Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.

eyeangle

macrumors regular
Original poster
Jan 2, 2014
153
19
Melbourne, Australia
Specifically the 6-Core model because that's what I own.

According to this table from OWC:
https://eshop.macsales.com/shop/mem...1.204008400.1181529146.1468163367#1333-memory
48GB (3x16GB) is the maximum ram I can have.

But I saw a video on YouTube from someone who has the same Mac Pro 5,1 as I do and they have 64GB (4x16GB) so I emailed OWC about this and what they had to say,

"64GB would only be possible using 8x8GB or 4x16GB. Your model has 4 slots and could physically fit 4x16GB but that is not stable, the ram processor cannot handle that amount. Per extensive OWC testing only the 3x16GB is stable."

I also emailed the person on YouTube and they said 64GB worked fine so I'm curious to know how much ram you have and what is your maximum? Have you found that the ram processor can handle over 48GB if you have so?
 

owbp

macrumors 6502a
Jan 28, 2016
719
245
Belgrade, Serbia
64GB with X56xx Xeon (4x16) and 56GB with W36xx Xeon (3x16 and 1x8).

Everything is listed here: https://forums.macrumors.com/threads/mac-pro-cpu-compatibility-list.1954766/

Capture.PNG
 

nbritton

macrumors regular
May 22, 2008
152
112
If you can find non load-reduced 32GB PC3L-12800R modules, the Mac Pro 5,1 can theoretically support up to 256GB. I have yet to see anyone who has done this. I was thinking about it, but the RAM costs like $2k. I recall Sk-Hynix makes non-load-reduced 32GB modules. Right now I have 128GB in mine, which is a dual socket Xeon X5690.
 
  • Like
Reactions: martyr

nbritton

macrumors regular
May 22, 2008
152
112
In theory the max usable memory is 96gigs on OSX and 128gigs if u r using windows on mac. This applies to most dual CPU Macs from 2010 upward

They upped it from 96GB, the current limit is unknown though. I've asked several times what the upper limit is in Mac OS and everyone replies back quoting the limit for virtual memory. However, virtual memory is not the same as how much physical memory Mac OS can address. I can confirm with absolute certainty that the latest versions of Mac OS can address 128GB, because I have 128GB (8 x 16GB) in my Mac Pro 5,1. Due to memory leaks in Safari and Flash I have on one occasion actually run out of memory (perf mon showed all 128GB of ram in use, plus it was using several gigabytes of swap space), so I can further confirm that Mac OS can utilize all 128GB of ram.
 
Last edited:

mastermamo

macrumors member
Jan 14, 2017
85
36
Cape Town South Africa
They upped it from 96GB, the current limit is unknown though. I've asked several times what the upper limit is in Mac OS and everyone replies back quoting the limit for virtual memory. However, virtual memory is not the same as how much physical memory Mac OS can address. I can confirm with absolute certainty that the latest versions of Mac OS can address 128GB, because I have 128GB (8 x 16GB) in my Mac Pro 5,1.
So at 128gig in do u notice any significant increase in speed/performance? If so, what software are these seen in?
 

nbritton

macrumors regular
May 22, 2008
152
112
So at 128gig in do u notice any significant increase in speed/performance? If so, what software are these seen in?

Nope, from a performance stand point it behaves no differently then if I had 64GB. In fact, when memory usage gets really high the systems starts to slow down a lot. I think this slow down could be due to memory compression being turned on, if I disabled memory compression I might see a large speed up, but I've never tried it yet. I'll have to try it, I think this disables memory compression:
sudo nvram boot-args="vm_compressor=1"
 

nbritton

macrumors regular
May 22, 2008
152
112
If performance is the same why go the 128gig route?
Memory compression is turned on by default if I remember correctly. Do u know what the purpose of this is?

Because I'm a power user, I usually have multiple virtual machines running, 30+ windows open, and 100 browser tabs open. Typically I idle at around 50GB of memory used. Also I happened to already have the memory, so it cost me nothing extra.

Memory compression is just what it sounds like. It compress the memory that applications use so that they take up less physical memory, this lets you run more applications on less memory. However, it comes with a performance penalty. If I disabled memory compression I would expect to see an increase in performance.
 

mastermamo

macrumors member
Jan 14, 2017
85
36
Cape Town South Africa
Can u post the terminal commands to enable and disable compression?
Is there any danger inherent in disabling compression?

Also what software do u use to check the amount of RAM in use at any given time?
 

nbritton

macrumors regular
May 22, 2008
152
112

martyr

macrumors member
Jun 19, 2007
34
12
If you can find non load-reduced 32GB PC3L-12800R modules, the Mac Pro 5,1 can theoretically support up to 256GB. I have yet to see anyone who has done this. I was thinking about it, but the RAM costs like $2k. I recall Sk-Hynix makes non-load-reduced 32GB modules. Right now I have 128GB in mine, which is a dual socket Xeon X5690.

I have 64 GBs in my dual socket X5690 and was wondering if I should upgrade to 96GB or 128GB. A few questions:
  1. Did you do any benchmarking between those two?
  2. Is your machine stable with 128GB?
  3. What version of macOS are you running?
  4. Does macOS recognize the full 128GB?
Thanks!
 

reukiodo

macrumors 6502
Nov 22, 2013
420
220
Earth
With dual X5690 cpus:
8x 16GB = 128GB : MacOS 10.14, Windows10, debian11 can all use all 128GB
4x 32GB + 4x 16GB = 192GB : MacOS 10.14 can boot, recognizes 192GB, but only uses 128GB, Windows10, debian11 can use 192GB
8x 32GB = 256GB : MacOS 10.14 locks up on boot, Windows10 can only use 192GB, debian11 uses all 256GB
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.