RAM Overclocking Guide: How (and Why) to Tweak Your Memory

Background

When most people talk about RAM overclocking, they tend to think of increased data rates, which requires boosting the memory's operating frequency. This is a multiple of the base clock rate (BCLK) and memory multiplier.

The memory multiplier (formerly called the memory divider) is the ratio between the RAM's clock speed and the BCLK. An example would be a 1:4 value for this ratio via a 200 MHz BCLK and 1600 MT/s DDR3 RAM (note that DDR3 RAM is double-rate, so that a 1600 MT/s data rate occurs at an 800 MHz clock rate).

Because “RAM speed” = BCLK x Multiplier, increasing the BCLK to overclock the processor increases the memory frequency automatically. But in addition to the base clock rate, a number of other parameters (often inter-related) affect memory performance and stability—operating voltages, latencies, channel/rank configurations, etc., and the task of optimizing all of these variables comes under the umbrella of memory “overclocking.”

Before we get into the details of this, we need to go over some memory-specific jargon, memory classifications, and what exactly each overclocking parameter represents.

Memory Naming And Classification

There is a wide variety of system RAM, including standard SDRAM, EDO SDRAM, RDRAM, and DDR SDRAM. Today we'll focus on the DDR3 and DDR4 variety.

DRAM is most commonly identified by four numbers in addition to its manufacturer and type (DDR, DDR2, DDR3, DDR4): the total memory capacity (8GB, for example), the data rate (say, 1333 MT/s), its bandwidth (e.g. 3200 MB/s), and its timing classification (7-7-7-21 is one such configuration).

DRAM (Dynamic Random Access Memory) is an integrated circuit (IC) with a two-dimensional grid of memory cells; each cell represents a bit, with the columns called bitlines and rows called wordlines, and the row/column number of a cell represents its memory address. The number of cells determines the DRAM chip's total capacity.

Physically, a memory cell is made up of a transistor-capacitor pair: the capacitor stores electric charge (a “0” if the charge is less than 50%, “1” if the charge is greater) and the transistor acts as a switch to couple charge in and out of the capacitor. But a capacitor does not stay charged for long; the charge leaks, and needs to be refreshed. This refresh cycle is what makes RAM dynamic. A memory controller (integrated into the CPU in many modern systems) reads the data, then writes it back. The frequency of the oscillator that controls these charge/discharge (read/write) cycles is the RAM's clock frequency, measured in number of cycles per second (usually MHz).

The clock rate shouldn't be confused with the speed at which information is read and written. The data rate, measured in millions of transfers per second (MT/s) determines this. Each generation of memory technology (DDR, DDR2, DDR3, DDR4) improves the maximum frequency capability, bandwidth, latency, and power usage of the chips. Double data rate RAM communicates twice per cycle, so with a clock speed of, say, 600 MHz, DDR RAM would yield 600 MHz x 8 bytes x 2 for a peak throughput of 9600 MB/s.

There is a bit of ambiguity when it comes to memory technology naming. For example, DDR3-1333 does not have a clock frequency of 1333 MHz, but half that.

The final differentiator between RAM chips is the timing classification, which represents how many clock cycles of delay there are between certain events. So, if an imaginary chip (that dispenses with architectural and basic physics limits) had a timing classification of 1-1-1-1, it would mean that it is returning data and carrying out each type of internal task once per clock cycle. The take-away: aim for the lowest possible numbers in the timing classification.

Memory Type And Configuration: DDR3 vs. DDR4

Differences in the core specifications of DDR3 and DDR4 RAM are of critical interest to overclockers. Overall, each subsequent generation of RAM brings improvements, and in this case the improvement from DDR3 to DDR4 confers lower power consumption, lower latency, and a far greater range of frequencies.

In terms of data rate, the maximum for a standard DDR3 RAM module (“standard” as defined by the Joint Electron Device Engineering Council, an independent semiconductor standards body) is 2133 MT/s, though various high-end chips surpass this limit quite easily. DDR4 does have a maximum as well, but it hasn’t been reached yet—every so often, RAM vendors crack the ceiling of data rates DDR4 RAM is capable of, and rates of more than 3000 MT/s are now sold off-the-shelf.

