Question: Why ain't I getting gigabit speed?

[Michael MacLeod]

Since it deserved recognition, here's the bit from if_rl.c that John 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.

Rock solid driver for a comically bad chipset. Thanks Bill!


On Tue, Feb 12, 2013 at 7:00 PM, John Nielsen <lists at jnielsen.net> wrote:

> On Feb 9, 2013, at 5:02 PM, Ronald F. Guilmette <rfg at tristatelogic.com>
> wrote:
>
> > P.S.  While I appreciate all the friendly advice people here have given
> > me, i.e. to go with a card based around some non-Realtek chip, I have to
> > say that up until now I have always and consistantly had -zero- problems
> > with the many other Realtek-based 10/100 cards that I have owned and
> used.
>
> A bit OT, but I would say that this is _because_ of the FreeBSD driver
> (rl, also by Bill Paul). Some of the hardware deficiencies documented in
> the manpage and in comments in the if_rl.c are almost comical..
>
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