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AW: What is the speed of PCI

Hi Jay,

Well, a lot of questions. I will try to give you an answer for the standard
pci system. This system can be build either with 32bit or 64bit and could be
run at 33MHz or 66MHz. There some limitations concerning the bus load of
such systems. 66MHz systems can be used inside a system witch us running
without connectors. At least for the standard PC connectors I haven't heard
about any 66MHz system. The organisation of the bus is similar to the ISA
bus. A short connector in the PC means 32bit, a long (extended) connector
means 64bit.

If you would calculate the performance of the system you ran in some
trouble, because this depends on a lot of things:
First, the bus system itself. For the calculation you need the number of bus
cycles for a transfer. For a single transfer you need a minimum of 4 cycles.
This means, if you transfers single bytes you would reach
33M / 4 cycles * 1 Byte = 8,25 MByte/s (66Mbit/s)
But this is an absolute worst case and only for very, very badly desinged
components.

The standard single beat transfer (one longword) results to
33M / 4 cycles * 4 Byte = 33 MByte/s (264Mbit/s)

If you would have high performance the components should strongly use burst
accesses over the bus. For this you need a DMA controller inside the network
chip, which can read data out of the mainboard. In this case the absolut
needed cycles reduce (cycles = one for arbitration, one for adressing, n
cycles for data, one cycle for termination).
So, if you would transfer your data in burst of 8 longwords (you need a fifo
with a minimum of 32 byte depth in the network chip) you need 11 cycles for
the transfer. This results to
33M / 11 cycles * 32 Byte = 96 MByte/s (768Mbit/s)

Ok, for infinit bursts you would reach the theoretical limit of 132MByte/s,
but never really think about it.

The 96 MByte/s is a could value for the PC market. In our tests we really
reach transfer rates of 80...100MByte/s (but for writing into the main
memory).

For 64bit systems you can double the transfer rate and for 66MHz systems you
double it again.

That's the truth for the bus system itself. The next point ist the real
length of the the transfers. During the transfer the initiator and the
target can force wait cycles and terminations with retry's. The depens, how
fast the initiator (in this case your network card) can handle the data.
Because you use gigabit cards, I think it will be very fast. But on the
other side you have the PC PCI-Bridge to the main memory. During an access
either the SDRAM could be in use by the processor or if it is free, it need
some time for the data access. This time results either in an termination
with retry or wait states. In both cases you loose time. How much this slow
down the transfer, depends on the design of the PCI-Bridge and the main
memory.

The transfer rate does primary not depends on the direction. But the problem
in the PC is the following:
A DMA controller normally is created on the side of the data source. In this
case the transfer would be initiated, if enough data is available. The data
could be stored in a fifo inside the target and stored to memory a few
cycles later (if the memory is ready). This result in a maximun bandwidth.
In the case of the network cards the DMA controller for sending should be
placed inside the PC based PCI bridge. But it is not available at this
point. The result is, that reads out of the main memory of the PC is
significant slower than writes to the memory. In this case I assume the
result would be better for data writing to memory rather than reading.

Question to the other guys: Is this currently true ? Or is a DMA controller
available in the north bridges ?

Unfortunatly I can give no black&white answer. If the system is designed
well, you could get transfer rates up to 100MByte/s for a 32bit/33MHz system
(400MByte for a 64bit/66MHz system). But the real transfer rate depens on
all (!) components in the system. Look for the bottlenecks.

Greeting,

Andreas Heiner




-----Ursprüngliche Nachricht-----
Von: Jay Cann [mailto:jcann@genuity.com]
Gesendet am: Montag, 23. April 2001 15:48
An: pci-sig@znyx.com
Betreff: What is the speed of PCI

Hi,

OK - let me first tell you that I am a network (Cisco) guy not a PCI
engineer.  I need layman's help.

I am trying to write up a Network presentation that explains the real speed
of network cards.  For example I have hard that a Gigabit PCI network card
can only support 160 mbps.

Instead of working with rumors I thought I would come to the source and ask
the experts.

What is the megabit through put speed of the following?

PCI-X
Mini PCI
PCI Local Bus
PCI to PCI
PCI 32bit
PCI 64 bit
PCI Mobile
Small PCI
PCI CD-ROM
PCI 33mhz
PCI 66mhz

What about duplex?  (example - if the PCI bus can support 160mbps is that 80
mbps in and 80 mbps out? )

If you have is the speed limited by the PCI bus.   (Example if I have 3 PCI
network cards they can only support 160 mbps together because of the bus
limitation)

I am sure by the above - you can tell that I have know idea what I am
talking about when it comes to PCI.

Also anyone know any of the above answers for ISA, EISA and AGP.  Not that
they use / have ISA and AGP network card today but who knows.   :-)

Any help would be greatly appreciated.

I would be more that happy to share my presentation with anyone who wants it
after I get all the required info (ISA, EISA PCI, AGP, Anything else I
missed ? )

Jay Cann
IT R&D Network Engineering
Genuity