[geeks] w00t! AlphaServer SMP goodness

[Eric Dittman]

> The PC164 runs in 128 mode. Unfortunately I have not enough SIMMs
> to run it in 256 bit mode. The memory access speed ist not that 
> different. The SDRAM runs with "83 MHz" = 12 ns. This is the speed
> of data transfers. But you have a latency of several cycles when 
> a new address is accessed. That is more or less in the magnitude
> of FPM / EDO RAM. Accesses of subsequent addresses are faster 
> than the first access with SDRAM and FPM RAM. The only difference
> is the interface, the "bit bucktes" that do the actual data storage
> are the same, regardless what type of DRAM. 

I just remembered that the PWS uses paired DIMMs, so it uses
128-bit mode, too, so that really won't be an issue.  The
Pyxis chipset on the PWS does have faster memory access, though,
which you can see if you check the specs for the PWS vs. the
PC164.

I prefer the chipset used in the PC164 and other systems to
the Pyxis as the Pyxis has some annoying bugs, one of which
results in losing some SCSI controllers after a reboot (the
ITI-6200 is one of the affected controllers).  Having to
power down a system doesn't make for easy remote admin.

> What SDRAM makes "faster" is the ability to pipeline. You can do 
> things like: "Command: read addr. A", "Command: read addr. B", 
> "Transfer: data A", "Transfer: data B".
> (At least a hardware designer explaind it that way to me.)

The design of the memory controllers is the issue here, not
the type of memory.

> But the different memory bandwith of the two systems can't make 
> a difference that is that big. I think it is really GCC, what 
> makes the difference. (Look at SGI/IRIX with MIPS Pro compiler
> and GCC. GCC code is knowen to be slower on that platform too.)

The GCC compiler is definitely slower than the Tru64 compiler,
but if you have a chance I'd like to see what the speed difference
is for the same binary run on both systems.
-- 
Eric Dittman
dittman at dittman.net
Check out the DEC Enthusiasts Club at http://www.dittman.net/