Often, folks post on topics such as maximum network performance or disk performance they should expect to see from their x86-based server, firewall, etc. And almost as often, some uninformed person posts a reply that says something to the effect of, "Your PCI bus is only 66MHz, which limits you to 66Mbit/sec", or something similar. This is wrong.
The most common PCI bus is 32 bits wide, and operates at 33MHz. Its maximum throughput is thus 32*33/8 million bytes/second. That's about 132MBytes/sec. Some PCI buses are 64 bits wide at 33MHz, such as on several popular Tyan Thunder models. Those have a maximum throughput of 264MBytes/sec. Other boards are 64 bits wide at 66MHz, which is limited to 528MBytes/sec. And numerous motherboard implementations have more than one PCI bus, so you could but high-bandwidth perhipherals on each of the two buses, and not substantially impact performance or cause them to compete for resources. Now, all card/driver combinations have some overhead associated with them. The bus isn't 100% efficient, but on many "consumer-grade" mainboards the 32 bit / 33MHz bus will push 110MBytes/sec or more in real-world use. If you don't believe me, check the 3ware RAID card reviews on storagereview.com (assuming SR is still up). This means a 100Mbit/sec network througput, which is 12.5MBytes/sec, will easily fit within the maximum throughput of the PCI bus. The real issue is kernel efficiency. Zero-copy TCP and things like that are going to improve linux network performance by leaps and bounds. Going from a 132MByte/sec bus to a 528MByte/sec bus will disappoint you :-) This is a popular form of confusion. Mr Billson is not the first person to give someone a misleading answer in this respect, nor will he be the last. I do not intend to put him down by correcting his answer, but I hope my post serves to better inform the readership of this list. On Sat, 2002-02-23 at 09:10, Peter Billson wrote: > There was some discussion last January (2001) about this type of > thing. The problem you will run into if you are using POTS Intel > hardware is the PCI bus speed, so you are going to have a tough time > filling one 100Mbs connection with an old Pentium - assuming an old > 66Mhz PCI bus. You can forget about filling two or more. Also, cheap > NICs will do more to kill your max. throughput. > That being said, I run old Pentium 133s with 64Mb RAM in several > applications as routers and can notice no network latency on a 100BaseT > network, but I have never benchmarked the machines. Usually the > bottlenecks are elsewhere - i.e. server hard drive throughput. Packet > routing, filtering, masquerading really doesn't require much CPU > horsepower. -- Jeff S Wheeler [EMAIL PROTECTED] Software Development Five Elements, Inc http://www.five-elements.com/~jsw/ -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]
