest change in several years. Scalar performance has gone up by 20% through microarchitectural and clock frequency bumps, but most importantly the DSP's vector unit count has been doubled up from two to four units, doubling the HVX performance of the new cores. This is something that will be particularly visible in the AI workloads we’ll cover shortly.