Some PCI motherboards contain DMA chipsets that are unable to support 100-Mbps Fast Ethernet. Because of this problem, the Solaris operating environment does not support 100-Mbps PCI network operation on systems containing the slow chipsets. This problem affects PCI cards only. Other buses are not affected.
The following chipsets are known to exhibit this problem:
82450GX (Orion) (A and B steppings only)
The following chipsets do not exhibit this problem:
82430HX (Triton II)
82450GX (Orion) (C0 stepping and later)
Some slow PCI motherboard chipsets do not support long data burst DMA transfers and are unable to transfer data from PCI cards to system memory sufficiently fast to sustain 100-Mbps throughput. When systems with these chipsets are connected to a 100-Mbps network, data can arrive at a PCI Ethernet card faster than DMA can transfer it from the card to system memory. When this happens, the card's FIFO begins to fill. If this condition persists long enough, the card's FIFO will overflow, causing loss of incoming network data.
When incoming data is lost, higher-level protocols such as TCP or NFSTM will time out and retransmit the lost data. These protocols ensure that all data is transferred, but performance is lowered. If only a few packets are lost, the performance impact may be small or moderate, but if many packets are lost, a very substantial and severe performance loss can arise.
In some cases, a drop in network FTP performance of two orders of magnitude has been seen when using such chipsets, rendering the network unusable. This case occurs when using 100-Mbps cards containing relatively small FIFOs. The cards are designed to be able to hold only a couple of packets, and they depend on the DMA mechanism to transfer data out of the FIFO in a timely way.
In other cases, cards with larger FIFOs are not as severely impacted by the problem, and under normal conditions perform as well on machines with slow chipsets as they do on speedy ones. However, under sustained 100-Mbps operation, this cannot continue indefinitely.
Because of this problem, the Solaris environment does not support 100-Mbps PCI network operation on systems containing the slow chipsets.
In particular, the PCI cards supported by the dnet, iprb, and elx drivers will not provide good performance on machines with the problem chipsets. If 100-Mbps operation is required on such a machine, it is best to use a non-PCI Ethernet controller. It is also possible that the PCI cards supported by the ieef driver, which have larger FIFOs, may function adequately. You must decide whether the performance on a particular machine is adequate for the intended purpose.