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Will DMI 2.0 limit the platform life of X99 based HEDT CPUs?

3K views 3 replies 4 participants last post by  CookieSayWhat 
#1 ·
As Skylake E (or whatever they call the HEDT equal) approaches us, one thing I have realized is that it is not the per core performance of the cores that may cause X99 to age poorly, but rather the lack of DMI 3.0. Skylake is only a few percentage points faster (5-10% depending on which benchmarks) and is perhaps 100-200 MHz slower in clockspeed.

The DMI link for those who do not know is the link between the X99 PCH and the CPU itself. Anyways, here is a typical block diagram for an X99 motherboard.



The problem is the DMI 2.0 link doesn't have a lot of bandwidth. Only 2 GB/s using an x4 link (it is actually 20 Gbit/s, but with a 20% overhead). Every chipset from P67 to X99 had this DMI 2.0 link. The reason why I am thinking this will be is because SSDs have proliferated since the standard was first released and in the case of SATA3 SSDs, have maxed out the SATA specification. It is not just SATA SSDs, it is also possible already with existing USB 3.0 drives to saturate DMI 3.0 if multiple drives are running at once.

It's also important because a lot of people have argued that X99 is more "future proof" than say, Z170 or a newer platform for various reasons (well, if you need the extra cores right now for something like file compression, video encoding, or something with embarrassingly parallel scaling then X99 is the only game in town right now, save buying a used system), due to the presence of the extra cores (and of course new cores will not get much faster because we are approaching the limits of silicon).

Even DMI 3.0 might not be adequate considering a pair of PCIe x4 M.2 SSDs in RAID 0 can already fully saturate the DMI 3.0 link and won't see the sequential gains that you would otherwise expect. It's about 8.0 GT/s (with the overhead, which has been reduced compared to DMI 2.0, IIIRC DMI 3.0 works out to about ~3.93GB/sec), which is essentially a doubling of standards. I had been hoping for more lanes (DMI 3.0 is like 4 lanes of PCIe 3, and I would like to see more - as many as 16 for an HEDT configuration if possible). Note though that the M.2 drives go through the CPU lanes on X99, as the block diagram shows, while on Z170 they go through the PCH and take advantage of DMI 3.0.

My question, then is, will the DMI 2.0, given its current limitations, affect the longevity of the X99 platform and the CPUs, even where CPU performance is not a bottleneck?
 
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#3 ·
I don't see this being a major problem, mostly because of the 40 PCI-E 3.0 lanes from the CPU.

Reasoning:
  • High end SSDs are M.2/PCI-E at this point because of SATA bandwidth limitations.
  • USB 3.1 PCI-E cards can be used if USB bandwidth is an issue.
  • RAID cards can be used for large arrays of SSDs which would saturate the DMI 2.0 link.
Of course, this is dependent on having some PCI-E slots free to install these add-on cards. 3/4 GPU setups in particular may run into this issue. This may be less of a concern on the Nvidia side, given that with the 10 series 3/4-way SLI setups are not officially supported in games.
 
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