Multicast Scaling

Multicast Scaling

[Todd Strader]

Hi,

I'm running ib_send_bw (version 1.20) over ConnectX HCAs on an 
Infiniscale III switch.  If I run:
ib_send_bw -s 256 -c RC
I get 790 MB/s
ib_send_bw -s 256 -c UD
still gets me 790 MB/s
ib_send_bw -s 256 -c UD -g 1
which runs over a multicast GID/LID gets 790 MB/s when there are two 
HCAs on the switch.  However:
3 HCAs - 726 MB/s
4 HCAs - 701 MB/s
. . .
12 HCAs - 205 MB/s

I'm trying to understand why this drops off for each new HCA I add to 
the switch.  All the HCAs and the switch are running the latest firmware 
(2.7.0 and 1.0.5 respectively).  Only one HCA is transmitting and only 
one is receiving.  All the other HCAs are quiet.

I'm wondering if anyone has any suggestions on what causes this effect 
and how I might deal with it.

Thanks.

Todd
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