Messages in this thread | | | Date | Wed, 23 Oct 2002 14:30:22 -0400 (EDT) | From | "Richard B. Johnson" <> | Subject | Re: [RESEND] tuning linux for high network performance? |
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On Wed, 23 Oct 2002, Nivedita Singhvi wrote:
> "Richard B. Johnson" wrote: > > > No. It's done over each word (short int) and the actual summation > > takes place during the address calculation of the next word. This > > gets you a checksum that is practically free. > > Yep, sorry, word, not byte. My bad. The cost is in the fact > that this whole process involves loading each word of the data > stream into a register. Which is why I also used to consider > the checksum cost as negligible. > > > A 400 MHz ix86 CPU will checksum/copy at 685 megabytes per second. > > It will copy at 1,549 megabytes per second. Those are megaBYTES! > > But then why the difference in the checksum/copy and copy? > Are you saying the checksum is not costing you 864 megabytes > a second??
Costing you 864 megabytes per second? Lets say the checksum was free. You are then able to INF bytes/per/sec. So it's costing you INF bytes/per/sec? No, it's costing you nothing. If we were not dealing with INF, then 'Cost' is approximately 1/N, not N. Cost is work_done_without_checksum - work_done_with_checksum. Because of the low-pass filter pole, these numbers are practically the same. But, you can get a measurable difference between any two large numbers. This makes the 'cost' seem high. You need to make it relative to make any sense, so a 'goodness' can be expressed as a ratio of the cost and the work having been done.
Cheers, Dick Johnson Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips). Bush : The Fourth Reich of America
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