[NTLUG:Discuss] Re: athlon 64 3200+ -vs- P4 3.4Ghz -- NUMA/HyperTransport v. AGTL+, /lib64 v. /lib

[Bryan J. Smith]

On Mon, 2004-11-08 at 15:08, David Simmons wrote:
> Had a thought and wanted to throw it out....I know that there will be a
> 'dedicated bus' for video

Actually, in most systems, the PCI is bridged off of AGP (which is a PCI
bus as well).

> and gigabit lan...

Correct.  More and more on-chipset NICs are using a dedicated
PCI-Express x1 channel right in the peripheral chip.

> but it seems like most of the 'contention' in a multi-disk system is
> data traffic.

Most 2-4 SATA ports in PCI-Express chipsets are also on a dedicated
PCI-Express x1 channel.

> What I mean is this - system has a few hard-drives in it - OS / Data
> (maybe on a supporting card, RAID, etc)

Which brings in another question.  How does the RAID work?

If it's "software" RAID (including those "FRAID" cards and on-mainboard
logic), then it's using the CPU.  So _all_ transfer goes over the
interconnect.  In the case of RAID-1, this means double the data
transfer through the interconnect.  In the case of RAID-5, _all_ data
written must go through the CPU first, meaning read through the
interconnect, so XORs can be calculated (the XOR operations themselves
are the least of the performance concerns -- it's the fact that the CPU
must read _every_ byte to compute that XOR).

If it's "hardware" RAID (ASIC+SRAM or microcontroller+SDRAM), then it's
localized to the on-board intelligence on the card itself.

> and a few CDRom's.  Currently, it seems that my CD-Rom's are averaging
> 15x (while their 'true' speed is much higher).

Actually, using CAV (constantly angular velocity), most of the _fastest_
CD drives are only capable of 12-20x on the innermost track.  They don't
reach 32-48x until you get to the outer tracks.

> While I know that a great deal of data/throughput is needed for video -
> and the dedicated bus might actually make true gigabit lan a reality -
> doesn't the main issue seem to be data I/O...or am I missing it?

Yes, and that's why most SATA channels are on their own PCI-Express x1
channel in these newer chipsets.

The legacy PCI bus is then left for audio, parallel ATA (largely optical
drives which are 1/10th the speed of a hard drive), KB/Mouse, USB,
etc...

-- 
Bryan J. Smith                                  b.j.smith at ieee.org 
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