DDR4 is also more power-friendly than DDR3, which employs a default voltage of 1.5V. Many overclocked setups show voltages in the 1.65 to 2.0V region, while DDR4 generally has an operating voltage of 1.2V with higher bandwidth. Overclocked DDR4 kits reportedly use anywhere between 1.4 and 1.8V. Having the same or higher data rate at lower voltages means less chance of permanent damage, lower cooling requirements, and more headroom for overclocking—all good things.

There are other core differences between DDR3 and DDR4, including maximum capacity, but these are peripheral for overclocking purposes. And this is obvious but bears mention: an overclocker using a previous-gen CPU cannot use DDR4 RAM—Intel’s Skylake design is the best bet when it comes to CPU/IMC compatibility.

Channel, Rank, and Side

Single- versus multi-channel, when it comes to memory, is a CPU-side mode. The IMC can use multiple parallel channels to access the memory, theoretically doubling the bandwidth, though real-world gains are typically less than ideal. The high-end CPUs used by overclockers now feature up to four-channel architectures, but the motherboard must support this.

A memory rank is a lower-level grouping of individual memory chips on a single memory module—a data block that is 64 bits wide. Single rank means that all the memory modules belong to one addressable block; dual rank means that the memory modules on a chip are divided into two groups. Rank does not depend on the number of physical memory chips on a memory module.

Depending on the CPU/IMC configuration, the maximum number of ranks supported per channel is limited; there are performance gains to be had by increasing the number of ranks in a configuration, but too many ranks negatively impact overall memory speed (though in general this will only come into play for servers, workstations, and other high memory-capacity configurations).

Single- versus double-sided RAM is a question of density. Single-sided memory sticks have all of the memory ICs in a high-density configuration on only one side of the physical module, whereas double-sided RAM has the packages on both sides. The two types can be one-, two-, three-, or four-rank. There is a lot of controversy regarding performance of single-/double-sided RAM, mostly because high-frequency, high-density, single-sided RAM is also often single-rank, and shows a noticeable reduction in benchmark scores compared to lower-clocked, double-sided, dual-rank RAM.

Interleaving is the process of dividing data blocks such that multiple targets can be addressed contiguously. Channel interleaving increases the potential read bandwidth of the system, and rank interleaving means one rank of memory can be addressed while another is being refreshed (for multi-rank configurations), reducing the chip's overall latency. Channel and rank interleaving parameters should be set to the highest possible supported by the motherboard to maximize memory performance.

Primary RAM Timings

A memory module’s timings are shown as a set of four numbers, for example 7-8-7-24. Each of these numbers stands for timing delay associated with an internal task type (identified by one of the acronyms defined below), and the order of the numbers is always CL-tRCD-tRP-tRAS.

In order to read or write data to a specific memory address (cell), first the row pertaining to that cell has to be activated, then the column. Each activation process is a separate task, and has various timings (or delays, in clock cycles) associated with it.

CL, the CAS Latency, which stands for Column Address Strobe Latency. It refers to the delay between sending a column address to the memory controller and receiving a result. This is arguably the parameter with the largest impact on RAM latency and performance.

tRCD, also known as RAS to CAS delay, where RAS stands for Row Address Strobe. This is the number of clock cycles it takes to activate a column of data (CAS) of a previously activated row (RAS).

tRP, also known as the RAS Precharge delay, is the delay between closing off read/write access to one row of data and opening access to a different one.

tRAS, the Row Active Time, is the number of cycles required to successfully retrieve data stored in a row; it can be thought of as a wait/delay (in number of clock cycles) before a new request to access a memory cell can begin.

There is a fifth timing classification, CMD, also known as the Command Rate. This is not often reported, but it is the delay between activation of memory and receipt of commands, and is often one or two cycles. There are also secondary and tertiary groups of timings. You can further improve performance by optimizing them, though the gains aren't as significant as what you get from the primary timings.

The timings listed above do depend on each other. For example, the amount of time it takes to access a new memory cell (after accessing a previous one) is tRAS (for a successful retrieval of data in the previous cell) + tRP (to switch to a different row) + tRCD (delay to access a column) + tCL (result from the cell itself). Also, the tRAS time is pre-configured to be greater than the sum of tCL, tRCD, tRP, though overclockers have posted benchmark configurations that defy this rule.

