Re: slapd hangs at 100% cpu in sched_yield (ITS#2030)

[Kurt@OpenLDAP.org]

Your suggestion will quite likely result in resource deadlock.
It will certainly spend huge amount of cycles unnecessarily
in a busy loop.  A loop which includes a back-off delay and
is finite might be acceptable.

A few additional comments...

At 06:58 PM 2002-08-20, steven.wilton@team.eftel.com wrote:
>So... as far as I can see, lock_id() will return EINVAL, ENOMEM or 0.

I'm looking at a newer version, it only returns 0, ENOMEM, and
under some odd circumstances, a range of other system result
codes.  Only one of concern here is ENOMEM.

>ENOMEM is returned when "Lock table is out of available locker entries". 

This code is also returned with memory allocation (malloc) failed.

>As far as I can tell (and please correct me if I am wrong), the reason that we run out of locks is because other threads are holding onto them.

Or this thread.

>Increasing the number of locks will possibly improve performance (as we don't need to wait for another thread to finish with it's lock),

Performance?  If you are waiting (not in a busy loop), you are
not significant hindering performance.  The issue is how to
prevent waiting forever... that is, how to prevent resource
deadlock.

>but as long as we are getting an ENOMEM error, the database is out of locks (because another thread is holding the lock)

or this thread.

>, and we should loop until the other thread frees the lock.

The other threads could be doing the same, looping for this
thread to free resources.

>This certainly fixes the problem on our system, as the first patch I submitted has been running for the past day or two without any problems.

You are just luckily in that you reached resource deadlock.

>What I am not sure about is how many locker entries may be being held by each thread, and how many are currently enabled in the slapd code.  The defaults should be 1000 (according to the db4 docs), which is a lot more that I thought slapd should use.

Lots of locks are needed for fine grain locking...  I believe
some guidelines for DB settings were posted to the software
list.