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OpenLDAP high CPU usage when performing mass changes

We are using openldap 2.4.26 with BDB 4.8 and have replication set up in mirror mode for our main ldap database. There are a couple of other replicas that have a subset of the data that the main cluster has but we are seeing the following behavior on all of them.

When performing mass updates via LDAP, lets say on the order of 30,000 entries being added to existing entries. We've noticed that the CPU use of the slapd instances goes through the roof (between 65% and 95% continuously), and seems to stay there until it is restarted.

The Problem is that this system has to be highly available, even for writing and when these updates "shock" the system, the response time goes way down when the process are turning like that. I don't think they are trying to catch up to the data changes because if I let them run a while after the updates are done. (Talking like 1hr) and then restart the instances, they go back to their normal state.

So far the only way I've been able to mitigate the issues is to reconfigure our ldap proxy instances to a machine that is having less trouble, restart the instances that are chugging along, then repoint the proxies back to the one just started, and start the others. Not exactly a quick operation.

I've played with cache settings for both OpenLDAP and BDB and have gotten the frequency of this issue reduced but I can't seem to get rid of it completely and it shows up quite often after large data manipulations. I'm at a loss of how to debug since nothing is crashing. Any suggestions on how to find out what's causing this would be very helpful. The logs are not throwing any warnings or posting messages that would seem out of the ordinary and I have played with the log settings but nothing seems to relate to anything that might explain why we are seeing CPU usage to go so high.

Thanks in advance

I fly because it releases my mind from the tyranny of petty things . . .

— Antoine de Saint-Exupéry

Jeffrey E. Crawford
ITS Application Administrator (IDM)