[realtek] What throughput can be expected

TD - Sales International Holland B.V. td@salesint.com
Tue Jan 8 06:59:01 2002

On Monday 07 January 2002 18:47, Donald Becker stuffed this into my mailbox:

I'm getting around 2.5MBps (MegaByte per second) on ftp from my linux with 2 
RTL8139's (one towards my internet router the other towards my LAN linux 
being my proxy/NAT machine). This is on a 2.4.5 kernel. The box at my mothers 
house does about 2.5 as well, only it peaks up till 6 sometimes (this is a 
2.2.?? kernel). I think it's partly disk performance and partly system load 
since my network is pretty dead overall so it shouldn't be my hub.

In addition to Donald's reply:
200MBps would mean 1600Mbps... The more technical people among us make 
difference between a small b and a capital B to point out the difference 
between bits and bytes. The confusion around this is also often used as 
marketing techniques. Don't let them fool you. A byte = 8 bits. Thus 1Mbit is 
only 1/8 MByte. So a 1Mbit line (1024kbps) has 1024/8 = 128kByte p/s max 
(theoretically, in real life you have overhead (headers of the packets n 
stuff, and collisions just to name a few i believe a thumb rule is you loose 
about 6-8% in overhead? this is just an estimate, it should be about right).

If my tech is still correct full-duplex means you can send and receive at the 
same time (whilst normally you'd be either receiving or sending not both at 
the same time). Now marketing people will make you want to think you're twice 
as fast because you can use the 100Mbps up and the 100Mbps down at once but 
this does not make 200Mbps.... it's still 100 down and 100 up and mainly 
you'll probably be downloading and if you'd get the max speed you'd use 100 
down (minus overhead) and like max 8Mbit up or so (i'm not good at these 
calculations as I do not know true numbers at the moment, i haven't really 
looked into them but these estimates should be about right) for the ack and 
error packets n stuff. the 200Mbps sortta makes me think of AMD athlons... 
They advertise 266MHz busses everywhere, but they're just 133MHz busses with 
DDR.... ok so in opposite to this they can infact transmit twice the data due 
to the DDR but this doesn't mean that the clock speed doubles... it's still 
at 133

Marketing is bullshit. Get facts :-)


> On Mon, 7 Jan 2002, Ambrus Michael wrote:
> > I'm having problems getting the throughput up on my recently upgraded
> > home LAN. The NIC's "non-branded" but PCI based with the RTL8139 chip.
> > I'm getting roughly 1.2MB/s on a LAN where the two clients used for the
> > tests are the only ones connected. The machines are P166 and AMD450
> > based.
> The packet copying required with the rtl8139 will have a significant
> impact with the P166 macine.
> > I've calculated the statistics simply by transferring large files with
> > (only a few however) application protocols - NFS and Samba. There seems
> > not to be a great difference in performance.
> Use 'ttcp' to test the TCP performance.  Transferring files adds a bunch
> of other issues to the test.
> > Since I'm quite new to FastEthernet my questions are as follows:
> > * What throughput can be expected both in general for FastEthernet and
> > specifically for RTL8139 based NIC's
> TCP Fast Ethernet: 10.9-11.5 MB/sec using the measurement of 'ttcp'.
> (TTCP counts only data payload bytes, and uses MB == 2^20, not 10^6.)
> > * Where can I expect to find the bottle-necks (please elaborate if
> > possible).
> The extra copying copy of packets to and from the memory used by the
> rtl8139, and various overheads depending on the kernel version.  The 2.0
> kernels were more efficient that the 2.2 and 2.4 kernels.
> > * I'm not using the most up-to-date driver, but I think the one that
> > comes with RH7.1 should be recent enough?
> The driver issues are almost always in dealing with media selection and
> error recovery.  The core packet transfer code has not changed.
> > * Is there a utility for managing the NIC EEPOROM without having to do it
> > from DOS?
> rtl8139-diag.c
> http://www.scyld.com/diag/index.html
> [[ This has been mentioned many times, and is on the rtl8139 page. ]]
> > * Can you get 200MB/s full-duplex with a hub-based LAN or do you need a
> > switch for this?
> You need a switch to direct connect to use full duplex.  For typical
> workstation traffic you will see a 3-4% performance improvement.  The
> "200Mbps" (not MBps, it's Mbps) is marketing nonsense.
> Donald Becker				becker@scyld.com
> Scyld Computing Corporation		http://www.scyld.com
> 410 Severn Ave. Suite 210		Second Generation Beowulf Clusters
> Annapolis MD 21403			410-990-9993
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