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Re: OpenLDAP high CPU usage when performing mass changes

Jeffrey Crawford wrote:

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.
When the CPU usage goes high like that it should be pretty easy to see where it's going, by getting a gdb stack trace of the running process.
At a guess, based on the minimal amount of information here, you've run into the glibc malloc fragmentation issue, and switching to tcmalloc might avoid the problem. 


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.
If you have the SYNC loglevel enabled, it should be obvious whether update traffic is the cause or not. 


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.
I would suggest you try out back-mdb in RE24. MDB uses 1/4 the total memory of BDB and it performs far fewer mallocs, so glibc malloc fragmentation should not be a problem. (I would have suggested 2.4.30, but the ITS#7190 fix is rather important if you have large volumes of delete operations. The other MDB-related ITSs, #7191 and #7196, are only crucial for non-X86 and non-Linux platforms.) 

--
  -- Howard Chu
  CTO, Symas Corp.           http://www.symas.com
  Director, Highland Sun     http://highlandsun.com/hyc/
  Chief Architect, OpenLDAP  http://www.openldap.org/project/