[openib-general] Re: [PATCH] IPoIB splitting CQ, increase both send/recv poll NUM_WC & interval

[Leonid Arsh]

Shirley,

  some additional information you may be interested:

  According to our experience with the Voltaire IPoIB driver,
  splitting CQ harmed the throughput (we checked with the iperf 
application, UDP mode.) Splitting the the CQ caused more interrupts, 
context switches and CQ polls.
 
  Note, the case is rather different from OpenIB mthca, since Voltare 
IPoIB is based on the VAPI driver,
  where CQ completions are handled in a  tasklet context,
  unlike mthca where  CQ completions are handled in the HW interrupt 
context.

  NAPI gave us some improvement. I think NAPI should improve much more 
in mthca, with the HW interrupt CQ completions.
 

Shirley Ma wrote:
>
> OK. I am going to split the patch without splitting CQ first.
>
> WC handler is called in the interrupt context, it is a myth
> to have bidirectional performance improvement with splitting CQ.
> More investigation is needed.
>
> If WC handler can be moved from interrupt context, splitting CQ
> is still an approach. Having a seperate thread or NAPI support
> can be implemented later.
>
> Thanks
> Shirley Ma
> IBM Linux Technology Center
> 15300 SW Koll Parkway
> Beaverton, OR 97006-6063
> Phone(Fax): (503) 578-7638
>
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