[openib-general] IB Query

[Dror Goldenberg]

> -----Original Message-----
> From: Majumder, Rajib [mailto:rajib.majumder at csfb.com] 
> Sent: Tuesday, August 02, 2005 6:05 AM
> 
 
> I have a query regarding SDP/IB. I have 2 processes. 1 TCP 
> client and 1 TCP server, both are running on the same 
> physical host. We have IB HCA installed on this host.
>  
> Both the process communicate using SDP. 
> 
> Will there be any performance gain? 
> 

These are the benefits of running the application locally over
SDP:
* When using BCopy SDP, then the HCA HW takes care 
    of all transport offloading - segmentation & reassembly,
    timers, transport checks, etc.
* When using ZCopy SDP, then the HCA HW can also take
   care of copying the data from one process address space
   Into other process address space.

When two processes communicate through TCP locally, all 
transport is taken care of by the TCP layer, which adds some 
overhead. Data is copied by the CPU. Looping back happens
at the IP layer and is internal to the OS. Nothing goes down
to the networking driver.

I'd also mention that SDP will transfer data through the PCI bus
where TCP will use the CPU bus move the data.

I don't have any performance numbers. It can be interesting to
measure bandwidth and CPU utilization at various message
sizes.

-Dror
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