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First Disappointment: Lynnfield Memory Controller

We decided to add a separate section into our today’s article devoted to the memory controller integrated into Lynnfield processor, because it differs from the memory controller in Bloomfield. The thing is that they in fact modified the entire Uncore block of the new LGA1156 CPUs, namely the way the bus and L3 cache frequencies are formed.

I have to remind you that Core i7 processors in LGA1366 package use one 133 MHz base clock generator (BCLK) and several independent multipliers that form the frequencies of the computational cores, QPI bus, L3 cache and memory controller as well as DDR3 SDRAM. New LGA1156 Core i7 processors have the same frequency forming principles, but fewer adjustable multipliers, because there is no QPI bus anymore, for instance, and hence its independent multiplier has been removed.

As a result, Core i7-800 and Core i5-700 use only three multipliers:

  1. CPU clock frequency multiplier. This multiplier is determined by the nominal CPU frequency and cannot be increased by the user over the nominal value. However, it may change automatically during work due to Turbo Mode and Enhanced Intel SpeedStep technologies.
  2. Memory frequency multiplier. Formally, Lynnfield processors support DDR3 SDRAM with up to 1333 MHz frequency. However, the list of supported multipliers for Core i7-800 processors includes 8x, 10x and 12x, which means that LGA1156 systems can work with DDR3-1067, DDR3-1333 and DDR3-1600 SDRAM. However, Core i5-700 processors, unfortunately, do not support 12x memory frequency multiplier that is why the maximum memory frequency for them is 1333 MHz.
  3. Uncore multiplier that determines the memory controller and L3 cache frequency. This multiplier is locked for all LGA1156 processors. For Core i7-800 it equals 18x, for Core i5-700 – 16x. As a result, the Uncore part of the Core i7-800 processors works at 2.4 GHz frequency, and Core i6-700 – at 2.13 GHz. I have to remind you that in LGA1366 processors this frequency could vary according to users’ needs, but by default it was set at twice the memory frequency.

So, the memory subsystem in Lynnfield processors turns out slower than in Bloomfield processors and not only because of fewer memory channels, but also because of lower memory controller and L3 cache frequencies.

Of course, all this affects the practical memory subsystem bandwidth and latency. If in LGA1366 systems we saw that the use of two active memory channels instead of three didn’t really cause any serious performance drop, then in LGA1156 the memory subsystem is slower, though not too much.

You can see it very clearly in memory subsystem synthetic tests. For example, we decided to compare the practical speed of the memory subsystem in LGA1366 and LGA1156 Bloomfield and Lynnfield processors working at the same frequency of 2.93 GHz. We used DDR3-1333 SDRAM in both systems with the same 7-7-7-18 timings.

For example, we can see the following results in Cachemem tests from the popular Everest diagnostic tool.


Bloomfield 2.93 GHz, three memory channels


Lynnfield 2.93 GHz, two memory channels

While the L3 cache memory of both processors works at not very different speeds, we can’t say the same about the memory. Triple-channel Bloomfield memory controller shows slightly higher performance during all memory operations than the dual-channel Lynnfield controller. The only consolation here could probably be the fact that the memory subsystem of the new Lynnfield platform shows slightly lower latency.

Overall, the same results were demonstrated by another utility measuring the memory subsystem practical parameters – MaxMem:


Bloomfield 2.93 GHz, three memory channels


Lynnfield 2.93 GHz, two memory channels

However, in this case we see not only a slight drop in practical bandwidth in LGA1156 systems, but also a little increase in latency.

So, the new LGA1156 platform cannot be regarded as a fully fledged replacement to the LGA1366. Of course, top Lynnfield processors will be able to compete against junior Bloomfields, but the fastest Core i7-900 will anyway remain undefeated. And this victory will be guaranteed not only by higher clock frequencies and support of CoressfireX or SLI 16x + 16x configurations, but also by a little higher performance of the memory subsystem.

 
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