Polling For 100 mbps Connections? (Was Re: Freebsd Theme Song)

Ted Mittelstaedt tedm at toybox.placo.com
Tue Dec 13 00:44:56 PST 2005

>-----Original Message-----
>From: Drew Tomlinson [mailto:drew at mykitchentable.net]
>Sent: Monday, December 12, 2005 12:30 PM
>To: Ted Mittelstaedt
>Cc: Michael Vince; danial_thom at yahoo.com; freebsd-questions at freebsd.org;
>Kris Kennaway
>Subject: Polling For 100 mbps Connections? (Was Re: Freebsd Theme Song)
>On 12/12/2005 8:13 AM Ted Mittelstaedt wrote:
>>  Fundamentally, here's the problem Danial is claiming exists:
>>it takes a certain amount of time to get the packet clocked in
>>from the network into the ethernet receiver.  This is hardware
>>dependent and cannot be changed.
>>It takes a certain amount of time to get the packet out of
>>the hardware in the ethernet card into main ram, this also
>>hardware dependent and cannot be changed. (unless the device
>>driver is terribly inefficient, which we will assume it's not)
>>Once in main ram, the information in the packet has to go through
>>a number of code statements.  The more code statements the
>>longer the information in the packet is sitting around in
>>the FreeBSD system's memory.
>>It then takes a certain amount of time to get the information
>>out of main memory into the other sending ethernet nic's buffers,
>>and it takes time to get it out of the sending nic back to the
>>Danial is claiming the slowness is in the main ram section of
>>things, not in the ethernet driver code.
>>polling makes the ethernet driver more efficient at high data
>>rates, but it does nothing for the speed of processing within
>>the TCPIP stack itself.  At low data rates polling is less
>>efficient than the interrupt method.  And unless the nic driver
>>is terribly inefficient to start with, the time it adds to the
>>packet path in the system is minor compared to the time spent
>>in the TCP/IP stack.
>Thanks for the explanation.  So would polling be beneficial or
>detrimental for a 100 mbps Ethernet card?

Yes, if you were running 100Mbt's of bandwidth through it.

>Not sure if 100 mbps is
>considered "high" or "low" speed.  I'm specifically interested in
>NetGear cards using the dc driver or DLink cards using the rl driver.

The rl chipset isn't known as a very good chipset. YMMV

Some of the Netgear cards use clone 21143 chipsets which are
extremely inferior to the real thing.  In particular if your
Netgear card is using a PNIC chipset it is pretty bad with serious
performance penalty.  This is documented in Section 4 of the dc manpage.

People seem to have good results with polling on the fxp cards.


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