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Old 2018-10-17, 10:53   #1
simon389
 
Aug 2013

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Default Which DDR4 RAM is best for LL on intel CPUs?

I’m seeing a lot of options and am getting confused about which to buy, from expensive 4700mhz DDR4 RAM with terrible CAS latency (28-45-45) to affordable DDR4 RAM at 3000mhz with great CAS latency (15-15-15). Is it all the same for LL crunching? Or is there a sweet spot for MHz/CAS?
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Old 2018-10-17, 14:09   #2
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Quote:
Originally Posted by simon389 View Post
I’m seeing a lot of options and am getting confused about which to buy, from expensive 4700mhz DDR4 RAM with terrible CAS latency (28-45-45) to affordable DDR4 RAM at 3000mhz with great CAS latency (15-15-15). Is it all the same for LL crunching? Or is there a sweet spot for MHz/CAS?
I read a paper sometime last year written by a ram manufacturer ( Micron, I think ) that said faster speed ALWAYS is better than lower CAS.

For LL crunching, assuming you have a decent number of fast cores, since memory access is the bottleneck, the faster the better. I believe George wrote a post within the past 10 days that his 7820x ( 8 cores ) using fast memory ( somewhere in the 3200+ range ) completely saturates the memory. This was to point out that additional cores wouldn't help thruput much.

Every post about memory I have ever read in the last x years ( 3 < x < 10 ) has reiterated that Samsung has THE best chips so look for dimms made using their chips.

Actually using 4700mhz memory would require other system components, particularly the motherboard, that supports it. Also, case ventilation to keep those dimms cool would help also.
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Old 2018-10-17, 14:36   #3
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Originally Posted by tServo View Post
... look for dimms made using their chips.
The companies that make chips are largely unknown to the public and are frequently exactly the same for quite a few large brand names.

<<Put you pet company name here>> just slaps a sticker on them.


The real question that good reviews are addressing is how a particular memory model is organized. Most of the time, though, this is a rather useless information because many sellers put a generic picture and description and don't pass the detailed information on to buyers - they know that 90%+ of the buyers will buy it anyway based on whim and rumors.
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Old 2018-10-17, 14:37   #4
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Remember to run dual-channel or quad-channel (or 6-channel for highend Xeons) RAM, whatever the best your motherboard and cpu supports, that is the biggest performance gain (not to be confused with dual-rank RAM)

Last fiddled with by ATH on 2018-10-17 at 14:37
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Old 2018-10-17, 15:59   #5
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In order or priority (highest first) if money is no objective:
1, fill the ram channels the system allows
2, get the fastest ram that is compatible
3a, if possible, get dual-rank ram.
3b, if you have single rank ram, aim for 2 modules per channel
4, timings - it makes a small difference compared to the above

The problem with 2 above, apart from cost, is knowing if your system will run stably with high speed ram. Compatibility is a minefield.

At a practical level, I'd just get something nice around 3200 and you get whatever performance you get.

My 6700k system has G.Skill Ripjaws V 3200C16 2x8GB, which are dual rank modules. That gave ~25% more throughput than my other system which only had 3000 single rank modules. I spent weeks questioning my sanity when I saw that difference, before I figured out it was due to rank. Note the difference wont always be of that magnitude, but it definitely helps a lot where ram bandwidth is the limiting factor. I believe the 3200C14 or C15 in that same series use Samsung B-die in single rank configuration though (on 8GB modules). Ram manufacturers may also change the chips used without changing the marketing name, so that's an extra complication. 4GB DDR4 modules are almost certainly all single rank. 8GB modules may be single or dual rank, but the trend has been towards single rank for a long time. I don't have any 16GB modules but suspect there is a good chance of dual rank there.
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Old 2018-10-17, 17:20   #6
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All the 16 GB modules I have dual rank. But that's an expensive way to buy ranks.
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Old 2018-10-17, 18:10   #7
simon389
 
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Quote:
get the fastest ram that is compatible
So, basically, ALWAYS prioritize MHz over CAS. So 4700mhz CAS 45 is the best?
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Old 2018-10-17, 18:19   #8
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Quote:
Originally Posted by simon389 View Post
So, basically, ALWAYS prioritize MHz over CAS. So 4700mhz CAS 45 is the best?
Yes, but if your motherboard or CPU will not run stably with memory that fast then you are just wasting money.

Another thing to consider is that memory prices escalate quite quickly once you get past the "sweet spot". I haven't looked at today's prices, but you might find that you could build two complete systems with DDR4-3200 for the same price as one system with DDR4-4700.
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Old 2018-10-17, 18:40   #9
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Yes, but if your motherboard or CPU will not run stably with memory that fast then you are just wasting money.

Another thing to consider is that memory prices escalate quite quickly once you get past the "sweet spot". I haven't looked at today's prices, but you might find that you could build two complete systems with DDR4-3200 for the same price as one system with DDR4-4700.
If I get the Corsair 4700mhz 19-19-19 on NewEgg (and compatible MSI Z370i Gaming Pro Carbon AC mobo), will the i3 8300 CPU be enough to saturate it, or should I get a faster CPU?

Last fiddled with by simon389 on 2018-10-17 at 18:43
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Old 2018-10-17, 18:55   #10
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Quote:
Originally Posted by simon389 View Post
If I get the Corsair 4700mhz 19-19-19 on NewEgg (and compatible MSI Z370i Gaming Pro Carbon AC mobo), will the i3 8300 CPU be enough to saturate it, or should I get a faster CPU?
Consider Xeons if you have the money. More cores, bigger cache, ECC ram and more memory channels, and maybe more sockets!

Last fiddled with by paulunderwood on 2018-10-17 at 19:00
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Old 2018-10-17, 19:06   #11
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Is the system primarily for doing work like this, or will it also be used for general tasks?

For the 8300 (4 cores, 3.7 GHz) if you want 16GB of ram, I'd go 4x4GB of 3200 of a recently launched set from a well known brand. Maybe a higher speed grade if the price difference isn't too much. 3600 would be better but it'll be a question of diminishing returns much above that.

You can look at the mobo's QVL if you want to try and check ram compatibility, but their lists will always be limited.

I would add, I'd be extremely cautious about the support and stability of 4700 speed ram. For this particular CPU I'd keep below 4000. I previously worked out a rule of thumb that for a quad core Intel, aim for a ram speed comparable to the CPU clock to be largely not limited by ram (>90% of potential). So for a 3.7 GHz CPU, look around 3700 ram speed. I forgot if that took into consideration rank or not though, too long ago. Since most of my quads run 3200 or slower, I knew they were all losing some performance from the ram but even 3200 speed was expensive then.

Note the overall throughput is dependent on the balance between the CPU and ram, and we're kinda in that transition state from one to the other, so don't over-think about getting the fastest ram.
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