lan bandwidth issue
cswiger at mac.com
Thu Nov 6 10:58:07 PST 2003
On Nov 6, 2003, at 10:26 AM, Jack L. Stone wrote:
> This "price advice" then implies that if Realtek simply raised their
> prices, the card would be just fine...??
No. The price advice implies that a NIC that is worth $5 is probably
not as good as a NIC which is worth $50. If Realtek raised their
prices, their cards would become overpriced "cheapo" NICs rather than
cheap "cheapo" NICs. :-)
> One should not just go by "expensive", but do some research not just
> on that "easy" benchmark. The "cheapo" measurement is very misleading
> considering some cards may just be "on sale" and are fine cards. ...or
> because they use the rlx driver....
Speaking of which, /usr/src/sys/pci/rl.c provides some very specific
technical details as to the design flaws with this chipset family:
* The RealTek 8139 PCI NIC redefines the meaning of 'low end.' This is
* probably the worst PCI ethernet controller ever made, with the
* exception of the FEAST chip made by SMC. The 8139 supports bus-master
* DMA, but it has a terrible interface that nullifies any performance
* gains that bus-master DMA usually offers.
* For transmission, the chip offers a series of four TX descriptor
* registers. Each transmit frame must be in a contiguous buffer,
* on a longword (32-bit) boundary. This means we almost always have to
* do mbuf copies in order to transmit a frame, except in the unlikely
* case where a) the packet fits into a single mbuf, and b) the packet
* is 32-bit aligned within the mbuf's data area. The presence of only
* four descriptor registers means that we can never have more than four
* packets queued for transmission at any one time.
* Reception is not much better. The driver has to allocate a single
* buffer area (up to 64K in size) into which the chip will DMA received
* frames. Because we don't know where within this region received
* will begin or end, we have no choice but to copy data from the buffer
* area into mbufs in order to pass the packets up to the higher
* It's impossible given this rotten design to really achieve decent
* performance at 100Mbps, unless you happen to have a 400Mhz PII or
* some equally overmuscled CPU to drive it.
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