[X4U] 64 vs 32 was...Re: move to Intel

[David Ledger]

>Date: Wed, 15 Jun 2005 09:41:09 -0400
>From: Richard Gilmore <rgilmor at uwo.ca>
>Subject: [X4U] 64 vs 32 was...Re: move to Intel
>To: "A place to discuss Mac OS X for the casual user."
>	<x4u at listserver.themacintoshguy.com>
>Message-ID: <BED5A835.1529%rgilmor at uwo.ca>
>Content-Type: text/plain; charset=US-ASCII
>
>What makes the G5 so much faster? I've got a 1.8GHZ G5 that seems way faster
>than my friend's Mini at 1.42GHZ I can't see how 380MHZ makes all that much
>difference.
>
>Richard
>
>On 12/6/05 3:29 PM, "David Ledger" <dledger at ivdcs.demon.co.uk> wrote:
>
>
>>>  From: Richard Gilmore <rgilmor at uwo.ca>
>>>  I just got a G5 and I personally do
>>>  not want to go back to a 32 bit machine.
>>
>>  64 bit machines are usually slower than their 32 bit counterparts,
>>  but they handle huge dataspaces. If you don't use > 4GB address space
>>  you don't gain anything, but all those extra bits have to
>>  sucked/squirted serially from and two the disc and other devices.
>>
>>  - - - - - - - -

At 11:09 -0700 15/6/05, x4u-request at listserver.themacintoshguy.com wrote:
>The memory's the same, DDR PC 3200, granted the iMac bus is faster 600 vs
>166 on the mini, L2 cache is the same, don't know about GPU etc...(will
>investigate) but to my mind that doesn't seem enough to account for huge
>difference in performance. If the bus/GPU etc...is all that's making the G5
>faster why put a new expensive chip in there in the first place? From a
>business point of view the profit margin would be much greater by simply
>using a cheaper G4 chip and not redesigning the whole machine around a new
>chip and architecture that's only 380MHZ faster. Why reinvent the wheel? I'm
>not a computer engineer but a 64 bit processor actually being slower than a
>32 doesn't make sense to me and contradicts my anecdotal evidence that the
>G5 is substantially faster. Have I had the wool pulled over my eyes by
>Apple?

Think of it as plumbing. It you want to get a lot of water from A to 
B, you need a wide pipe. A bit of microbore (1/4") pipe in line with 
a 2" pipe gives microbore flow. Mac Mini is microbore in line with 
1.5", G5 Mac is 1.75" in line with 2". (Just a rough idea, don't put 
figures to this). The computer example is different because a CPU 
doesn't need a constant supply of data to do useful work whereas the 
plumbing does.

All a 64bit CPU has over a 32bit one is a wider address bus, and in 
some cases a wider data bus.

A wider address bus means that the memory address space is bigger. 32 
-> 64 = vastly bigger. If you're running a large Oracle database you 
can hit the 4GB limit of a 32bit CPU. This happened years ago on 
Sun/HP/IBM and they went to 64bits. If Apple is going to go after 
this market, it has to use 64bit CPUs.

A wider data bus means that more bytes can be transferred into/out of 
the CPU at once. This does increase speed if the data is in a form 
where this is useful. Usually these bytes are going to/from a disc 
which is a serial device at the read/write head level and is much 
slower and so getting more bytes in/out of the CPU doesn't help. For 
fast source/dest devices it can help if all the bytes are needed. If 
the device can only handle 8/16/32 bytes at once, it still takes 
8/4/2 shots to get all 64 bits there.

When Sun went 64 bit, a lot of their users thought it was faster - 
until they timed it.

David

-- 
David Ledger - Freelance Unix Sysadmin in the UK.
Chair of HPUX SysAdmin SIG of hpUG technical user group (www.hpug.org.uk)
david.ledger at ivdcs.co.uk
www.ivdcs.co.uk