virtualbox.org

I am going to install some guest Linux distros and I use a 64 bit PC. Though I would like to believe in the benefits of 64 bit OS's over 32 bit OS's, the general consensus seems to be that 64 bit OS's only outperform 32 bit OS's when there is more than 4GB of RAM available to the OS. My computer has 8GB of RAM available total, so the maximum RAM I can give a guest OS while still having enough RAM to have my host OS (Windows 8 x64) perform fully is ~4GB, unless I only want to use 1 OS at a time. If I want to use multiple (2) OS's at the same time, is it still worth installing 64 bit distros in Virtualbox?

The benefit of 64-bit OSes is that they can run 64-bit applications. If you don't use them then you would not actually see an improvement in speed.
As for the 4GB I run 64-bit virtual guests at 512MB and suffer no ill effects. The only time you need more is if you want to do more things in the guest at one time or use something that requires more memory.

That said it will not always be that way. They are already closing down the i386 and moving to i686. (PAE) support

M0rtiferrimus wrote:the general consensus seems to be that 64 bit OS's only outperform 32 bit OS's when there is more than 4GB of RAM available to the OS.

Given the choice of the two on the same PC: the CPU clock speed is the same, bus speed the same, number of cores available the same... but the 64bit OS has twice as much data to shift - why would the latter ever be faster?

Of course you may have an application that requires access to more than 2GB of RAM, but those would be very unusual and in any case this isn't a matter of performance really, just capacity.

Now I'm not saying that 64bit OS's serve no purpose: both OS's and apps are becoming more bloated, so having a 64bit host allows you to do more at once - but I wouldn't expect it to go any faster.

ps. a lot of people confuse 64bit OS with 64bit operations in the CPU. In fact an application can take advantage of most CPU performance features regardless of the bittedness of the OS. Likewise disk size limits have nothing to do with the OS being 64bit or not.

All good points above, and as previously mentioned, no advantage unless you NEED to run something that requires a 64 bit OS on a guest.

I read somewhere that the 64 bit architecture doubled the register count, allowing the compiler to generate code with fewer accesses to memory. From what I recall, this was the basis for most of the 64 bit speed gains on code which did not need the larger amounts of memory.

As noted above, 32 bit has been mostly orphaned, so installing a 64 bit OS would be more future-proof. Unless you have a compelling reason to install a 32 bit guest, it might make sense to install 64 bit.

martyscholes wrote:I read somewhere that the 64 bit architecture doubled the register count

The number of registers is the same, though of course 64bit registers are twice as wide. However that is the CPU, not the OS.

mpack wrote:

martyscholes wrote:I read somewhere that the 64 bit architecture doubled the register count

The number of registers is the same, though of course 64bit registers are twice as wide. However that is the CPU, not the OS.

No, it's double the general purpose registers (and in double width) alright:

https://en.wikipedia.org/wiki/X64#Architectural_features wrote:Additional registers: In addition to increasing the size of the general-purpose registers, the number of named general-purpose registers is increased from eight (i.e. eax, ebx, ecx, edx, ebp, esp, esi, edi) in x86 to 16 (i.e. rax, rbx, rcx, rdx, rbp, rsp, rsi, rdi, r8, r9, r10, r11, r12, r13, r14, r15).

Leak wrote:No, it's double the general purpose registers (and in double width) alright:

Interesting, thanks for that. Most of my assembly level programming these days is done on Arm, Blackfin, DSPs, I've never needed to code for 64bit Intel. I didn't realise Intel had introduced new regs, I thought they had simply renamed and extended the role of the ones previously used for SIMD. [*]

However, we are still discussing the capabilities of the CPU instead of the OS.

[*] The engineer in me wonders how the opcodes have changed. The old opcode fields wouldn't be wide enough to select from a wider range of registers. Please god no more prefix codes...