Why MacBook Pro's Memory Size is Limited 16GB

To the sounds of fanfare, the MacBook Pro was released. And then, to the disappointment of many, his biggest limitation became known: no more than 16 GB of memory for all models.

The public reaction was strong enough that Phil Schiller himself [Apple's senior vice president for global advertising] responded to it:

Placing more than 16 GB of fast memory in a laptop would require using a memory system that consumes much more power, and this would be extremely inefficient. I hope that you pay attention to the new generation of MacBook Pro, this is an incredible system.

And although the logic here is clear, let's try to dig deeper.
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The Intel chipset used supports the following RAM types: DDR3 RAM, Low Power DDR3 RAM (LPDDR) and DDR4 RAM. However, LPDDR3 operates at frequencies of no more than 1600 MHz, but Apple uses 2133 Mhz RAM. How is this possible? There is just another memory standard, LPDDR3E (E - improved), which accelerates to 2133 Mhz. All DDR3 memory types are limited to 16 GB.

LPDDR4 is not supported by Intel's CPU, and another standard, DDR4L, low voltage memory, is not yet ready. So, DDR4 desktop memory would be the only chance to go beyond 16 GB.

The question is: how much energy is actually saved when using LPDDR3E memory instead of DDR4, which could be used with the same CPU?

Briefly and roughly - under normal conditions, the memory uses 1.5 W, and when using DDR4, it would consume 3-5 watts. This would be 20% of the total system consumption, as opposed to 10% used by LPDDR.

Using the excellent menus iStats can be calculated by loading the system in a normal way, how much amperes use memory. And then count the watts.



In this case, 1.49 A at 1.2 V = 1.8 W of power is used. Since MacBook Pro does not use DDR4, test it in this way will not work. We'll have to trust the results of DDR4 checks to people from the PC world. It is important to note that, despite the opposite statements, the difference in the consumption of DDR3 and DDR4 is almost imperceptible .

Some comments on the difference in consumption of DDR and LPDDR:

Peak reduction is more noticeable. From 4 watts to 6 watts.
www.tomshardware.com/reviews/lovo-ddr3-power , 2650-7.html

The difference in energy consumption between systems equipped with conventional memory and low voltage memory is only 2 watts in idle and 3 watts under load.
www.xbitlabs.com/articles/memory/display/kingston-lovo-hyperx_3.html#sect1

Although both articles prove a slight difference when using LPDRR memory, they make these comparisons on more powerful computers, and in some cases they compared desktop computers and LPDDR and DDR memory installed on them. In our case, only the bare numbers are important, and they show that DDR uses 2-3 watts more than LPDDR.

We will not get absolutely exact numbers, but, depending on the use of the laptop, we can estimate that we save from 2 to 5 W when using LPDDR3E RAM instead of DDR4. And since the system consumes an average of 20 watts during normal use, it can be said that the memory consumes 2 watts instead of 3-6 watts in the case of DDR4. In terms of total usage, it turns out that memory accounts for 10% of energy, instead of 20-30% in the case of DDR4.

RAM: Background Memory Consumption

It is important to note that the estimates were conservative because they do not consider what is difficult to calculate in reality: after using RAM, a lot of energy is spent on going to sleep, which leads to background memory consumption. As a result, there is a decrease in efficiency, which is not visible on consumption tests, but it is believed that on an average system this effect takes up to 50% of total consumption when using DDR4 RAM, and only 20% when using LPDDR3.

The background energy of DDR4 makes a large contribution to the overall consumption due to the lack of a fast transition mode to the off state, which affects energy savings.
www.cs.rochester.edu/~ipek/micro15.pdf (pdf)

Considering that a 75 W * h battery is used on 15 "machines, it was possible to make the battery up to 24% more to meet the limitations of the FAA, and still they would not have been able to achieve the same operating time as possible using LPDDR.

RAM: Sleep Consumption

In sleep mode, LDDPR RAM uses 80% less energy than DDR RAM. The guys from Micron, the manufacturer of memory, conducted a large-scale analysis and published the results in a very detailed PDF , and made the following conclusion:

As follows from the equation, a Mobile DRAM device that does not use PMM consumes an average of five times more power than a device that uses PMM.

The previous example shows the consumption calculation for a specific device with Mobile DDR SDRAM in a specific operating environment. The memory consumption of a device can vary greatly, depending on how the system organizes memory access and how the PMM features are used.

