[SI-LIST] Re: Motherboard recommendation request
- From: Bob Drzyzgula <bob@xxxxxxxxxxxxx>
- To: "Martin.J Thompson" <Martin.J.Thompson@xxxxxxx>
- Date: Tue, 2 Oct 2001 08:20:37 -0400
On Tue, Oct 02, 2001 at 08:21:38AM +0100, Martin.J Thompson wrote:
>
> <Doug Brooks <doug@xxxxxxxxxx>>
> >
> >We need to buy a new PC. RAM capacity is the MOST important factor.
> >
> >We need at least 4 GB of RAM (is it possible to have more?)
> >Prefer PC100 or PC133 SDRAM
> >prefer P3 or P4 processor
> >
How much do you want to spend? :-)
Take a look at the various boards based on the ServerWorks
HE (Xeon/Slot 2) and HE-SL (PIII/S370) chipsets. SuperMicro
(http://www.supermicro.com) has an HE board that will take
up to four 900MHz Xeons and 16GB of buffered ECC memory. They
also have some HE-SL boards that will take up to two 1+GHz
PIII chips in Socket 370, and 4GB of buffered ECC memory.
These chipsets do two-way interleaving on memory as long as
dimms are installed in pairs. The ServerWorks LE boards are
cheaper and can hold up to 4GB of memory, but they don't
interleave memory. Tyan (http://www.tyan.com) also makes
ServerWorks boards.
> Why not DDR SDRAM @ effectively 266 MHz? I can't recommend a P4 at the =
> moment, my quick hack benchmarking last week showed about 20% inprovement =
> from my 800MHz P3 w/ P100 to 1.7GHz P4 w/ PC800 Rambus DRAM. I hope to =
> get an Athlon with DDR in to evaluate...
I'm not sure, but I think the Athlon boards sort of top out
at 3GB, at least that's the capacity of the AMD760-based
dual-processor Tyan boards. Also, the 20% improvement
doesn't do much good if you wind up having to page.
> OTOH if you are really memory limited (as it sounds like you are) =
> processor speed may not matter - In which case the bandwidth of Rambus may =
> help, if the extra latency doens't hinder you.
>
> Regarding amounts of memory, NT4 and I assume Win2K can only address 2GB =
> from user space - you used to be able to get a version of NT which split =
> the memory 3GB user and 1GB kernel, but I don't know whether that's sitll =
> available. If you are a Linuxer, then I think I remember something about =
> 2GB there as well, but don;t quote me on it - anyone else got a better =
> answer for Linux?
The base Pentium architecture uses 32-bit addressing,
limiting both physical and virtual address spaces to 4GB.
Operating systems usually grab some major portion of the
virtual address space for themselves -- it appears from what
you say that NT takes 2GB. The Linux kernel takes 1GB for the
OS, leaving 3GB for the user process.
This is different than the physical addressing issue, however,
although the OS may lock some physical pages to be mapped to
the same place in all virtual spaces. Since the Pentium Pro,
I believe, Intel chips have had support for Physical Address
Extensions (PAE) that effectively allow the processor to
use 36-bit addresses when accessing *physical* memory. On
a system with more than 4GB of physical memory, this
allows an OS to run multiple multi-GB processes without
having to page. It does not, however, allow a single process
to have more than a 4GB virtual address space. The big downside
to using PAE is that it works by using indirect pointers to
higher memory locations, and you take a CPU hit from the
extra address calculations.
In Linux 2.4.x, you can compile the kernel to use PAE if you
have a big-memory machine such as the Supermicro ServerWorks HE
board. In Windows 2000, you need at least Advanced Server
to get support for PAE. Server (and I presume Professional,
I'm not certain) support only the basic 4GB. Advanced Server
supports 8GB, while Datacenter Server supports up to 64GB.
Datacenter Server is only available through high-end OEMs that
manufacture big servers with more than eight processors, and
from what I've read Microsoft has only sold about 500 copies
of it world-wide. Thus, as a practical matter, I think that
it's fair to say that, short of spending a small fortune,
Windows is limited to 8GB of physical memory. Which, assuming
the 2GB user address space is correct for Windows, is in any
event probably about as much as is useful unless you are
running more than three simultaneous 2GB user processes on
more than four processors.
If you need more than that, you probably want to look at some
other operating system.
Hope this helps,
--Bob Drzyzgula
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