Does faster RAM really make a difference?
Darien Graham-Smith
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Nov 17, 2008 2:35 PM
Will splashing out on expensive RAM make a real difference to PC performance? And if so, how much should you buy? We supply the benchmarks and lay the details bare
According to legend, Bill Gates once concluded that “640KB of memory ought to be enough for anyone” (a legend he’s at pains to repudiate). These days it takes more than 800 times that much RAM just to make Windows Vista work smoothly. But as the idea takes hold that more RAM is always good, we’re increasingly seeing home systems armed with a vast 4GB of storage. Indeed, with 64-bit Vista gaining traction, we’re sure the 8GB home PC can’t be far away. Is there any need for this much memory, or is it a waste of money beyond a certain point?
This month we set out to discover the truth. Armed with a comprehensive set of benchmarks and a big stack of DIMMs, we’ve tested performance on both XP and Vista to find out how much memory you really need.
But, of course, quantity of memory isn’t the only consideration. Does faster RAM make a difference? And what about issues such as latency and dual-channel architecture? Do they matter, or are they mere technical curiosities of no importance in the real world?
We’ll lay the details bare, along with explanations of the difference between DDR, DDR2 and DDR3, and how to decode RAM speeds and timings. We’ll even delve into the black art of memory overclocking. If you’re thinking of upgrading an existing system – or building one from scratch – don’t buy a DIMM until you’ve read this.
What’s the ideal amount of memory?
According to Microsoft, Windows Vista requires 512MB to install and run, while Windows XP requires 64MB. In practice, systems with these memory allocations will work, but won’t be able to keep all your active programs and data in RAM at once, so they’ll be slowed down by constant hard disk access.
The solution is to add more memory – but how much do you need for a smooth Windows experience? To find out, we used our standard 2D benchmarks, which run numerous real-world tasks in common applications including Word, Excel and Photoshop. We ran them on an Intel Core 2 Duo E8500 system, with varying amounts of RAM, and both XP Professional and Vista Home Premium. You can see the results in the graphs.
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| Vista vs XP: The RAM benchmarks |
Our results show Vista is distinctly slower than XP. With its plethora of extra bells and whistles, the newer OS makes heavier demands on the system than its predecessor, particularly the RAM. On a 512MB system, our benchmark tasks were bogged down by constant paging, and even a 4GB system couldn’t keep up with XP on an eighth as much RAM.
While adding RAM helps, the ideal amount depends on how you use your PC. For pure number-crunching, boosting memory has only a small benefit. In our audio-encoding benchmark, quadrupling Vista’s RAM from 512MB to 2GB yielded only a 1.4% speed increase. Video encoding derived a mere 5% benefit from the same upgrade. But when it came to multitasking, the benefit of more memory shone. Going up to 2GB sped things up by an astonishing 35%, and in Photoshop the improvement was more than 40%.
When we added RAM beyond 2GB, several applications saw no benefit at all, and though multitasking and graphical applications continued to run more smoothly, there was nothing like the dramatic leap you’d see in going from 512MB to 1GB, or from 1GB to 2GB. In conclusion: 2GB is the sweet spot for Vista, but if you make heavy use of your computer, you can always add more.
Is there an upper limit of RAM?
Before you splash out on a bumper pack of DIMMs, check you’re not wasting money on more RAM than your system can use. XP won’t recognise more than 3GB, so there’s no point adding extra.
With Vista, the situation is more complex: 32-bit editions can address up to 4GB of RAM, but that has to include not only your system RAM but also the memory in your graphics card and other system resources. If you install a 4GB DIMM, you’ll find less than 3.5GB available to the system. Given the diminishing benefits of adding memory above 2GB, you might want to consider saving money and sticking at 3GB.
64-bit editions of Vista can take advantage of far more RAM – the 64-bit version of Home Basic will take up to 8GB, while Home Premium supports up to 16GB and the Ultimate, Business and Enterprise editions can accommodate a massive 128GB of RAM – though good luck finding a motherboard with enough DIMM slots.
What sort of modules should I choose?
Unless your computer is a real antique, buying RAM means buying DIMMs: dual in-line memory modules. DIMMs come in several types, and which sort you need depends on your motherboard. If your computer’s more than four years old, it might take plain old DDR DIMMs (known as “double data rate” because memory can be accessed twice per clock signal). More recent PCs will most likely use the faster DDR2 DIMMs, while high-end desktop systems might use the latest DDR3 standard.
The different types of DIMM aren’t cross-compatible: if your motherboard demands DDR2, it won’t work with DDR3. To ensure you don’t make mistakes, different types of DIMM have notches in different places, making it impossible to plug the wrong sort of DIMM into a slot. To find out which type of DIMM you need, look at the modules already installed on your PC: odds are they’ll be labelled either DDR, DDR2 or DDR3. Alternatively, go online and look up your motherboard on the manufacturer’s website, or use a system-analysis tool such as SiSoft Sandra.
A few motherboards based on Intel’s P35 and P45 chipsets provide slots for both DDR2 or DDR3, offering some flexibility. Be warned, though: you can’t use both types at once.
If you have a laptop, you’ll need to use compact modules known as “small outline DIMMs”, or SODIMMs. As with desktop modules, SODIMMs come in DDR and DDR2 flavours, with notches to ensure they’re only plugged into the right sort of slot. DDR3 SODIMMs are starting to appear on the market too, though only the very newest laptops use them.
