RE: LGWR, EMC or app cursors?

Apart from any i/o issue, you CAN at least put the rows into different blocks and avoid the gcc block contention.  

You won’t miss the extra 2 blocks.    

From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Herring, David (Redacted sender "HerringD" for DMARC) Sent: Thursday, October 10, 2019 1:03 PM To: exriscer_at_gmail.com; dmarc-noreply_at_freelists.org Cc: oracle-l_at_freelists.org
Subject: RE: LGWR, EMC or app cursors?  

The code in question is probably going to raise some eyebrows but here goes since I knew I'd have to explain at some point (and then give the excuse it's a 3rd party app):  

• The app can run from any number of app servers, in our case 3 are needed. Each app server issues heartbeats and when the heartbeat appears to be separated by 30 sec or more, it assumes a lost connection and restarts the app server. The check previously was 15 sec. but I got them to adjust to 30 sec.
• The heartbeat sessions are persistent, running an update that uniquely identifies each app server (and hence session) with a timestamp, commit, wait 5 sec, the SELECT current time (within DB session) minus heartbeat timestamp, then repeat. I know the 2 SQL_IDs for the heartbeat and set tracing on those IDs. Parsing the tracefiles I see that the heartbeats repeat every 5 sec consistently, until the last one I the file that hits 25+ sec gap which causes an app restart and hence a restart of the DB session.
• I don't have an extensive OLTP background but to me the heartbeat process is asking for trouble. The table has 1 row per app server, so in this case 3. I dumped the rowid details and confirmed all 3 rows are in the same block. This means every 5 seconds, potentially concurrent, 3 different sessions will try to update the same block.
• The DB is running on a 2-node RAC and heartbeat sessions may show up wherever (connect via scan) so we get plenty of GC waits. I would think they'd want a separate object per app server doing a heartbeat to (nearly) completely eliminate contention but again, the app isn't in my control.
• This env was converted from MySQL and apparently worked fine there. There's a QA env where the heartbeat processing works fine too.

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From: oracle-l-bounce_at_freelists.org <oracle-l-bounce_at_freelists.org> On Behalf Of Ls Cheng Sent: Thursday, October 10, 2019 7:24 AM To: dmarc-noreply_at_freelists.org
Cc: oracle-l_at_freelists.org
Subject: Re: LGWR, EMC or app cursors?  

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Hi  

Can you try to identify the number of commits when that happens plus the IOPS?  

I had a similar problems years ago, it turns our in the code a loop ran 2500 commits per sec  

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On Mon, Oct 7, 2019 at 5:20 PM Herring, David <dmarc-noreply_at_freelists.org> wrote:

Folks, I've got a bit of a mystery with a particular db where we're getting a periodic 25-30 pause between user sessions and LGWR processes and can't clearly identify what's the cause.  

· The database is 11.2.0.4, RHEL 7.5, running ASM on EMC.

· Sometimes once a day, sometimes more (never more than 5) times a day we see user processes start waiting on "log file sync". LGWR is waiting on "log file parallel write".

· At the same time one of the emcpower* devices shows 100% busy and service time 200+ (from iostat via osw). mpstat shows 1 CPU at 100% on iowait. It's not always the same disk (emcpowere1, a1, h1, …), not always the same CPU. EMC and sysadmins have confirmed there are no disk errors and from their standpoint the disks are waiting on Oracle.

· During this time LGWR stats in ASH are all 0 - TIME_WAITED, DELTA* columns. Only after the problem goes away (about 25 secs) these columns are populated again, obviously the DELTA* columns 1 row later. LGWR's session state is WAITING so I assume the column value observations are due to LGWR waiting, as it won't write stats until it can do something.  

I am stuck trying to find out, really prove who is the culprit or what exactly the wait is on. Is LGWR waiting on user sessions and user sessions are waiting on LGWR and all that causes the disk to be 100%? Can I enable some sort of tracing on LGWR and would that point to exactly what he's waiting on to prove where the problem is?  

Regards,  

Dave

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