problems connecting to ldaps:// under high load with ppc64 client

[Matthias Leopold <matthias.leopold@meduniwien.ac.at>]

hi,

i'm operating an owncloud server that connects to an IBM Tivoli 
Directory Server as LDAP backend. the ldap admin tells me he is seeing 
"null binds" from my owncloud server in his logs:

2016-05-24T14:32:56.349452+2:00 srvr_ssl_read: EIO in handshake. 
EWOULDBLOCK timeout. Read: -2 of 0
2016-05-24T14:32:56.350445+2:00 GLPSSL019E The SSL layer has reported an 
unidentified internal error, SSL extended error code:406.
2016-05-24T14:32:56.351813+2:00 GLPSRV022E Failed to initialize secure 
connection from client (connection ID: 61786, IP address: x.x.x.x, Port: 
59921).
2016-05-24T14:32:56.357220+2:00 GLPSRV044W Client connection from 
x.x.x.x bound as NULL closed by server.

i investigated on my server and noticed that it has problems connecting 
to the ldaps://ldap.example.com uri (which is the ITDS server) under 
high client system load, whereas connection to ldap://ldap.example.com 
is ok.

$ ldapsearch -v -x -z 0 -H ldaps://ldap.example.com -b 
"ou=groups,dc=example,dc=com" -v "objectClass=posixGroup"
ldap_initialize( ldaps://ldap.example.com:636/??base )
ldap_sasl_bind(SIMPLE): Can't contact LDAP server (-1)

my server (RHEL 7 on a ppc64 LPAR) is using the openldap 
clients/libraries. the high load that is causing the problems is on _my_ 
server. is there any specific tuning (besides increasing RAM/CPU) i can 
do to optimize ldaps client queries? i'm thinking of tuning the tcp 
stack or something similar, but i'm not an expert on this. where can i 
look for debug info? i have strace and tcpdump output

thx
matthias