Re: 64-bit apps [was Re: NSUInteger question]
Re: 64-bit apps [was Re: NSUInteger question]
- Subject: Re: 64-bit apps [was Re: NSUInteger question]
- From: "Michael Ash" <email@hidden>
- Date: Wed, 9 Apr 2008 23:41:19 -0400
On Wed, Apr 9, 2008 at 5:17 PM, Nick Zitzmann <email@hidden> wrote:
> > currently, i believe there are small performance penalties for 64-bit apps
> as we still are in a primarily 32 bit world. i imagine this will shift over
> the next few years, but will 32 bit apps no longer work?
> >
>
> The performance penalties are on PPC64 only. X86-64 doesn't have
> performance penalties, in fact, X86-64 apps will run faster because function
> arguments no longer have to be 4-byte aligned on the stack. They aren't even
> stored on the stack anymore, unless they're very large data structures.
This is not really true. x86-64 can run faster than IA-32 because of a
better overall instruction set. There are more registers available,
and that makes a big difference. However it can also run slower. The
same thing that makes PPC64 apps slower also makes x86-64 apps slower.
Pointers are wider, integers are often wider. All of this takes up
more RAM which in turn consumes more memory bandwidth, allows for
fewer objects to be held in cache, more overhead for page table
management, and more swapping.
> 2. Since there aren't a whole lot of 64-bit apps out there, when some users
> see them using more memory, they tend to think something's wrong with the
> app. This is especially true if GC is also turned on, as it is in 64-bit
> Xcode, and the VM requirement swells to 30+ GB. I don't remember why turning
> GC on does this.
We need to educate users that the VM number is utterly meaningless.
There are a lot of people around from the OS 9 days who still think
that "virtual memory" is a synonym for swap, but in reality it has
nothing to do with performance, RAM usage, or swap.
Mike
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