Re: Problem with e100 driver and latency on different packet sizes

[Corey Minyard <cminyard@mvista.com>]

Thanks for the response.

I looked at the code, and there doesn't seem to be a way to turn it off
dynamically or with a bootline option.  I have to have the driver
compiled into the kernel (because I'm netbooting with NFS) so I can't
really use a module parameter.

A dynamic field in /proc would be quite nice.

-Corey

Feldman, Scott wrote:

>>I've attached a small program to measure latency of 
>>round-trip time on UDP.  If I send 85-byte packets between 
>>two of my machines, I get 170us round-trip latency.  If I 
>>send 86-byte packets, I get 1329us latency. 
>>This seems quite odd.  If I test on the eepro100 driver, I 
>>get expected linear increase in round-trip time as the packet 
>>size increases, and it never gets close to 1300us.
>>    
>>
>
>This sounds like a side-effect of the "CPU Cycle Saver" feature to
>bundle Rx packets per one interrupt.  See
>Documentation/networking/e100.txt.  I haven't tried your setup, but I
>would guess that you can play around with the BundleSmallFr module
>parameter, or better yet, if you want the lowest latencies, turn off CPU
>Saver (ucode=0).
>
>CPU Saver trades latency for reduced interrupts, resulting in CPU
>savings, hence the name.
>
>Hope this helps.
>
>-scott
>
>
>  
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