CPU & Memory Subsystem

As we mentioned earlier, the Snapdragon 845 is the first SoC employing ARM’s new cluster technology DynamiQ. The different CPU cores are no longer hosted in dedicated cluster subsystems but are now integrated in a larger DynamiQ cluster. This change means that the cluster cache which to date was referred to as the L2 cache becomes the L3 cache and the individual CPU cores now receive a new private per-core L2. In the case of the Snapdragon 845, the Kryo 385 performance cores – derivative of ARM’s new A75 – are configured with 256KB L2 caches. The Kryo 845 efficiency cores, which are derivative of the A55, use 128KB L2 cache configurations.

The L3 cache of the DynamiQ Shared Unit (DSU) is configured at 2MB. At the launch of the Snapdragon 845 Qualcomm advertised three voltage and clock domains – unfortunately we haven’t had time to look deeper into the system of the QRD to find out how this is partitioned, however it is still my unconfirmed belief that the third clock/voltage domain is dedicated to the DSU and not part of the CPU cores. An important characteristic that is totally new to the Android SoC ecosystem is the introduction of a system cache – this 3MB cache seems to sit at the memory controller / interconnect level above the CPU subsystem, something we’ll get back to in just a bit.

Qualcomm’s performance projections seemed relatively conservative as they claimed a performance uplift of only 25-30% which seemed lower than ARM’s projections. We have to keep in mind that beyond the microarchitectural improvements expected from the transition from A73 to A75 based CPU cores we also have a flat 14% frequency increase from 2.47GHz to 2.8GHz on the side of the performance cores. Unfortunately in the limited testing time we had with the QRD we couldn’t make use of long-running CPU benchmarks such as our SPEC suite, so for the scope of this article we had to base our synthetic analysis on GeekBench4 results.

Geekbench 4 - Integer Performance
Single Threaded
  Snapdragon 845 Snapdragon 835 % Increase
 
AES 1160 MB/s 942.5 MB/s 23.1%
LZMA 4.15 MB/s 2.98 MB/s 39.3%
JPEG 20.8 Mpixels/s 16.6 Mpixels/s 25.2%
Canny 32.1 Mpixels/s 24.9 Mpixels/s 28.8%
Lua 2.18 MB/s 1.75 MB/s 24.0%
Dijkstra 1.90 MTE/s 1.62 MTE/s 16.9%
SQLite 70.3 Krows/s 53.4 Krows/s 31.8%
HTML5 Parse 12.9 MB/s 8.97 MB/s 44.1%
HTML5 DOM 3.00 Melems/s 2.27 Melems/s 31.9%
Histogram Equalization 67.3 Mpixels/s 52.5 Mpixels/s 28.2%
PDF Rendering 66.4 Mpixels/s 48.5 Mpixels/s 37.0%
LLVM 321.2 functions/s 257.3 functions/s 24.8%
Camera 7.96 images/s 5.64 images/s 40.9%

For the integer workload results we see a healthy performance across the various tests. Qualcomm’s 25-30% increase here seems to be justified as this is the most common increase in most tests. Workloads such as LZMA, HTML5 parsing, PDF rendering and the Camera substests see larger increases into the 40% range. The overall improvement in absolute performance for the integer tests is 31%.

Geekbench 4  (Single Threaded) Integer Score/MHz

If we revisit performance per clock across recent microarchitectures we see the Snapdragon’s A75 based cores increase by only a meagre 15% which is below our expectations. We move on to the floating point benchmarks to see if we see a similar story.

