[Date Prev][Date Next] [Chronological] [Thread] [Top]

Re: openldap 2.4.16 hangs with dncachesize smaller than max number of records

Rodrigo Costa wrote:


I tried bigger caches but I do not have enough memory to apply them.
This was the reason I only tried the dncachesize to speed up search queries.

I also have this same database running in a old openldap version(2.1.30)
even with a little more records on it. In this version, as I understand,
there isn't any cache at openLDAP but only at BDB.

False. back-bdb was originally written without entry caching, but it was never released that way, and entry and IDL caching are both present in 2.1.30.

See how it is
behaving in terms of memory and CPU from the openLDAP 2.1.30:

  3651 root      24   0  136M 135M  126M S     0.0  1.1   0:00   1 slapd

See a really small memory consumption and I really reasonable
performance. The only issue I have with this version is about the
replication mechanism which I would like to increase its availability
using syncrepl unless slurpd.

The problem is that for version after 2.1 looks like we need to have
enough memory to cache all database since there are many situations
where slapd appear increase CPU or memory usage and considerably reduce

Not all, but as the documentation says, the dncache needs to be that large. None of the other caches are as critical.

I tried to remove from slapd.conf any cache constraint seeing if the
previous version performance directly from disk reads would be
reproduced. I saw some good startup, like 500 returns per second, but
after sometime slapd hanged and did not return any more records to
ldapseach. Also it consumed all 4 CPUs :

  4409 ldap      15   0  400m 183m 132m S  400  1.5  27:22.70 slapd

And even after I stop the ldapsearch the CPU consumption continues
consuming all CPUs processing. I believe it entered in a dead loop.

You should get a gdb snapshot of this situation so we can see where the loop is occurring.

I do not have a heavily load system but based in records and DBs I have
some memory resource constraints.

Performance is directly related to memory. If the DB is too large to fit in memory then you're stuck with disk I/Os for most operations and nothing can improve that. You're being mislead by your observation of "initially good results" - you're just seeing the filesystem cache at first, but when it gets exhausted then you see the true performance of your disks.

I also tried some smaller caches, like :

cachesize         500000
dncachesize     500000
idlcachesize    500000
cachefree       10000

But it also hangs the search after sometime.

I was wondering if there is a way to run slapd without cache, reading
from disk(like first time read to insert record in cache), what is
enough for small/medium systems in terms of consulting. In this way I
could use the behavior as the 2.1.30 system and the new syncrepl

The 2.4 code is quite different from 2.1, there is no way to get the same behavior.

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