Re: blocking enq-ss wait events

The doc which chris has pointed says the SMON can cause such sort segment contention but it doesn't say anything about big rollback. But as you said you were seeing a lot of "wait for a undo record" post DB reboot i.e. most probably the same rollback was getting resumed (which was running before reboot) and this rollback process is also managed by SMON only , so i would say the sort segment contention was somehow related to the Big transaction Rollback only. Though I can't explain it logically at the moment. Others may throw some light.

Thanks and Regards
Pap

On Fri, May 7, 2021 at 8:51 AM Lok P <loknath.73_at_gmail.com> wrote:

> Thank You Chris.
>
> Both of them are tempfiles. I see the CONTENTS column in dba_tablespaces
> of both tablespaces are TEMPORARY. But i am wondering how this can be
> related.
>
> Also is that big rollback which was happening before DB reboot and resumed
> even after DB reboot is anyway having relation with the sort segment
> contention which we saw post reboot? Or the sort segment contention after
> reboot is independent of the rollback which was still in play by the SMON
> post reboot?
>
> Regards
> Lok
>
> On Fri, May 7, 2021 at 2:27 AM Chris Taylor <
> christopherdtaylor1994_at_gmail.com> wrote:
>
>> It's interesting that you have different TEMP space assigned to SYS and
>> application users. Are both TEMP spaces actually TEMPFILES or is one
>> dictionary managed?
>>
>> (DBA_TEMP_FILES vs DBA_DATA_FILES)
>>
>> Also there used to be a scenario where FAST_START_PARALLEL_ROLLBACK
>> parameter would negatively impact performance but in 11.2, I would expect
>> that to not be an issue unless you're CPU bound where CPUs are pegged (100%
>> busy or near 100% busy).
>>
>>
>>
>> Chris
>>
>>
>> On Thu, May 6, 2021 at 2:52 PM Lok P <loknath.73_at_gmail.com> wrote:
>>
>>> I see we have different temp space aligned to SYS and different for
>>> application users. So why would SMON(which is a SYS process) create the SS
>>> or SORT SEGMENT contention on a tempspace i.e. aligned to application
>>> users/queries and that too after a DB reboot?
>>>
>>> On Thu, May 6, 2021 at 11:44 PM Lok P <loknath.73_at_gmail.com> wrote:
>>>
>>>> Thanks a lot. So here is what happened and I am still struggling to
>>>> clearly understand how logically these are related. We had killed one long
>>>> running transaction as it was reading from UNDO from a long time(~8+hrs),
>>>> that resulted in a rollback and it was keep going and we use to see wait
>>>> event "wait for a undo record" from multiple SYS sessions (which most
>>>> probably SMON doing the cleanup in multiple parallel slaves). But till that
>>>> time we were all okay because other application queries/sessions and
>>>> everything was going fine and that was not blocking anyone.
>>>>
>>>> Then when above was going on the infra team did another planned
>>>> activity in which the database had to be rebooted. They did that and
>>>> brought the database back online, and after this we started seeing the same
>>>> "wait for undo record" wait event and thought it may be that SMON is
>>>> resuming its rollback/cleanup and it should not impact other application
>>>> queries. But then suddenly it appears "enq:ss contention" for multiple
>>>> application sessions and the blocking session was waiting on "sort segment
>>>> request" and those application queries were just stuck.
>>>>
>>>> I am trying to understand why even the database shutdown happened and
>>>> it started up seamlessly but the old rollback/cleanup reinitiated again by
>>>> the SMON. And even then, why after the DB reboot , the application session
>>>> was stuck on "sort segment request" causing other sessions to hang in
>>>> "enq-ss contention"? How is it related to the big rollback?
>>>>
>>>> Regards
>>>> Lok
>>>>
>>>>
>>>> On Thu, May 6, 2021 at 7:02 PM Chris Taylor <
>>>> christopherdtaylor1994_at_gmail.com> wrote:
>>>>
>>>>> No killing the recovery process isn't a great idea as I'm fairly
>>>>> certain SMON is involved which is a critical component of the database
>>>>> operation. Kill it, you kill the instance.
>>>>>
>>>>> Are you CPUs on the server very busy, very low idle ?
>>>>>
>>>>> Find SID & SERIAL for the SMON process and query GV$PX_SESSION for
>>>>> QCSID=<sid of SMON> and QCSERIAL# = <serial# of SMON> and see how many
>>>>> parallel server processes its using.
>>>>>
>>>>> If you kill (or have to restart the database) be aware that the
>>>>> startup of the database will "pause" while SMON cleans up that dead
>>>>> transaction before it opens the database (most likely) but SMON should do
>>>>> that in parallel and might be your best bet to get the database back to
>>>>> normal operating procedure. Though it might take a while for SMON to
>>>>> finish and thus delay the opening of the database which you can monitor by
>>>>> tailing the alert log file if you do kill & restart the database instance.
>>>>>
>>>>> Chris
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> On Thu, May 6, 2021 at 1:14 AM Lok P <loknath.73_at_gmail.com> wrote:
>>>>>
>>>>>> Thank you . It seems to match our symptoms. (Big Rollback causing
>>>>>> enq-ss contention)
>>>>>>
>>>>>> We were thinking about killing the system processes which are trying
>>>>>> to perform the rollback as we don't need that job anymore. Is that safe?
>>>>>> Or we have to either wait to let that finish and the new transactions
>>>>>> may move on by increasing the pga_aggregate_target as suggested in the
>>>>>> note. else drop and create temp tablespace.
>>>>>>
>>>>>> On Thu, May 6, 2021 at 10:13 AM Chris Taylor <
>>>>>> christopherdtaylor1994_at_gmail.com> wrote:
>>>>>>
>>>>>>> I had to look up ENQ-SS on Oracle support. That's a sort segment
>>>>>>> contention, usually encountered when SMON is really busy cleaning up a dead
>>>>>>> / killed transaction.
>>>>>>>
>>>>>>> Oracle support has a few notes on this but the most applicable seem
>>>>>>> to stop at 11.1 .
>>>>>>>
>>>>>>> There is one note that says to try to increase PGA_AGGREGATE_TARGET
>>>>>>> SS Sort Segment Enqueue: 'enq: SS - contention' (Doc ID 2601825.1)
>>>>>>>
>>>>>>> Chris
>>>>>>>
>>>>>>> On Wed, May 5, 2021 at 11:56 PM Lok P <loknath.73_at_gmail.com> wrote:
>>>>>>>
>>>>>>>> Hi All, Need some help, we had killed one of the long running
>>>>>>>> sessions which was running since ~8hrs+ , but after we kill that we see a
>>>>>>>> lot of "wait for a undo record" wait events but we ignored that thinking
>>>>>>>> that will run for sometime as because it will do rollback. But suddenly now
>>>>>>>> we are seeing "enq-SS contention" wait event in addition to "wait for a
>>>>>>>> undo record" and that is blocking all other application queries. So
>>>>>>>> wondering how we should mitigate this issue?
>>>>>>>>
>>>>>>>> The version is 11.2.0.4 of Exadata.
>>>>>>>>
>>>>>>>> Regards
>>>>>>>> Lok
>>>>>>>>
>>>>>>>

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