Re: Could use some thoughts on a problem with a query in production

[Bergquist, Brett]

Thanks Bryan.  I am looking at all things :)   The fact that it is not 
reproducible with a copy of the database is a hinderance but also can indicate 
something external.

The interesting thing is that the same query with the same parameters was run 
40 times over about an 8 hour period with about 30 minutes apart.  Each of 
these hit the exactly the same problem.   At the same time, the same query 
requesting different time periods worked perfectly.   Insertion rates into the 
database also were not affected.

Does anyone know anything about the “cache cleaning”?   I have started to peak 
and poke at what it does but I am wondering if there is something in there that 
might be triggering the issue.   The reason that it is peaking my interest is 
that within the code where the BLOCKED threads are occurring are checked for 
the “cleaning”.   These are the areas that the code is waiting.   it is almost 
such that something gets lock stepped in which the code cannot progress because 
of something holding a monitor and the monitor is is being released and 
acquired almost once per result.   Also the fact that the CPU utilization of 
derby went from about 5% to about 45% during this time indicates something is 
being worked on or a busy wait being performed.

On Jun 6, 2017, at 9:28 AM, Bryan Pendleton 
> wrote:

Hi Brett?

What's the underlying storage system like? In particular, is it an SSD
storage system? Is there any chance that you happened to hit a spot
in time where there was some non-Derby event which was causing the
storage system to misbehave?

I understand that certain SSD implementations, and certain file
systems, have to perform automatic maintenance activity on the
SSD media, such as "wear leveling", which can impose significant
overhead on the file system and cause normal use of the file system
to be VERY slow for a significant period.

Here's a relatively old article on the subject; you might try some web searching
to see if you can get more recent information.

http://www.computerworld.com/article/2524589/data-center/analysis--ssd-performanceis-a-slowdown-inevitable-.html

Anyway, I guess what I'm suggesting is that you consider investigating
whether a non-Derby external factor is at work on the machine which
is suffering the performance problems.

thanks,

bryan




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Re: Could use some thoughts on a problem with a query in production

[Bryan Pendleton]

Hi Brett?

What's the underlying storage system like? In particular, is it an SSD
storage system? Is there any chance that you happened to hit a spot
in time where there was some non-Derby event which was causing the
storage system to misbehave?

I understand that certain SSD implementations, and certain file
systems, have to perform automatic maintenance activity on the
SSD media, such as "wear leveling", which can impose significant
overhead on the file system and cause normal use of the file system
to be VERY slow for a significant period.

Here's a relatively old article on the subject; you might try some web
searching
to see if you can get more recent information.

http://www.computerworld.com/article/2524589/data-center/analysis--ssd-performanceis-a-slowdown-inevitable-.html

Anyway, I guess what I'm suggesting is that you consider investigating
whether a non-Derby external factor is at work on the machine which
is suffering the performance problems.

thanks,

bryan

RE: Could use some thoughts on a problem with a query in production

[Bergquist, Brett]

Thanks Katherine, I will look into that.

It is interesting that this one query had the issue repeatedly over about a 12 
hour period whereas others querying performing the same query with different 
time periods during the same time did not experience any issue.   One would 
think if the statistics were bad or being updated, they also would have a 
similar issue.

The stack traces seem to show that the indexes are being used; I don’t see any 
table scans being performed which is usually a good indicator that the indexes 
are not being use.

From: Katherine Marsden [mailto:kmarsdende...@sbcglobal.net]
Sent: Monday, June 05, 2017 3:39 PM
To: derby-dev@db.apache.org
Subject: Re: Could use some thoughts on a problem with a query in production

Hi Brett,

I just happened to see your post on my phone and what came to mind was I wonder 
how the automatic statistics update is implemented and if that might somehow be 
at play here.  I may be totally off base but remember at one point this was 
implemented and there was discussion of how and when to do it and concern that 
at some point, some query was going to get hit with the cost (for the team so 
to speak) . It might be worthwhile looking at what triggers the statistics 
update and if it might be at play here.

Thanks

Katherine

Sent from my iPhone

On Jun 5, 2017, at 12:04 PM, Bergquist, Brett 
> wrote:
Background:


· A large database approximately 750G

· derby.storage.pageCacheSize=64000

· Inserts going into the database about 125/second

· Other database updates and deletes are being performed at a lower 
rate.

A query is run by the customer that does not complete in a reasonable amount of 
time.   Unfortunately the customer’s program retries this query up to 40 times 
compounding the problem.

The query retrieves a set of records for a period of time.  The query is 
constructed using the indexes of the table such that the rows should be in 
sorted order and an external sort not required.   Basically the query fires off 
periodically and requests the records for a specific hour of the day.   The 
next time it fires off and retrieves the next hour, etc.

The query has run successfully with other parameters repeatedly through out the 
week with no issues.  Only one set of query parameters caused the issue.   The 
same problem occurred exactly one week later again for the same time period of 
the day.

I was able to retrieve the database locally from the customer and I configured 
my server to time and time zone and ran the query with no issue.   So it does 
not appear to be an optimizer problem with it choosing the wrong path.

There is a script that is run at the customer’s site to retrieve some data that 
I use for debugging these types of problems as access to the customer site is 
not possible due to security reasons.   One of the things that is captures is 
stack traces every 5 seconds for 1 minute.

The stack traces show no deadlocks and also seem to show that the query path is 
not doing a large external sort or anything like that.   Many of the threads 
are blocked however and they seem to be blocked at a low level.For example 
many threads look like this:

"DRDAConnThread_98" #123 prio=5 os_prio=64 tid=0x0001027b3800 nid=0x94 
waiting for monitor entry [0xfffd37afe000]
   java.lang.Thread.State: BLOCKED (on object monitor)
at 
org.apache.derby.impl.store.raw.data.FileContainer.pageValid(Unknown Source)
- waiting to lock <0xfffd70d5d598> (a 
org.apache.derby.impl.store.raw.data.AllocationCache)
at 
org.apache.derby.impl.store.raw.data.FileContainer.getUserPage(Unknown Source)
at 
org.apache.derby.impl.store.raw.data.FileContainer.getPage(Unknown Source)
at 
org.apache.derby.impl.store.raw.data.BaseContainerHandle.getPage(Unknown Source)
at 
org.apache.derby.impl.store.access.conglomerate.OpenConglomerate.latchPage(Unknown
 Source)
at 
org.apache.derby.impl.store.access.conglomerate.GenericConglomerateController.fetch(Unknown
 Source)
at 
org.apache.derby.impl.sql.execute.IndexRowToBaseRowResultSet.getNextRowCore(Unknown
 Source)
at 
org.apache.derby.impl.sql.execute.ProjectRestrictResultSet.getNextRowCore(Unknown
 Source)
at 
org.apache.derby.impl.sql.execute.NestedLoopJoinResultSet.getNextRowCore(Unknown
 Source)
at 
org.apache.derby.impl.sql.execute.NestedLoopJoinResultSet.getNextRowCore(Unknown
 Source)
at 
org.apache.derby.impl.sql.execute.NestedLoopJoinResultSet.getNextRowCore(Unknown
 Source)
at 
org.apache.derby.impl.sql.execute.ProjectRestrictResultSet.getNextRowCore(Unknown
 Source)
at 
org.apache.derby.impl.sql.execute.SortResultSet.getRowFromResultSet(Unknown 
Source)
at