Don't run Snow Leopard in 64-bit

[mmccaskill]

Contrary to belief, Snow Leopard by default (except for Xserves) runs in 32-bit mode. Check for yourself.

uname -a

You should see something similar

root:xnu-1456.1.25~1/RELEASE_I386 i386

I forced it to 64-bit on my '08 Mac Pro and things were slower! Spaces was very slow. Dashboard was quirky. I say avoid it.

 

[Switz213]

Hmmmmm.
iiiiiiiiiinteresting.

 

[thunderclap]

It's my understanding that it's not necessary to enable the 64-bit kernel in Snow Leopard because it's primary function is to allow greater than 32GB RAM. Is this incorrect?

 

[Krafty]

It's my understanding that it's not necessary to enable the 64-bit kernel in Snow Leopard because it's primary function is to allow greater than 32GB RAM. Is this incorrect?

Thats correct but you can't get that across to everyone for some reason. People want to run 64 bit for the sake of saying they're running 64 bit.

 

[mmccaskill]

It's my understanding that it's not necessary to enable the 64-bit kernel in Snow Leopard because it's primary function is to allow greater than 32GB RAM. Is this incorrect?

Oh that is interesting information that I didn't know. I didn't think it was possible to access more than 32GB in 32-bit kernel without PAE.

 

[TheSpaz]

Oh no! Don't do something that isn't supported??? I had no idea! Thanks for the warning?

Okay... I was being sarcastic there.

Most people don't even care about the 64bit kernel.
64bit kernel isn't compatible with a lot of third party drivers yet
64bit kernel is slower in some cases
64bit kernel isn't supported on most Macs

Why in the heck would I want a 64bit kernel? Sounds like trouble to me.

 

[Amdahl]

Oh that is interesting information that I didn't know. I didn't think it was possible to access more than 32GB in 32-bit kernel without PAE.

It IS using PAE. It can't use more than 32GB of RAM, the other poster says. PAE allows 64GB of RAM to be used, but the actual limit is going to be OS dependent.

 

[Zimmy68]

Hmmm, I did everything with the defaults on my clean install of a MacBook Pro 2008.
When I go into Activity Monitor, it is showing Intel(64 bit) under the Kind column for system apps.
Not sure what you are talking about.

 

[CWallace]

Thats correct but you can't get that across to everyone for some reason. People want to run 64 bit for the sake of saying they're running 64 bit.

And then complain that all the programs they use, which have 32-bit kexts, won't work in SL (not understanding because they're running it in 64-bit mode).

Apple knows most current apps are still using 32-bit kexts which is why SL is booting by default into 32-bit mode to maintain as much application compatibility as possible.

 

[Catfish_Man]

I forced it to 64-bit on my '08 Mac Pro and things were slower! Spaces was very slow. Dashboard was quirky. I say avoid it.

Definitely did not see this on my Mac Pro at work; everything felt exactly the same and the limited set of benchmarks I had time to run turned up the same results.

 

[shakenmartini]

It's my understanding that it's not necessary to enable the 64-bit kernel in Snow Leopard because it's primary function is to allow greater than 32GB RAM. Is this incorrect?

Close, it is only for faster access to memory over 32 GiB

Under the 32-bit kernel you can access over 32 GiB just fine, there is just a bit of performance hit for address translations

 

[Catfish_Man]

Close, it is only for faster access to memory over 32 GiB

Under the 32-bit kernel you can access over 32 GiB just fine, there is just a bit of performance hit for address translations

While that's true, it does miss an important point: Many structures in the kernel grow with increased memory (the page table in particular). When these hit the 4GB limit *for the kernel process*, the system will just crash immediately. I don't know where the limit is, but my understanding from others is that it's somewhere around 40GBish.

 

[shakenmartini]

While that's true, it does miss an important point: Many structures in the kernel grow with increased memory (the page table in particular). When these hit the 4GB limit *for the kernel process*, the system will just crash immediately. I don't know where the limit is, but my understanding from others is that it's somewhere around 40GBish.

That is wrong too. We've tested 32-bit SL with kernel extensions accessing up to the full 32 GiB and it works just fine because of address translation. However, in the real world there shouldn't be any kernel derived code trying to access this amount of memory. This is poor coding to do anything like this. Applications should run as applications where they can access all the memory they need under SL (and leopard for that matter).

I can't remember the hard limit for memory for 32-bit kernel mode, but we have only tested up to 32 GiB since there aren't any apple desktops that support more than 32 GiB memory at the moment.

 

[Catfish_Man]

That is wrong too. We've tested 32-bit SL with kernel extensions accessing up to the full 32 GiB and it works just fine because of address translation. However, in the real world there shouldn't be any kernel derived code trying to access this amount of memory. This is poor coding to do anything like this. Applications should run as applications where they can access all the memory they need under SL (and leopard for that matter).

I can't remember the hard limit for memory for 32-bit kernel mode, but we have only tested up to 32 GiB since there aren't any apple desktops that support more than 32 GiB memory at the moment.

When did I say anything about 32GB? I said "40ish", and it could be higher than that. Are you implying that the page tables are O(1) in the number of pages they track? That would be an... interesting data structure indeed.

 

[bearcatrp]

Applications should run as applications where they can access all the memory they need under SL (and leopard for that matter).

Don't think so under leopard. I would get page outs while working in imove due to its 32 bit limit. Less than a gig but still get them. And I have 20gb ram in my pro. Will see this weekend if SL corrects this when I do my next project.

 

[Wraz]

i just restarted my macbook. and held down 6 and 4, nothing happend? maybe i should be glad that nothing changed? atleast i think... how can i tell i still have 32bit?

 

[powerbook911]

i just restarted my macbook. and held down 6 and 4, nothing happend? maybe i should be glad that nothing changed? atleast i think... how can i tell i still have 32bit?

Good question. How can we tell what mode we're in, after playing around with 64?


Will it go back to 32 at next normal restart?

Thanks.

 

[Scottsdale]

I read on another site that Apple has disabled ability to boot into 64-bit for all MacBooks (the consumer models - not the MBPs). Anyone know why they would do that? That's why people cannot hold down 6-4 when booting to SL with their MB. And it works with MBPs... all others that are 64-bit CPUs.