PMM is the most powerful of all LPDDR power saving features. Another possibility is to consolidate the memory used by a computer into as few physical chips as possible in sleep mode, so that the remaining ones can be completely turned off.

Considering that LPDDR memory is made for mobile devices, and DDR RAM for devices that are plugged into an outlet, it’s no wonder that 70% -90% less energy is spent on a sleeping laptop using LPDDR RAM. PDF .

Apple has been using LPDDR for several generations of laptops, and partly because of this, laptops have a long sleep time. Switching to DDR4 would dramatically reduce this time from 30 days to an interval of less than a week in length. With DDR4, their laptop would completely use up the entire battery in a few days if you closed it when the charge level was 50%. This would not only be an annoying factor, but would also harm the batteries, since they would often be discharged to 100%. It is harmful to batteries, and sometimes it just kills them .

Other sources of information

According to the Apple scale, even if 0.1% of people suffer from a problem, they consider it to be a scandal. Apple gets a huge amount of frustrated consumers. If Intel decided to support LPDDR4, Apple would use them. The iPhone 7 uses LPDDR4 memory because it uses Apple’s own processor .

On LinusTechTips, we tried to analyze how much additional energy would be needed to support more memory, but they compared only 16 Gb DDR4 to 32 Gb DDR4, and this is a mistake. Surprisingly, but usually the information from this resource is very high quality.

Comparison with other laptops with 32 Gb RAM support

In the last article, I mentioned that the restriction of the FAA would prevent Apple from using DDR4 and making the battery work long enough, and as a result received a wave of reviews where they say that if other laptop makers could achieve results, Apple could. I walked through laptops with support for more than 16 Gb of memory, and, as expected, they all suffer from the limitations of battery life.

Dell xps 15

The only disappointing feature of the notebook is the battery life, which can withstand only 5 hours and 40 minutes of video playback with a battery of 84 Wh. This is very far from the promotional 17 hours. Here .

With its powerful graphics and CPU, a 15 "4K display takes a lot of energy to power everything hidden in its 2kg case. As a result, the Battery Eater battery stress test took only an hour and a half.
But if you use the video resolution of 1080p, it lasts almost 5 hours.
hothardware.com/reviews/dell-xps-15-late-2015-review?page=8

Lenovo Thinkpad p50

The review mentions "epic battery life", but the specifications indicate 8 Gb of RAM, 4 DIMM slots, and a maximum of 64 Gb.

Why did he have only 8 GB? Because it takes up only one slot, instead of the standard two, which increases battery life.

Unlike most other laptops, ThinkPad P50 is able to work on a single charge all day. In the Laptop Mag Battery Test, the laptop stood for 8 hours and 25 minutes of web surfing on WiFi with a bright display. From here .

Simple web surfing even without switching between applications does not at all reflect the real use of a professional laptop. Most of the memory is not involved at all.

The replayed video from 50% brightness of the 4K display and at 100% volume spun just over 5 hours. To ease the laptop, on the ThinkPad P50, Lenovo decided to use a 60 W * h battery instead of 96 W * h on a P70 or 90 W * h on a ZBook. The result is not bad - the same as that of Dell or Samsung, but it will disappoint those who want to work all day without an outlet. From here .

Lenovo ThinkPad T460

This laptop is more appropriate to compare with the products of Apple, because it is also light and thin. It has 32 GB of RAM installed, and in most cases it runs for a long time on battery. But if you look closely, you can see that it has a very weak percentage, Intel Core i7-6600U with only 2 cores. Such processors are used in mobile devices, and not in serious professional laptops. So comparing them is wrong.

Personal opinion

Apple made a 17 "MacBook Pro, but stopped producing it because of low sales. I had one, and I always like to own the most powerful machine of all available. Even if it weighed 10 kg and cost $ 10K, I would I bought them anyway, but I certainly don’t imagine their target audience ...

The market voted with a wallet, and the volumes of lighter and thinnest laptops led Apple along this path.

To illustrate my personal needs - I recently rendered one clip in HD via the Style app neural network. Because of the 16 Gb limit, it took several days, and the process wrote 20.44 Tb in a swap (yes, more than 20,000 gigabytes). All this would be much faster if I had 32 GB.



Apple never tended to compromise, and switching to DDR4 simply because of the support of 32 GB of RAM would be absurd, judging by the conclusions in the article. They would rather replace the Intel CPU with their ARM CPU to support LPDDR4. In addition, as Phil Schiller himself commented, using DDR4 would have required a different motherboard, which could leave even less space for batteries.