Voltages That Affect Memory

We covered a number of voltages in our processor overclocking article, but some processor-side and motherboard-side voltages are also relevant to memory overclocking. CPUs from both Intel and AMD have integrated memory controllers, and not all of the following voltages will be relevant for all chipsets/processor generations.

Obviously, the most important voltage with respect to memory is the memory voltage, called a variety of names depending on the motherboard firmware and chipset, including VDDQ, SSTL (Stub Series Termination Logic) voltage, DIMM Voltage, DRAM Voltage, VDIMM Select, etc. This value is set to 1.5V (usually) for DDR3 and 1.2V for DDR4 as a default.

The VTT (termination voltage) goes by many names. It can be found listed as IMC Voltage or QPI/VTT Voltage and others, but it is the voltage fed to the IMC on-board the CPU. There are differences between Intel pre-Sandy Bridge, Intel post-Sandy Bridge, and AMD terminology here: modern Intel processors call this the VCCSA (also called “system agent”), and AMD calls this the VDDNB. This is the voltage that can be tweaked if changes in BCLK introduce instabilities.

The Reference Voltage, also called the VREFDQ, DRAM Ctrl Ref Voltage, DDR_VREF_CA_A, etc., sets the threshold for a voltage level to be considered a “0” or a “1.”

The VDDNB is the voltage supplied to the IMC, but the “NB” part refers to northbridge, and other memory options may look very similar; it is best to look up the chipset and motherboard specific definition of this term before changing the values.

The DRAM voltage and perhaps the voltage supplied to the IMC (whatever the variable is named in a specific system motherboard/processor configuration) are the only values that need to be touched in the beginning stages of memory overclocking.

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30 comments
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  • Sakkura
    Perfect timing, I was just about to get to work on my DDR3-1600 kit. Gotta squeeze out all the performance possible from my aging Ivy Bridge build. Especially now that I'm running VR.
  • Amdlova
    No i don't want oc the memory if i can't oc the ssd :)
  • BobsKnob
    You know why there's no benchmarks? Because overclocking memory offers such small 'real world' gains that it's not even worth it. You might as well try to overclock your usb ports. This is just a lame attempt to advertise products, in this case Kingston and Arctic heat spreaders and try to pass it off as reporting.
  • derekullo
    They also mentioned Mushkin, Crucial and G.Skill.

    Heaven forbid they mention the manufacturer of the ram in an article about overclocking ram.

    Now if this would have been an article about Farcry 5 or Elder Scrolls 6 and they had a picture of an Arctic Heat Spreader then you would have been on to something.
  • BobsKnob
    Thank you for proving my point. It's just advertising masquerading as reporting.

    You know, there used to be a time when Tom's did real, unbiased reviews and reporting. When something sucked...it sucked and the review wasn't candy coated.
  • damric
    Well done. Will there be a Part 2 that dares to tweak secondary timings and beyond?
  • dish_moose
    I'm with Bob on this one - until you show real wold benchmarks showing real improvement from OC RAM, I won't waste my time.
    -Bruce
  • Maxxtraxx
    Check out the youtube channel Digital Foundry, this video specifically: https://www.youtube.com/watch?v=Er_Fuz54U0Y
  • Maxxtraxx
    Proof of increased performance from Ram Overclocking from real world benchmarks can be found here:
    https://www.youtube.com/watch?v=Er_Fuz54U0Y

    Am I not allowed to edit my own posts? or am i really missing something here? I wanted to say more after I posted but could not find any way to edit my own post.
  • BobsKnob
    What GPU is being used for that test?
  • BobsKnob
    Also, when I skip through that video, I see a lot of the FPS's are similar regardless of ram speed.
  • Maxxtraxx
    Watch the whole GTA5 section of the video, also here is a second video from DF on the same subject:

    https://www.youtube.com/watch?v=qksXthUcbiQ

    You'll see the same FPS results there the gains are anywhere from a few FPS more under light CPU loads to 20FPS more with heavier loads.

    The amount of FPS gained by RAM overclocking is much less consistent when compared to GPU overclocking due to it being bound to the CPU job of keeping the GPU fully fed without bottle-necking.

