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Re: [Discuss-gnuradio] MAC layer development and USRP2
From: |
Jeff Brower |
Subject: |
Re: [Discuss-gnuradio] MAC layer development and USRP2 |
Date: |
Wed, 7 Apr 2010 11:54:13 -0500 (CDT) |
User-agent: |
SquirrelMail/1.4.2-1 |
John-
>> Which part of the Linux issue... sustained throughput or latency? I
>> wouldn't be surprised to find that latency
>> hasn't
>> improved substantially because it's not a priority for server software.
>> Even VoIP applications are not concerned
>> about a 1 msec improvement... whereas that makes or breaks a wireless MAC.
>
> I know that in the early days of Linux development, David Miller spent
> a lot of time making sure that the Ethernet layer could reliably send
> and receive more than 1 MByte/sec via TCP over 10 megabit Ethernet,
> and more than 10 MBytes/sec over TCP on 100 megabit Ethernet. I watched
> his measurements and his kernel evolve to make it happen (learning from and
> improving on Van Jacobson's early work making 68000-based Sun-2's move
>>1MByte/sec over TCP on original Ethernet).
>
> You might say, "That's only 90%, surely he can do better," but
> that's 90% of the raw bit rate, delivered flow controlled and error-free
> at the TCP socket layer (all the overhead, from interframe spacing to
> preambles to CRCs to packet headers, goes in the 10%).
>
> As you might expect, pumping the data through required keeping all
> parts of both systems working in overlap. "One packet being assembled
> to transmit, one received packet being picked apart, and one packet
> flying on the medium", at all times. If these two software jobs can
> both run in one packet time, you win (and don't need much if any
> buffering, keeping latency very low). These code paths were heavily
> scrutinized and optimized for the common cases.
>
> I haven't kept track of who's measuring Linux kernel GigE thruput
> recently. Here's a pointer to a 2001 study:
>
> http://www.csm.ornl.gov/~dunigan/netperf/bulk.html
>
> Most people care about TCP speed, but making fast paths for TCP
> usually makes even faster paths for the UDP packets that USRP2 will be
> using soon.
I can believe Linux Ethernet handling is fast and gets faster all the time...
but with most of the emphasis on
throughput. I'm still looking for studies that focus on latency; i.e.
turn-around time (or RTT), and use recent Linux
and motherboards. Probably such data is harder to find becuase in most
applications (over long routes), improving RTT
at the motherboard + kernel level won't be worth the effort.
-Jeff
- Re: [Discuss-gnuradio] MAC layer development and USRP2, Charles Irick, 2010/04/06
- Re: [Discuss-gnuradio] MAC layer development and USRP2, George Nychis, 2010/04/06
- Re: [Discuss-gnuradio] MAC layer development and USRP2, Jeff Brower, 2010/04/06
- Re: [Discuss-gnuradio] MAC layer development and USRP2, Charles Irick, 2010/04/06
- Re: [Discuss-gnuradio] MAC layer development and USRP2, Jeff Brower, 2010/04/06
- Re: [Discuss-gnuradio] MAC layer development and USRP2, John Gilmore, 2010/04/06
- Re: [Discuss-gnuradio] MAC layer development and USRP2, Marcus D. Leech, 2010/04/06
- Re: [Discuss-gnuradio] MAC layer development and USRP2, Jeff Brower, 2010/04/07
- Re: [Discuss-gnuradio] MAC layer development and USRP2,
Jeff Brower <=
- Re: [Discuss-gnuradio] MAC layer development and USRP2, Per Zetterberg, 2010/04/07
- Re: [Discuss-gnuradio] MAC layer development and USRP2, George Nychis, 2010/04/07
Re: [Discuss-gnuradio] MAC layer development and USRP2, Veljko Pejovic, 2010/04/06
- Re: [Discuss-gnuradio] MAC layer development and USRP2, Jeff Brower, 2010/04/06
- Re: [Discuss-gnuradio] MAC layer development and USRP2, Veljko Pejovic, 2010/04/06
- Re: [Discuss-gnuradio] MAC layer development and USRP2, George Nychis, 2010/04/06
- Re: [Discuss-gnuradio] MAC layer development and USRP2, Philip Balister, 2010/04/06
- Re: [Discuss-gnuradio] MAC layer development and USRP2, Jeff Brower, 2010/04/06
- Re: [Discuss-gnuradio] MAC layer development and USRP2, George Nychis, 2010/04/06
- Re: [Discuss-gnuradio] MAC layer development and USRP2, George Nychis, 2010/04/06