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Q: Internal bus bandwidth ( Answered 5 out of 5 stars,   1 Comment )
Question  
Subject: Internal bus bandwidth
Category: Computers > Hardware
Asked by: cerebrate-ga
List Price: $15.00
Posted: 03 Oct 2002 04:17 PDT
Expires: 02 Nov 2002 03:17 PST
Question ID: 71970
I'm looking for information on the internal bandwidth available on
various PC buses (ISA, PCI, AGP, etc.). I have the theoretical
bandwidths available already, so what I'm looking for isn't that, but
the real-world bandwidth available allowing for command overhead,
etcetera.

Also, if possible, I'd like to get some idea of how much of that
bandwidth is used by different devices - 10 and 100 mbps network
cards, video adapters, sound cards, &c.

Request for Question Clarification by alienintelligence-ga on 03 Oct 2002 05:40 PDT
Hi cerebrate,

Would you like for us to provide comparisons
from hardware testing sites based on different
motherboards? Have a particular motherboard or
manufacturer in mind?

Will the bus bandwidth usage information from
manufacturers of video, sound, and network
cards suffice for the devices?

thanks,
-AI

Clarification of Question by cerebrate-ga on 03 Oct 2002 08:20 PDT
I'm just looking for averages, really, for planning - most of the
machines I use are Compaq/HP, so theirs would be good
manufacturer-wise, but as that can vary depending on management's whim
I'd like to get an idea of the typical situation.

Regarding the devices, that should be fine, but again I'd like to be
sure that I'm getting an average/typical view of things for the class
of device, rather than a potentially atypical manufacturer.

Thanks,
cerebrate-ga
Answer  
Subject: Re: Internal bus bandwidth
Answered By: haversian-ga on 03 Oct 2002 13:23 PDT
Rated:5 out of 5 stars
 
Hello cerebrate-ga,

Performance of internal buses is dependent on many factors, such as
latency within a chipset, depth and implementation of buffers, size of
data transfers, maximum concurrent transfers, etc.  For these reasons,
the only way to know for sure what sort of performance your
application will see is to benchmark specific configurations.  With
that in mind, I can give you an idea of what maximum transfer rates
you might see are.

Here I omit a discussion of the ISA bus.  It is similar enough to PCI
that conclusions can be drawn based on the trends seen in my treatment
of the PCI bus.  Further, ISA devices are rarely if ever used where
performance is a concern, and an ISA slot has not been standard on
most motherboards for several years.

A standard PCI bus runs at 33 MHz and transfers 32 bits of data on the
rising edge of each clock pulse.  This gives a maximum theoretical
burst transfer rate of approximately 125MiB/sec.  However, there is
overhead on each command which will lower the overall data rate,
depending on the ratio of commands to data transferred.  In the
general case, the performance of the PCI bus cannot be determined
except by direct measurement.  However, I suspect you are primarily
concerned with the performance of a single device in the absence of
other PCI devices, or in the presence of devices making a minimum of
PCI bus transfers.  In real-world tests on a single active PCI device,
you can expect a maximum of approximately 110MiB/s.  See for example
StorageReview.com's review of the Adaptec 2400A RAID controller at
http://www.storagereview.com/articles/200107/200107037410_2.html. 
Scroll down to the Disk/Read Transfer Rate line under the table
labeled RAID 0 with 4 Drives.

64 bit, 33MHz; 32 bit, 66 MHz; 64 bit, 66 MHz, and PCI-X (64 bit, 100
or 133 MHz) all have a higher theoretical peak burst speed but will
show similar efficiency (220 – 440MiB/sec) if other factors are held
constant.  When dealing with these faster and wider PCI buses, it is
important to keep in mind that it becomes more difficult for a single
device to saturate the bus.  This means that either the device in
question becomes the bottleneck, or that you must consider multiple
devices operating concurrently in order to bump up against the PCI
bus' limitations.  In either case, it becomes increasingly difficult
to provide meaningful data from the point of view of the PCI bus
itself.


AGP, which stands for Accelerated Graphics Port, is not a bus at all,
but rather a port.  The difference is that a bus allows multiple
devices to share available bandwidth, whereas a port is a
point-to-point connection between two devices, typically a graphics
card and the chipset.  AGP video cards are extremely well-engineered
to eke the maximum performance out of the port, and with few
exceptions are extremely close to the theoretical value.  For hard
numbers, including the effect of Sideband Addressing (SBA), see this
page at Reactor Critical:
http://www.reactorcritical.com/review-geforce2gts/review-geforce2gts_4.shtml.


The amount of bandwidth consumed by different devices would depend on
the device in question.  10Mbit ethernet cards will transfer
approximately 1.25MiB/sec.  100Mbit cards will transfer about 12.5
MiB/sec.  Gigabit cards about 125MiB/sec (though at this speed, few
cards can achieve over about 800Mbit, even in a 64-bit PCI slot).

Video cards can utilize nearly all the AGP bandwidth provided, but the
specific utilization would depend on the size and number of textures
being transferred, and the number of polygons in each frame being
displayed.  A high-end card such as the Radeon 9700 still will not use
much bandwidth if it is displaying a simple spinning cube, for
example.

Sound cards do not transfer much data at all.  CD quality sound at
44.1khz with 32-bit samples requires a fraction of a megabyte per
second.  Sound cards though are making 44,100 PCI bus transfers every
second, and so are extremely sensitive to PCI bus latency, rather than
bandwidth.

If you require links to more sites with supporting documentation or
quotations, please request a follow-up.

Thank you,
  Haversian
cerebrate-ga rated this answer:5 out of 5 stars
Many apologies for delay in getting back to you on this one.

Exactly the information I was looking for at that stage, were I still
working there to use it - hence my delay - but well worth it anyway
just out of interest. Thanks muchly.

Comments  
Subject: Re: Internal bus bandwidth
From: answerbod-ga on 03 Mar 2005 09:57 PST
 
RAID isn't as great as some reviews make it out to be. Before you
setup a machine in RAID you may want to check this out:
http://www.bestpricecomputers.co.uk/reviews/Home-PC-RAID/

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