Low amount of cache in upcoming i9 processors

Vespill · Jun 9, 2017

I'm not the most experienced in this, but how come they have reduced L3 cache amount in the new i9 CPUs? for example, the latest iteration of the i7-6900k is the i9-7900x. It has 2 more cores, but 16MB cache compared to the Broadwell-E counterpart's 20MB. Why is it reduced like this? Surely more is better?

kanewolf · Jun 9, 2017

Cost. The competition (AMD) doesn't have as much cache in their CPUs so Intel can cut back on cache on the i9 and if you want more you buy a Xeon.

USAFRet · Jun 9, 2017

Possibly not needed, due to better efficiency/speed in other areas.

Can't just look at one factor of the overall specs.

Sakkura · Jun 9, 2017

kanewolf :

Hmm... Ryzen, and IIRC Threadripper, has 2MB L3 cache per core. Intel is stepping down from 2.5MB (Broadwell-E) to 1.375MB. Seems a bit much of a downgrade if they were just matching AMD. And I'd have thought the competitive pressure to provide as much as possible would be higher than ever.

But I think you may be spot on about Xeons. Maybe they're scared of cannibalizing Xeon sales by proving i9 chips with so many cores, so they nerfed the L3 cache to keep enterprise customers on Xeons?

Vespill · Jun 9, 2017

USAFRet :

Sakkura :

Sorry, I do not understand. Do you mean 20MB, 25MB, and 13.75MB?

Vespill · Jun 9, 2017

USAFRet :

Besides core count, GHz, and cache, is there anything that significantly impacts performance? I have to ask as I'm not the most knowledgeable in this regard.

k1114 · Jun 9, 2017

I think you all missed the much larger amount of faster L2 cache. No need for more L3 when you have more L2. I don't know about L1 but chances are, I bet that's more too.

Even if you didn't see the L2 amount, it wouldn't be cost because they're going to use the same dies as the xeons and it's unlikely cache will be a cause for binning. But seeing L2, that's more expensive than L3 so that kills that theory. Also since amd is pushing back, they would want to up them however they can and widen the gap. If they are just getting by, people will see it as intel faltering and that's not an image they want. Intel knows exactly what they are doing.

I'd put architecture as the significant performance factor. Cache isn't something you even look at. But in the end, performance matters and you don't really need to pay such close attention to most specs.

USAFRet · Jun 9, 2017

Vespill :

IPC - Instructions per Clock. How much can it do for each clock cycle.

But generally, overall performance.

GHz is particularly confusing.
Which is better/faster/more efficient?
A 13 year old Pentium 4 at 3.4GHz, or an i7-6700 at 3.4GHz?

Sakkura · Jun 10, 2017

Vespill :

No, I mean the total amount divided by the number of cores. The more cores a CPU has, the more cache it would need to keep those cores fed properly.

k1114 :

Good point. Though Skylake-X still works out at less combined L2+L3 per core than Broadwell-E, it's by a much smaller margin. I wouldn't be surprised if the L2 in Skylake-X is slower than in Broadwell-E, but... it's surely better than L3. So a little less overall cache but considerably faster -> probably better for a lot of workloads.

I'll still be interested to see how much cache the Xeons come with though.

theonerm2 · Jun 10, 2017

Sakkura :

Vespill :

No, I mean the total amount divided by the number of cores. The more cores a CPU has, the more cache it would need to keep those cores fed properly.

k1114 :

Good point. Though Skylake-X still works out at less combined L2+L3 per core than Broadwell-E, it's by a much smaller margin. I wouldn't be surprised if the L2 in Skylake-X is slower than in Broadwell-E, but... it's surely better than L3. So a little less overall cache but considerably faster -> probably better for a lot of workloads.

I'll still be interested to see how much cache the Xeons come with though.

L2 cache is a good amount fast than L3. I'd rather have more L2 than twice the L3.

Sakkura · Jun 10, 2017

theonerm2 :

Yeah, though the larger L2 may be slower than the old, smaller L2. It's (usually) a tradeoff.

Vespill · Jun 10, 2017

Sakkura :

Vespill :

No, I mean the total amount divided by the number of cores. The more cores a CPU has, the more cache it would need to keep those cores fed properly.

k1114 :

Good point. Though Skylake-X still works out at less combined L2+L3 per core than Broadwell-E, it's by a much smaller margin. I wouldn't be surprised if the L2 in Skylake-X is slower than in Broadwell-E, but... it's surely better than L3. So a little less overall cache but considerably faster -> probably better for a lot of workloads.

I'll still be interested to see how much cache the Xeons come with though.

Im buying very soon, if I were to go for it and by the 6950x, will it be able to compete with the performance of the equivalent i9, perhaps after some overclocking? Meaning, if I got it, would I be good for something like 3-4 years? Like mentioned, I work with 4k video and slightly complex 3d models.

k1114 · Jun 10, 2017

The i9 7900x should be out this month so unless you need it right now, it would be advisable to wait. It will be cheaper, $1000 vs $1500 for the 6950x, and be better performance.

Sakkura · Jun 10, 2017

Yeah paying $4-500 less for the 7900X that runs higher clocks is worth waiting for. Plus you get the more modern X299 platform, particularly nice if you want to have this system for a while.

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Low amount of cache in upcoming i9 processors

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