[openib-general] CQ peek

[Kevin Reilly]

Thanks Sean and Roland for your response. You right it's not a primitive in
the IB verbs or iWARP unless you think the verbs define a
non-consuming poll(). Which is a stretch for sure.  We fell into the trap
of structuring with it because it was in VAPI and maybe other
that coded over VAPI fell into this deadend too.

I guess you can make arguments that using a peek is sub optimal performance
vise but since there is a peek in sockets and there is a
peek function in VAPI some ULPs may be structures to use it.

Kevin J. Reilly
STSM, HPC Architecture
-Federation/HPS  Chief Engineer
-HPC interconnect architect
(office) 845-433-7976  (tieline) 8-293-7976



                                                                           
             "Sean Hefty"                                                  
             <sean.hefty at intel                                             
             .com>                                                      To 
                                       "'Roland Dreier'"                   
             07/08/2005 06:57          <rolandd at cisco.com>, Kevin          
             PM                        Reilly/Poughkeepsie/IBM at IBMUS       
                                                                        cc 
                                       <openib-general at openib.org>         
                                                                   Subject 
                                       RE: [openib-general] CQ peek        
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           
                                                                           




>First, a general point about peek CQ.  For all RDMA device
>architectures that I know of, doing "peek CQ" is just as expensive as
>doing "poll CQ," since the driver needs to do the same locking and
>incur the same cache misses either way.  And then the app will always
>have to do "poll CQ" later anyway.
{snip}
>This is moderately compelling, and if someone has a concrete use for
>poll CQ I'll probably end up implementing it someday (unless someone
>beats me to it).  But I'd be somewhat surprised if a ULP uses a
>primitive that is not defined in either the IB or iWARP verbs.

I vaguely remember discussing peek CQ before, but I don't remember what the
outcome was.  I tend to agree with Roland though.  There are likely
mechanisms
to perform the same tasks that can provide better performance.  I would
lean
more towards removing peek CQ from the API than implementing it, especially
if
it leads an application to be less efficient.

- Sean