    The CPU can go from doing very little to supply the GPU to doing a great deal at different times during the game. The GPU, unless it hits a frame limiter like v-sync will be running at 100% all the time unless it is bottle-necked by the CPU.
  • blackmagnum
    Did you know that overclocking the memory might corrupt the data? That's why you should only overclock ECC memory.
  • inmyrav
    How about some measurements to show the alleged benefits of overclocking memory? I have never seen a significant impact on gaming performance even from significant overclocks / reduced timings in memory. I'd be interested to see how much the benefits are and where exactly they exist, if they do?
  • Sabishii Hito
    Memory OC is mainly for fun and competitions, and I thoroughly enjoy it.
  • daglesj
    Buy the ram that looks the coolest in your motherboard. That's the only valid advice you need these days.
  • g-unit1111
    That's very interesting. I had no idea you could identify that much from a RAM module. Kind of like how you can identify everything about a car from the VIN number.
  • JackNaylorPE
    Quote:
    But current-generation heat spreaders from reputable vendors do make a difference in operating temperatures, and when the same kit is provided in two different packages—one with a heat spreader and one without—going for the heat spreader makes technical sense.


    What is the difference ? Why is it significant ? Here I see the aesthetics as the primary driver here. Yes, of course, two sets of modules, one with and one w/o the HS, take the one with ... but what is the thermal issue here ?. I have not heard of an issue with RAM temps since DDR2. As an analogy, while I saw the reasoning behind putting a hybrid cooler on a FuryX, what is the point on a AIB GTX 1070 ? The card operates well below (10C) it's throttling point on air so, exactly, what is the hybrid doing for me ? What do I get out of the extra $100 ?

    The only cooling effect of these big tall toothy coolers is that they "look cool". While they served a purpose (when they were effective) w/ DDR2, they were absolutely useless on DDR3. And DDR4 runs even cooler.

    http://benchmarkreviews.com/index.php?option=com_content&task=view&id=773&Itemid=67&limit=1&limitstart=1
    Quote:
    At more than 2" tall in certain areas the Corsair Vengeance could pose a problem for users like me who use large coolers such as the Scythe Mugen 2. I was able to use the Corsair Vengeance only after I mounted the fan on my cooler on the backside. Size is definitely a concern with heat spreaders of this size and therefore I encourage users to check that they will have enough space under their heatsinks before purchasing the Corsair Vengeance kit.


    http://benchmarkreviews.com/index.php?option=com_content&task=view&id=773&Itemid=67&limit=1&limitstart=6
    Quote:
    The problem I have with the Corsair Vengeance is the same I have with many kits of RAM on the market. Companies insist on putting large coolers on their RAM and it limits the choice in CPU heatsinks that can be used within users system. DDR3 does not require these elaborate coolers with its lower voltages which translate to lower temperatures then RAM saw during the DDR, and DDR2 era. Corsair is correcting this with low profile versions of its Vengeance line but ultimately I would like to see the average size of coolers drop instead of having to look for specific low profile versions of a memory line.


    When such a "makes a difference" statement is made, as a reader, I want to see the data behind the conclusion.. 1) what are the differences in temps ? 2) how is this significant ? and 3) what are the performance impacts in something we actually do (besides benchmarks).

    With regard to the "glued on" comment, this is an issue that we never see mentioned in reviews / comparisons. Some RAM HSs can be removed with a screw ... some are glued on, why not mention this in a comparison review ? GFX card reviews typically include, RAM manufacturer, specs, how it is being cooled and yet this is rarely done for system RAM.

    Would love to see time and effort put in to showing more than "same ole same ole".

    For example, gaming average fps is limited primarily by the GFX card performance... tho there is some general acceptance that CPU can be limiting in certain games, especially when in multi-player mode. What is less accepted is that memory speed can affect performance... and when this subject is argued, we oft see links to tests that "prove" the hypothesis via "google something that shows this". Sometimes that results in a link that can prove both opposing arguments

    http://www.tomshardware.com/reviews/32-gb-ddr3-ram,3790-10.html

    Metro 2033 shows no performance gain going from 1600 to 2400 DDR speed, and yet F1 shows an 11% increase in average fps. Like anything else, system performance is impacted by the weakest link in the chain. If RAM is not the weakest link, then the impact is zero. So a "test" that "proves this" is misleading. To make testing useful, the script should involve determining under what circumstances RAM becomes the bottleneck and then seeing if faster RAM or overclocking can have in impact.

