G.Skill release DDR5-8400 Cl40 kit

Habib-CFD · April 18, 2024, 00:26

G.Skill release DDR5-8400 Cl40 kit with 130 GB/s of Bandwidth in dual channels. I am curious about any CFD benchmark using these insane rams. Considering the DDR5-5200 kit can provide a maximum of 332.8 GB/s at octa channels, it must be a big step for PC territory.

Link

flotus1 · April 18, 2024, 16:25

The thing with these super high transfer rates is: they are mostly for show.
First, you need to get lucky with the CPU. Not all IMCs will be able to run these transfer rates.
And if you get it to run, (sub-)timings will usually be so terrible, that they negate most if not all of the benefits of the higher transfer rate.
Or to put it very bluntly: they are halo products for bragging rights. You spend a lot of money for worse results.

enthony · April 19, 2024, 05:18

It will be interesting to see if motherboards can even handle this high RAM speed. I just hope it doesn't come at a ridiculously high price point

Habib-CFD · April 19, 2024, 06:03

Quote:

Originally Posted by enthony

It will be interesting to see if motherboards can even handle this high RAM speed. I just hope it doesn't come at a ridiculously high price point

Most releases of the Z790 board can support, however as the flotus1 mentioned the limitation usually occurs by CPU IMC. Based on the YouTube reviews, the Intel 14 generation K series CPU supports up to 7800 MHz without out blue screen

.

wkernkamp · April 19, 2024, 18:51

The 7800x3d can probably support it. The nice thing about the high MCLK (4200) is that you can run FCLK and UCLK at 2100. CFD performance pretty much scales with FCLK unless the ratios are unfavorable.

My 7800X3D runs fine at FCLK 2133 so that should not be a problem. Cannot get the UCLK stable at 3200 for DDR5-6400 (DDR5-6200 is solid). The bottleneck on this CPU is FCLK. However, there is no efficient FCLK that goes with DDR5-6400 and above with UCLK==MCLK. For a good result, you would need a UCLK==FCLK and MCLK=2*UCLK. So, 2100, 2100 and 4200 matching DDR5-8400. I expect that to work out of the box with the XMP/EXPO profile, from what I have seen about memory overclocking on this CPU. Might even go a bit higher.

The 7900X3D and 7950X3D have double the Infinity Fabric bandwidth (controlled by FCLK) because they have two CCDs. At 2100 FCLK that is 134.8 GB/s total for these dies. This will be a match for the 2xDDR5-8400 memory bandwidth. The second die does not have the large 3D cache, so memory performance will not double relative to the 7800X3D. However, you should be able to do well with 7000 series dual CCD cpus. In theory faster than the intel i9 13900 and 14900 which max out at DDR5-7800.

April 18, 2024, 00:26	G.Skill release DDR5-8400 Cl40 kit	#1
Habib-CFD Member Join Date: Oct 2019 Posts: 65 Rep Power: 7	G.Skill release DDR5-8400 Cl40 kit with 130 GB/s of Bandwidth in dual channels. I am curious about any CFD benchmark using these insane rams. Considering the DDR5-5200 kit can provide a maximum of 332.8 GB/s at octa channels, it must be a big step for PC territory. Link

April 18, 2024, 16:25		#2
flotus1 Super Moderator Alex Join Date: Jun 2012 Location: Germany Posts: 3,428 Rep Power: 49	The thing with these super high transfer rates is: they are mostly for show. First, you need to get lucky with the CPU. Not all IMCs will be able to run these transfer rates. And if you get it to run, (sub-)timings will usually be so terrible, that they negate most if not all of the benefits of the higher transfer rate. Or to put it very bluntly: they are halo products for bragging rights. You spend a lot of money for worse results. Habib-CFD likes this.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Heat Release vs. Apparent Heat Release // Comparison with GT Power	MFGT	CONVERGE	10	August 17, 2016 03:38
Openfoam Ubuntu 12.04 Unmet dependencies	slls33	OpenFOAM Installation	10	April 9, 2013 05:16

April 19, 2024, 05:18		#3
enthony New Member Anthony Join Date: Jan 2024 Posts: 1 Rep Power: 0	It will be interesting to see if motherboards can even handle this high RAM speed. I just hope it doesn't come at a ridiculously high price point

April 19, 2024, 18:51		#5
wkernkamp Senior Member Will Kernkamp Join Date: Jun 2014 Posts: 372 Rep Power: 14	The 7800x3d can probably support it. The nice thing about the high MCLK (4200) is that you can run FCLK and UCLK at 2100. CFD performance pretty much scales with FCLK unless the ratios are unfavorable. My 7800X3D runs fine at FCLK 2133 so that should not be a problem. Cannot get the UCLK stable at 3200 for DDR5-6400 (DDR5-6200 is solid). The bottleneck on this CPU is FCLK. However, there is no efficient FCLK that goes with DDR5-6400 and above with UCLK==MCLK. For a good result, you would need a UCLK==FCLK and MCLK=2*UCLK. So, 2100, 2100 and 4200 matching DDR5-8400. I expect that to work out of the box with the XMP/EXPO profile, from what I have seen about memory overclocking on this CPU. Might even go a bit higher. The 7900X3D and 7950X3D have double the Infinity Fabric bandwidth (controlled by FCLK) because they have two CCDs. At 2100 FCLK that is 134.8 GB/s total for these dies. This will be a match for the 2xDDR5-8400 memory bandwidth. The second die does not have the large 3D cache, so memory performance will not double relative to the 7800X3D. However, you should be able to do well with 7000 series dual CCD cpus. In theory faster than the intel i9 13900 and 14900 which max out at DDR5-7800.