Re: Server slowing down over time - Mailing list pgsql-performance

Hi,

On 08/27/2015 07:21 PM, Jean Cavallo wrote:
> Hi,
>
> I am currently working on a data migration for a client.
> The general plan is :
>    - Read data from a postgresql database
>    - Convert them to the new application
>    - Insert in another database (same postgresql instance).
>
> The source database is rather big (~40GB, wo indexes), and the
> conversion process takes some time. It is done by multiple workers
> on a separate Linux environnement, piece by piece.
>
> When we start the migration, at first it looks good.
> Performances are good, and it ran smoothly. After a few hours,
> we noticed that things started to slow down. Some queries seemed
> to be stuck, so we waited for them to end, and restarted the server.
>
> After that it went well for some time (~10 minutes), then it slowed
> down again. We tried again (a few times), and the pattern repeats.

If you're moving a lot of data (especially if the destination database
is empty), one possible problem is statistics. This generally is not a
problem in regular operation, because the data growth is gradual and
autovacuum analyzes the tables regularly, but in batch processes this is
often a big issue.

The usual scenario is that there's an empty (or very small) table, where
indexes are inefficient so PostgreSQL plans the queries with sequential
scans. The table suddenly grows, which would make indexes efficient, but
the planner has no idea about that until autovacuum kicks in. But before
that happens, the batch process executes queries on that table.

Try adding ANALYZE after steps that add a lot of data.

>
> My postgresql specific problem is that it looks like the server gets
> stuck. CPU usage is <10%, RAM usage is under 50% max, there is no
> noticeable disk usage. But, there are some (<10) active queries, some
> of which may take several hours to complete. Those queries work
> properly (i.e < 1min) right after the server restarts.

That's a bit strange. Essentially what you're saying is that the
workload is neither CPU nor I/O bound. To make it CPU bound, at least
one CPU would have to be 100% utilized, and with 4 CPUs that's 25%, but
you're saying there's only 10% used. But you're saying I/O is not the
bottleneck either.

> So my question is : What could slow the queries from ~1min to 2hours
>  which does not involve CPU, Memory, or disk usage, and which would
> "reset" when restarting the server ?

A lot of things, unfortunately, and the fact that this is a migration
moving data between two databases makes it even more complicated. The
virtualization does not make it less complex either.

For example, are you sure it's not stuck on the other database? I assume
you're running some long queries, so maybe it's stuck there and the
destination database is just waiting for data? That's be consistent with
the low CPU and I/O usage you observe.

Locking is another possibility, although it probably is not the only
cause - it'd be utilizing at least one CPU otherwise.

>
> For information, the number of processes does not seem to be the
> problem, there are ~20 connections with max_connection set to 100.
> We noticed at some point that the hard drive holding the target
> database was heavily fragmented (100%...), but defrag did not
> seem to change anything.

If it was a problem, you'd see high I/O usage. And that's not the case.

>
> Also, the queries that appear to get stuck are "heavy" queries,
> though after a fresh restart they execute in a reasonable time.

Does the plan change? If not, check waiting locks in pg_locks.

>
> Finally, whatever causes the database to wait also causes the
> Windows instance to slow down. But restarting Postgresql fixes
> this as well.

That's a bit strange, I guess. If you're not observing light CPU and I/O
usage, then the instance should not be slow, unless there's something
else going on - possibly at the virtualization level (e.g. another busy
instance on the same hardware, some sort of accounting that limits the
resources after a time, etc.)

> Configuration :
>
> The Postgresql server runs on a Windows Virtual Machine under
> VMWare. The VM has dedicated resources, and the only other
> VM on the host is the applicative server (which runs idle while
> waiting for the database). There is nothing else running on the
> server except postgresql (well, there were other things, but we
> stopped everything to no avail).
>
> PostgreSQL 9.3.5, compiled by Visual C++ build 1600, 64-bit

You're 4 versions behind.

regards

--
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services