Geekbench 4 - Floating Point Performance
Single Threaded
  Snapdragon 845 Snapdragon 835 % Increase
SGEMM 16.6 GFLOPS 11.4 GFLOPS 45.1%
SFFT 4.23 GFLOPS 2.86 GFLOPS 47.9%
N-Body Physics 1400 Kpairs/s 872.2 Kpairs/s 60.5%
Rigid Body Physics 8524.2 FPS 6130.5 FPS 39.0%
Ray Tracing 354.0 Kpixels/s 232.7 Kpixels/s 52.1%
HDR 11.9 Mpixels/s 8.31 Mpixels/s 43.2%
Gaussian Blur 34.5 Mpixels/s 23.9 Mpixels/s 44.3%
Speech Recognition 17.9 Words/s 13.6 Words/s 31.6%
Face Detection 752.4 Ksubs/s 532.8 Ksubs/s 41.2%

The FP subtests of GB4 show a noticeably larger increase than the integer tests. Besides the switch from a 2-wide decode front-end to a 3-wide one, the largest changes of the A75 microarchitecture was found in the floating point execution pipelines and is likely the cause for the larger FP performance improvement. The boost here comes at an overall 45% in GB4.

Geekbench 4 (Single Threaded) Floating Point Score/MHz

In terms of performance per clock, the 45% overall boost translates into a much larger 26% increase in IPC which is nearer to what we had expected.

Revisiting the performance claims from ARM’s TechDay release of the A75 we notice that we had been promised larger improvements such as up to a 34% increase in GB4 performance per clock, which I interpreted with the frequency increase of the Snapdragon 845 to result in a 52% overall increase, which did not materialise. I reached out to ARM on the topic and got back several points of consideration: The projections ARM published were made on a A75 simulation with 512KB L2 caches and 2MB L3. The L3 matches the configuration of the Snapdragon 845 however Qualcomm’s choice of going with smaller L2 caches will have a certain performance hit. ARM didn’t have a number at hand for GB4 but quotes a 2% performance degradation for SPEC2000, and claims for GB4 it should be lower. Another consideration point is the memory subsystem of the SoC which ARM can’t control but heavily impacts the performance of the CPU, so let’s have a look at that.

Running our internal memory benchmark on the QRD we see several expected characteristics of the Snapdragon 845: Compared to the Snapdragon 835’s A73 based cores we see the shift from shared cluster L2’s to private ones as well as the integration of sort of an L3 and L4 cache. The new L2 caches are very visible in our benchmark as memory latency up to the 256KB barrier (or rather, the 320KB barrier as the L1D and L2 are exclusive) sees a vast reduction compared to the L2 region of the A73 cores. The A75 cores promise 8-cycle hits for the L2 versus 19 cycles on the A73. In our test the difference is far larger as see a reduction from ~30ns down to ~4.5ns (not forgetting a clock frequency increase of the new cache). After the 256/320KB test size boundary we enter the DSU’s L3 cache. ARM describes the L3 as pseudo-exclusive so the outer boundary should end around or shortly after the 2048KB mark, the transition here is much harder to make out in the limited data we had time to collect so hopefully we’ll get to revisit it on a commercial device.

On the Snapdragon 835 the transition between L2 cache and DRAM is very sharp and visible in the graph. On the Snapdragon 845 however we see a far more gradient latency transition stretching out to up to the 5MB test depth. This is confirmation that Qualcomm’s system cache is indeed applied to the CPU subsystem and acts as an exclusive L4 cache to the processors. I think this new system cache is a true SoC-wide cache lying high up at the interconnect or memory controller level.

One of the worries of such a configuration for the CPU subsystem was increased latency to DRAM and it seems my fears were realised as the Snapdragon 845 shows a 30% increase in main memory latency from the CPU subsystem. Previously the Snapdragon 835 seemed to have by far one of the best memory controller implementations which directly resulted in higher performance of memory latency sensitive workloads. The latency increase in the 845 thus must be counteracting some of the microarchitectural improvements on part of the CPU cores. For GB4 in particular I made a remark that I didn’t notice any performance impact at all on the part of the Kirin 970’s memory latency, however we’re talking about different platforms and CPUs so I can’t be certain.

We reserve final conclusion on synthetic benchmarks until we get more time with a Snapdragon 845 device and able to investigate more and run SPEC. For now it looks like the Snapdragon 845 does not reach ARM’s projected performance levels, and falls well short of the claims. Among one of the other performance claims was Octane. We retired Octane some years ago and Google shortly followed up with official retirement, but as an added data-point the Snapdragon 845 reached a score of 15969 versus the Snapdragon 835’s 11879, also well short of a 20000 target that a projected 1.48x per clock performance increase would have resulted in.

