How important is L3 Cache?

[BrettT 63]

I am considering doing a new system but am presently weighing my options.  I presently have a 6850K @ 4.2 GHz     1080 GTX   and am using it to run an HTC Vive.  I am considering getting the RTX 2080 Ti, HTC Vive Pro and either 9900K or the 8086K.  I am not in a rush.  My system runs decent with locked frames at 30 and able to maintain that in all but the most difficult areas.  In those areas it dips down to 20.  I am using Fly Inside so though my simulator is operating at 30 FPS my video output is running at 90 FPS which makes for a smooth experience.

What I am hoping to accomplish is to bolster the FPS in the difficult areas and be able to add in more dynamic lighting and shadows and to increase my overall resolution with the Vive Pro

So the 3 processors in the discussion are:

6850K - 4.2 GHz - 6 Core - L3 15MB

8086K - 5.0 (?) GHz - 6 Core - L3 12MB

9900K - 5.0 (?) GHz - 8 Core - L3 16MB

My main question is how important is the L3 cache with regards to overall performance (FPS, information handling, texture loading).  With the 8086 I am giving up 3MB of L3, maintaining the same number of cores and gaining 0.8 GHz in speed.  I have always suspected that the L3 cache had a subtle but noticeable impact but have nothing to back it up

Interested in any thoughts

Regards,

 

Brett

 

[Bob Scott 7,715]

I think the impact of memory performance is probably one of the least understood facets of hardware performance w/r/t P3D.

Your 6850K has significant advantages in that department over the mainstream consumer CPUs and chipsets because of the larger (and 20-way set-associative) L3 cache and also quad-channel DDR4.  That won't make up for a clock speed 24% slower than an 8086K or 9900K at 5.2GHz, but it certainly makes the most out of the clocks you're getting.

L3 cache is important, but there is a point of diminishing returns, which would explain why intel has landed on 2MB per physical core and not gone beyond that in several generations of "mainstream" CPUs.  If it made a big difference, we'd see movement to add per-core L3 over time I think.  Can't deny that more is generally better, and since L3 cache is shared across the cores, that 16MB on the 9900K is something I wouldn't mind having

That said, I'm happy with what I'm getting out of an 8086K and its 12MB together with fast 8.3ns RAM.  I do wish there was more data on the effects of memory performance.

Regards

[BrettT 63]

Thanks Bob,

The cost was one of the things that had me curious with regards to the 6850K.....i.e what makes it more expensive.  When I originally purchased it the 6700K had not come out yet and I was wanting something more than 4 core anyways.  I believe I bought it for about $600 USD on sale a couple years ago.  That puts it more expensive than both the 8086K which has less L3 and the 9900K which has slightly more L3 and an extra 2 cores.  Is it the PCI lanes?   Just kind of thinking out loud.  Of course it could be a quantity thing where it is more of a niche CPU with capabilities beyond some of the limitations of a mainstream CPU and thus not sold in the same quantities as the mainstream CPU and thus more expensive. 

[Bob Scott 7,715]

At the time you bought your 6850K, it was a current-generation Extreme Edition CPU, a hexacore when 6-core CPUs weren't common, had a quad-channel IMC, more cache, 40 PCIe lanes, etc.  I spent $1000 on a quad-core i7-975 Nehalem in 2009 and the same on a dual-core X6800 Conroe CPU in 2006.  The advantages were there at the time, but the edge you get at those premium prices is but a temporary, fleeting thing.  My 2011-vintage $280 Sandy Bridge i7-2600K beats both of those, and the i7-4790K, i7-7700K, and i7-8086K CPUs that came after absolutely eclipse them.  All are capable processors still, but no longer cutting edge.  I still have my Conroe in a working PC and use it occasionally with a few niche WinXP hardware devices/software titles that never made it to 64-bit.

The dilemma always is whether to pay the price for cutting edge now, or wait a few years and get it at mainstream prices.  FWIW, the Conroe was the biggest single-step improvement in the sim I ever had...never regretted the few good years that one gave me before something better came along, as it always does.

Cheers

[DaveCT2003 1,820]

I was once a computer hardware engineer before making my way into Program Management.  From my perspective, one should always get as much Cache (L2/L3) as one can get! 

With the above in mind, I couldn't possibly agree more with Bob.  In fact, I built a i7-4770K (I lucked out in the lottery with an incredible processor that beats most i7-4790Ks I've worked on), and I'm still using it today and have no need to upgrade as of yet.  Well, I moved to an SSD and replaced my NVIDIA 900 Series with a 1080, but the same 5+ year old motherboard and processor.  Watercooling has helped to keep the processor in great shape (just a simple Corsair H110, but it keeps my processor at excellent temps even though I've overclocked to 4.5GHz).

I hope this helps, and I wish you the very best!