    Among the areas of relevance therefore, based upon past articles on the topic.

    -what games / programs can be impacted ? (CAD, video editing ...)
    -what actions within those games / programs can be impacted ?
    -what parameters need to be analyzed ... for example min fps vs avg fps ?
    -Is the test bed adequate to perform the evaluation ... for example, what happens to previous tests when 2nd GFX card is added ? ... 3rd and 4th

    Was never a popular undertaking, my guess cause of the amount of effort involved, but here's some old links which did look into some of these aspects

    http://www.anandtech.com/show/2792/12

    22.3 % (SLI) increase in minimum frame rates w/ C6 instead of C8 in Far Cry 2
    18% (single card) / 5% (SLI) increase in minimum frame rates w/ C6 instead of C8 in Dawn of War
    15% (single card) / 5% (SLI) increase in minimum frame rates w/ C6 instead of C8 in World in Conflict

    Also see http://www.bit-tech.net/hardware/memory/2011/01/11/the-best-memory-for-sandy-bridge/1

    http://www.anandtech.com/show/7364/memory-scaling-on-haswell/10
    http://www.anandtech.com/show/6372/memory-performance-16gb-ddr31333-to-ddr32400-on-ivy-bridge-igp-with-gskill/14
  • gamebrigada
    You should mention that some games do not like memory overclocks. Ubisoft is pretty famous in this regard some of their games will run very low frame rates with any overclock no matter how stable. FarCry 4 is a good example.
  • RedJaron
    Everyone asking for benchmarks showing the benefits of OC'd RAM, look back at any number of SBMs, particularly any Codemasters racing games. You will see big fps gains when RAM bandwidth is increased. Other games, hardly anything. This is nothing new. Most games don't care about CPU OCing either.

    However, games aren't the only thing computers are used for. Many professional apps, particularly Adobe products like Premiere and After Effects, are very RAM sensitive.

    330381 said:
    You should mention that some games do not like memory overclocks. Ubisoft is pretty famous in this regard some of their games will run very low frame rates with any overclock no matter how stable. FarCry 4 is a good example.
    I'd love to see the proof of this. It's not difficult to OC your RAM poorly, resulting in lower performance all around, not just in one game. My suspicion if that is the case you saw here, if indeed you did see it and aren't simply repeating something told you.
  • anbello262
    Everyone who is asking for benchmarks, and saying that it makes no sense for gaming:
    Go and read the articles referenced on the first page. There are 4 articles that explain everything you're complaining about.
    It would make no sense to write everything again, that's exactly the point of referencing previous articles.

    It is even explicitly said that OCing ram makes little sense for gaming (except for a very small number of games), but that it can have quite a big impact on other applications, like WinRAR, CAD, Media editing or MATLAB, even more if heavily multitasking. Benchmarks included.


    By the way, right now I'm celebrating my mix&match 24GB of DDR3 (Sniper 2x4 2400@11 + RipjawsX 2x8 2133@9) running at 2200@9 with little effort. I was worried I would have to losen the timings, but it has worked better than expected, with even a mild OC.
  • damric
    Experts know that memory latency has always been bottleneck for gaming. Overclocking timings/frequency/memory controller adds significantly to minimum FPS, which can mean the difference between a stutterfest or smooth rendering. The uninformed user only looks at maximum FPS, which is the GPU bound portion.
  • dish_moose
    looking at these benches : http://techbuyersguru.com/gaming-ddr4-memory-2133-vs-26663200mhz-8gb-vs-16gb?page=4 You don't get a lot of bang for the buck by using the fastest RAM - When buying RAM, I generally go for the fastest RAM(speed & Timing) that has the best price/performance ratio. I rarely OC memory unless my CPU OC also speeds up my memory clock.
    -Bruce
  • anbello262
    Why would you not OC memory, when you don't actually pay any extra for it?
    It's completely free, and it comes with a small performance upgrade (depending on the application).
    More performance, same price = very good deal, unless your time is too valuable and you dislike overclocking, in which case probably this article isn't for you.