I am a video editor and have been waiting to upgrade my 4790k CPU. Initially, the 7900x looked like a great fit but I am concerned about heat (and fan noise). My hope is to have the 7900x running at 4.5GHz for all cores when I encode and when I am editing for timeline playback. Right now I have a Corsair H110 AIO.
I don't have a Skylake-X myself. But based on the reviews I have read so far, my understanding is: Under full multicore load, the processor will lower its clock automatically to stay in the package power limit or/and to stay below temperature limits. You can raise either or both in the BIOS and then might be able to encode at 4.5 GHz on all cores if you run the cooler at full tilt and have good airflow in the case to cool the mainboard appropriately too. But only if your encoding software is not heavy on AVX.
If your encoder makes good use of AVX, then simply dial in a suitable AVX clock offset. Meaning, automatically and temporarily clock those cores down which detect an AVX workload. I used this feature on Broadwell-E and it is great. Despite the lower AVX clock, your encoding will still go plenty fast due to its very high utilization of the hardware available to it. And your non-AVX workloads will not be affected by the AVX clock offset.
I am planning on buying from Silicon Lottery in hopes to get the temps down.
Do you mean to buy a delidded processor? If so, this will get core temps down dramatically. But it means either direct-die cooling, which the cooler base and mounting system need to support, or the potential need to delid + relid again if/ when the 3rd party thermal interface has degraded.
Am I unrealistic to expect the 7900x to run at 4.5GHz with my cooler and keep encoding temps below 70C?
The H110 is merely a slim 280 mm radiator, and that is not very much if we talk about power dissipation far beyond 150 W. You will definitely need high fan speeds.
On the other hand, if the CPU is
not delidded, then core temperatures will quickly and often reach the configured thermal limit, independently of radiator size and fan speeds, simply because of the high thermal resistance of Intel's stock TIM (a layer of paste much much thicker than a soldered interface, and thicker than with properly relidded CPUs).
And as noted it depends a lot on whether or not the encoder uses AVX.
Should I be looking at the 7820x for 4.5GHz and cooler temps?
Going with 7820X instead of 7900X means several things:
- 8 instead of 10 cores: Under full load at same clock simply means somewhat lower power draw but somewhat longer encoding times. How much so depends on how well the encoder scales with core count. If the encoder scales ideally, then of course the 7820X would draw 80 % of the power but take 125 of the time (again, if both processors were compared at same clock).
- Rather than 2 AVX-256 ports and 1 AVX-512 port per core on the 7900X, you'd get just the 2 AVX-256 units on the 7820X. Whether or not your encoder would use the additional AVX port of the 7900X depends on the encoder. It is more likely that it does not. Maybe the software vendor can tell you. If it is prepared to use the additional AVX port of the 7900X, then this would mean increased speed (but by far less than double because of imperfect scaling) at increased power draw of course.
- Only 28 instead of 40 PCIe lanes, which is possibly irrelevant for your use case.
Related to the first bullet point: If 7820X and 7900X were compared not at same clocks but at same power budget, and the encoder scales well with core count, then the 7900X would be faster. This is because you could run its 125 % cores at
more than 80 % the clock compared to 7820X.
While it would be easy to say "run encoding at 3.5GHz" but CPU upgrades are about saving time in my world. So I would want as fast a clock speed I can get away with!
Encoding on all cores, even if no AVX is involved, will likely mean that clocks oscillate between stock all-core turbo and base clock, unless you lift both package power limit and temperature limit, and cool heavily. Much depends on your software, and some on how well you can (and are willing to) optimize/ oversize the cooling.
Again, these are just my conclusions based on the reviews I have read so far. My own overclocking experince is limited to Sandy Bride, Ivy Bridge-E, and Broadwell-E. But behavior of Skylake-X when overclocked appears to be highly similar to Broadwell-E but with the frequency wall of the transistors shifted past the limits imposed by power and temperature.
(Edit: But temperatures regressed relative to BDW-E due to thermal paste instead of solder.)
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BTW, Tom's Hardware will apparently publish another socket 2066 related review this week, based on additional testing of the 7900X in their German lab (probably not yet with 7820X results) [
source, in German]. AFAIU this review will include analysis at different package power levels, and also investigate topics more or less loosely related to those raised by der8auer, but from a different perspective.