Is there an optimum number of DIMMs?
We tend to think of a computer’s memory as a contiguous space, but in the physical world it’s normally spread across multiple DIMMs, filling up some or all of the motherboard’s slots. This raises a few practical and technological issues.
One obvious consideration is how many memory slots your motherboard has. Most come with four DIMM slots, though a few have six or even eight, while lightweight systems and laptops often have only two. If you have just two slots free, it makes little sense to fill them with two low-capacity DIMMs, as you’ll be closing off options for future upgrades. Due to the dynamics of supply and demand, it may not make economic sense, either: a pair of 512MB DIMMs can cost more than twice as much as a single 1GB module.
However, two smaller DIMMs can be faster than one large one – as long as your motherboard supports dual-channel architecture. This is a similar system to RAID0 striping, but for memory rather than hard disks: two identical DIMMs run in parallel, acting as one large module with twice the bandwidth of a normal DIMM.
To quantify the benefits of dual-channel architecture, we tried our standard benchmarks with a single 2GB DIMM, then repeated them with two 1GB DIMMs in dual-channel mode. You can see the results in the bottom-left graph on page 102.
We found the benefits varied hugely between apps: dBpoweramp and Photoshop gained no advantage, but Canopus ProCoder completed its tasks 1% more quickly with dual-channel RAM, while 3ds Max gained a 1.5% boost. Our Office tasks ran a whole 5% faster, and the multi-applications test reflected a 3% performance improvement.
If your motherboard supports dual-channel operation, the DIMM slots will be colour-coded in pairs. To take advantage of it, you simply need to install two identical modules in slots of the same colour – which is one reason why DIMMs are often sold in matched “kits”. You may also have to enable dual-channel architecture in your BIOS.
What does RAM speed mean?
The next issue to consider is speed. RAM speeds can be quite confusing, as they can be expressed in several ways. Starting with the oldest DDR modules, the basic models run at an internal frequency of 100MHz, while more advanced modules increase the internal clock speed to 133MHz, 166MHz and up to 200MHz.
It might seem logical to refer to these different modules by their internal speeds but, thanks to the double data rate that gives DDR its name, a 100MHz module can carry out a theoretical maximum of 200 million transfers per second, while the 200MHz module can carry out 400 million transfers per second. For this reason, 100MHz DDR is known as DDR-200, 133MHz modules are labelled DDR-266 and so forth.
This is a fairly obvious system, but RAM transfers aren’t very convenient units to work in. It’s much more common to talk about data in terms of bytes. So to make DIMM speeds more easily understandable, they’re also given a “PC-rating”, which expresses their bandwidth in megabytes per second.
PC ratings can be calculated very simply. Each RAM transfer consists of a 64-bit word, or eight bytes. So to convert transfers-per-second into bytes-per-second, you simply multiply by eight. DDR-200 is thus equivalent to PC-1600.
DDR2 uses almost the same naming conventions, but the chips communicate with the CPU at twice the speed of DDR. The slowest DDR2 is therefore capable of 400 million transfers per second, and is designated DDR2-400, or PC2-3200. As you’d expect, DDR2 goes up to DDR2-800, also known as PC2-6400, and above this there’s a high-end part, based on 266MHz chips, to give DDR2-1066. Its PC-rating is rounded down to PC2-8500 for convenience – its peak bandwidth is more like 8533MB/sec.
DDR3 extends this process, running the I/O bus at four times the speeds of DDR – so the basic part can handle 800 million transfers per second, earning the labels DDR3-800 and PC3-6400, with faster chips being named accordingly.
The maximum standard RAM speeds approved by JEDEC – the body behind the three DDR standards – are DDR-400, DDR2-1066 and DDR3-1600. You may also hear of modules with higher speed ratings, such as DDR2-1250 and DDR3-2000, designed to run at overclocked speeds in enthusiast motherboards.
What are the benefits of buying extra speed?
Not every motherboard will support every RAM speed. Older boards may be unable to run fast DIMMs at their full rated speed, while more recent models can refuse to boot if you use memory that’s too slow.
But if you do splash out on super-fast RAM, what benefit can you expect to see? In theory, the fastest DIMMs can communicate with the CPU at more than twice the speed of slower modules. In practice, however, very few systems spend half their time waiting for RAM transfers, so expensive DIMMs won’t magically double your performance.
To find out what difference RAM speed makes, we ran our standard benchmarks on a Vista system equipped with 4GB of DDR3-800, then repeated the test with 4GB of DDR3-1600 RAM.
For many applications, the performance advantage gained through faster RAM is nominal – dBpoweramp, Photoshop and 3ds Max gained no benefit at all, while the multi-applications test ran a mere 0.5% faster. But the benefit wasn’t negligible: our Microsoft Office tests received a 1.5% boost, and the Canopus ProCoder video suite completed its tasks an impressive 5.5% more quickly.
Games can receive a 2-4% boost. So if you use your PC for heavy gaming or entertainment, where every second counts, investing in the fastest RAM can pay off – just. You may even want to overclock your RAM.
But for everyday usage, RAM speed is almost a non-issue, and investing in faster DIMMs won’t give a noticeable performance boost. You’ll see far greater benefits from adding more RAM, or investing in a faster hard
disk or CPU.