Introduction System Performance
POST A COMMENT

140 Comments

View All Comments

  • gamertaboo - Monday, February 12, 2018 - link

    Well, you sure see wildly different scores if you compare them using Geekbench. 4260 vs 1998 on single core, and 10,221 vs 6765 on multi-core I believe. Apple's chips are always faster, always. It's literally the one thing Apple at the very least, always does well. Reply
  • Gk12kung - Tuesday, February 13, 2018 - link

    Dude your extremely mistaken , the processors in the x and 8 are differently binned . Since apples GPU is inhouse this year they couldn't get enough high quality GPU in time so the x uses higher binned gpu than 8 so the higher performance and the x uses faster ram than 8 which is also a factor in GPU scores . I've used a 8 and x there is rarely any throttling the d220AP is the lower GPU revision used in 8 and d221ap is the x version with higher bin GPU Reply
  • BronzeProdigy - Tuesday, February 13, 2018 - link

    The GFX bench is also wrong. The A11's GPU is behind. If you look at the sections for the X you'll see N/A. Look at the SD835/5T scores and you'll see they match up but the score on that test either doesn't exist or doesn't match the X's score. This is because they failed to note that the GFXBench has different versions (e.g. 3.1, 2.7, 3.0) so they wrote the wrong scores thinking that the scores on the X were something they're not.

    https://uploads.disquscdn.com/images/9c0ebbebc7b3c...
    Reply
  • peevee - Tuesday, February 13, 2018 - link

    Except in real life users do not run SPEC for long time. They load and start apps, or process photos. It all takes less than a second. Reply
  • rocky12345 - Tuesday, February 13, 2018 - link

    Yea because no one ever plays any games on theirs mobile devices which can go from 5 minutes to 5 hours of use and will heat up any device no matter who makes them. So if you have devices from a mobile maker that is known to heat up & then throttle from that heat then yep it won't be as fast as in the benches. I don't care if Apples makes them or Samsung it happens to every device but it happens worse on some than it does to others. Reply
  • Stochastic - Monday, February 12, 2018 - link

    This looks like a nice, albeit not earth shattering overhaul to the 835.

    Any chance we'll see a Google SoC in the Pixel 3 this year? Or is it more realistic to expect that in the Pixel 4/5? It's a bit boring seeing Snapdragon SoCs in practically all Android flagships.

    Also, what's up with Chrome's lackluster Javascript performance? You mention that the Nitro engine Apple uses is much better optimized. You would think with all the competition in the browser space these days and Google's vested interest in the future of the web that they would push Javascript performance further.
    Reply
  • Jon Tseng - Monday, February 12, 2018 - link

    Not going to happen for a bunch of reasons, not least bc Google doesn't have baseband IP. Reply
  • Dr. Swag - Monday, February 12, 2018 - link

    I don't think we'll see Google SoCs unless pixel really gains market share. Not worth it to design and fabricate soc if you only sell a few millions phones. Reply
  • Stochastic - Monday, February 12, 2018 - link

    Yes, but this would help them gain marketshare. They could perhaps even license the SoC to other OEMs in order to advance the Android hardware ecosystem as a whole. See this: https://www.androidauthority.com/google-dabbling-s... Reply
  • techconc - Wednesday, February 14, 2018 - link

    Regarding Javascript performance, this article is placing far too much emphasis on the Javascript engine. Yes, Apple's Nitro engine is ahead of Google's V8 engine. However, the majority of the speed difference comes down to the fact that Javascript is inherently single threaded. (Yes, I know work is being do to attempt to address this, but it's not there yet). That, coupled with the fact that Apple's single core performance is WAY ahead of everyone else on ARM is why you see such a big difference in performance. Reply

Log in

Don't have an account? Sign up now