Re: [tlug] hardware

[Tod McQuillin <devin@example.com>]

On Tue, 20 May 2003, Jonathan Q wrote:

> On Monday 19 May 2003 20:02, Sam Tilders wrote:
>
> > Is there a specific reason you condemn realtek?
>
> Realtek 8129 and 8139 chipsets have had a lot of problems,
> particularly under load and more particularly if NFS is
> involved.  The driver code for one or more of the BSDs
> described the level of brokenness in the the 81x9 chipsets
> in fairly unflattering terms.

Yes, there are some nice comments in there.  Here's what Jonathan is
referring to:

/*
 * The RealTek 8139 PCI NIC redefines the meaning of 'low end.' This is
 * probably the worst PCI ethernet controller ever made, with the possible
 * 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, aligned
 * 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 large
 * 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 packets
 * 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 protocol
 * levels.
 *
 * 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.

[...]
-- 
Tod McQuillin