Re: (ITS#8958) 2nd cn=config update blocks slapd while adding subordinate index

[ondra@mistotebe.net]

On Tue, Jan 22, 2019 at 05:49:12PM +0000, h.b.furuseth@usit.uio.no wrote:
> On 1/21/19 7:49 PM, OndÅ?ej Kuzník wrote:
>> Except there are no locks as you know being the author of parts of
>> code that deals with what I'm about to outline anyway:
>>
>> Whenever a cn=config op is about to be processed, a pause is requested.
>> That tells worker threads to stop picking up new work and waits until
>> they're all quiet. The indexing task is run by one of these worker
>> threads.
> 
> Duuh, right.  I got stuck looking for what's special about the
> indexing task and couldn't find it:-(  I need to make it special.
> 
> So, let tasks declare their expected speed until finish or
> between pausechecks.  At FAST=1 (default) and SLOW=0.
> A pause only stops tasks with speed < ltp_pause.
> In thread_pool_pause(), replace the WANT_PAUSE stage with
> 
>    while (++ltp_pause <= max speed) {
>      wait until no more tasks with speed < ltp_pause;
>    }
> 
> Then fast tasks should breeze past slow ones when preparing
> to pause.  Until all threads have slow tasks, anyway.
> 
> To mitigate that, we'd need to predeclare the speed when
> submitting a task, and limit the number of parallel slow
> tasks.  pool_submit() could stash the rest in a "slow queue"
> instead of submitting.  But I don't want to go there yet.

The problem is not that it's still scheduled when the pause is
requested, scheduled tasks don't prevent a pause (and I think some
of them need to be a bit more aware of that fact), but that it's been
picked up by a worker thread and is running while cn=config waits for
them to pause.

What I was suggesting was that it could check every so often whether it
might be holding up a pause and join it just like some other tasks do.
But that's only a good idea if it's able to handle the changes that
happen during the pause (indexes reconfigured again, the database going
away, ...) as well as being able to release any locks temporarily.

-- 
OndÅ?ej Kuzník
Senior Software Engineer
Symas Corporation                       http://www.symas.com
Packaged, certified, and supported LDAP solutions powered by OpenLDAP