Re: Momentary Delay

On Wed, 17 Jun 2015 10:33:37 -0300
Anderson Valadares <andervalbh(at)gmail(dot)com> wrote:

> 2015-06-15 18:19 GMT-03:00 Bill Moran <wmoran(at)potentialtech(dot)com>:
>
> > On Mon, 15 Jun 2015 17:48:54 -0300
> > Anderson Valadares <andervalbh(at)gmail(dot)com> wrote:
> >
> > > 2015-06-12 19:56 GMT-03:00 Bill Moran <wmoran(at)potentialtech(dot)com>:
> > >
> > > > Please do not remove the mailing list from replies. See below.
> > > >
> > > > On Fri, 12 Jun 2015 09:21:19 -0300
> > > > Anderson Valadares <andervalbh(at)gmail(dot)com> wrote:
> > > >
> > > > > 2015-06-08 20:33 GMT-03:00 Bill Moran <wmoran(at)potentialtech(dot)com>:
> > > > >
> > > > > > On Mon, 8 Jun 2015 11:59:31 -0300
> > > > > > Anderson Valadares <andervalbh(at)gmail(dot)com> wrote:
> > > > > >
> > > > > > > Hi
> > > > > > > We are experiencing an intermittent problem in a GIS database
> > from a
> > > > > > > client. Some times of the day is as if the PostgreSQL executed
> > the
> > > > > > slowest
> > > > > > > operations see below an example. The query was performed three
> > times,
> > > > > > twice
> > > > > > > I canceled and the third time I left it to end. The consultation
> > > > took 10
> > > > > > > seconds to finish, but the measured execution time is 20 ms. As
> > we
> > > > have
> > > > > > > several queries that run every 2 seconds when this momentary
> > delay
> > > > occurs
> > > > > > > queries accumulate and burst the limit of 203 connections
> > allowed.
> > > > The
> > > > > > > interval of "momentary stop" are approximately 2 seconds
> > occurring at
> > > > > > > random times and during these stoppages occur no locks and no
> > > > increased
> > > > > > > load on the server is identified. There is a job 2/2 seconds
> > locks
> > > > > > > collection of information, running queries, etc., nmon also
> > collects
> > > > > > > information every 2 seconds. My client asks what causes these
> > > > momentary
> > > > > > > stops? because it affects all operations of the database? etc.
> > How
> > > > do I
> > > > > > > identify what is causing these delays in executions of
> > operations in
> > > > the
> > > > > > > database?
> > > > > >
> > > > > > Number of points to consider:
> > > > > > * Check the contents of pg_locks and see if something is waiting
> > on a
> > > > > > lock when the query is slow.
> > > > >
> > > > > There is nothing waiting when the query is slow, see:
> > > > >
> > > > > snap_datetime | waiting | count
> > > > > -------------------------------+---------+-------
> > > > > 2015-06-05 09:25:00.954731-03 | f | 74
> > > > > 2015-06-05 09:26:00.249187-03 | f | 205
> > > > > 2015-06-05 09:27:00.826874-03 | f | 207
> > > >
> > > > I don't know what that means. Since you don't show the query that
> > > > generated that output, I have no idea if your statement is valid, or
> > > > if you're running a query that will inherently produce incorrect
> > > > results.
> > >
> > > Sorry the information was incomplete. Below is the query that was
> > > performed to extract information from tables that used to monitor the
> > > database.
> > > The snap_datetime column indicates the time that occurred monitoring,
> > > the waiting column tells you whether any connection was on hold and the
> > > column
> > > count tells how many connections existed at the time.
> > >
> > > select
> > > s.snap_datetime
> > > ,a.waiting
> > > , count(*)
> > > from stat_snapshot s, stat_activity a
> > > where s.snap_id = a.snap_id
> > > and s.snap_datetime >= '2015-06-05 09:25:00'
> > > and s.snap_datetime <= '2015-06-05 09:28:00'
> > > group by
> > > s.snap_datetime
> > > ,a.waiting
> > > order by s.snap_datetime
> > > ;
> > > snap_datetime | waiting | count
> > > -------------------------------+---------+-------
> > > 2015-06-05 09:25:00.954731-03 | f | 74
> > > 2015-06-05 09:26:00.249187-03 | f | 205
> > > 2015-06-05 09:27:00.826874-03 | f | 207
> > > (3 rows)
> >
> > Given what I'm seeing above, there's still a lot of information
> > missing. Those tables are not standard PostgreSQL tables, and I'm
> > not aware of any tool that creates them, thus I have to assume
> > it's a cron job or something similar created in-house. Given the
> > query you ran and the resultant data, my first guess is that the
> > data in stat_snapshot and stat_activity is worthless: i.e. it's
> > captured once per minute, and therefore there are 59+ seconds
> > worth of detail that aren't captured, thus the actual liklihood
> > that the those tables will contain any indication of the problem
> > is very low.
>
> Yes it is a cron, created at home, running every 1 hour gathering
> information from PostgreSQL tables (pg_stat_activity, pg_locks, etc).
> How to improve the collection of information PostgreSQL?

Look at the log file, details of what is going on are being recorded
as they happen.

However, given your statement below (claiming that nothing happened
during the observed slowdown between 9:26 and 9:27) you may want to
find someone who is actually capable of interpreting the log output,
as there are dozens of indicators of what's wrong. Immediately after
the problem disappears, a number of queries finish after spending a
lot of time executing, in addition to a number of complaints of long
lock waits. So all those queries were fighting each other and when
they finally untangled themselves and got their work done, everything
started running smoothly again.

I would look at fct_sinotico_simplificado(), as it seems to be putting
an undue load on the system. FCT_PAINEL_CCO_VEICULOS_GMAPS() might
also be a problem, but it seems less prevalent. I don't know what
language those function names are in, so I can't really guess at
what they do to make any more detailed recommendations.

Additionally, there is a checkpoint in the middle of the problem
period that took 44 seconds to write 600M to disk. That seems to
contradict your earlier statement that IO isn't being maxed out and
makes me doubt that your IO monitoring is effective.

My overall guess is that you have some poorly written code, both on
the Java side and on the database trigger/function side; but that
under normal usage the hardware is sufficient to keep up. However,
when you have a brief spike in activity, the inefficiencies create
a cascading performance problem. I've seen this before at a previous
job, and the high-level problem was that most developers are good
enough to throw something together that will work, but not talented
and experienced enough to write something truely efficient.

Lock down your connection pool to keep it smaller. Look at where you
can do caching on the Java side to avoid rapid-fire duplicate queries.
Look at limiting inserts to busy tables to a single worker thread that
has a queueing structure so that it can absorb spike loads and spread
the load on the DB out more evenly. Implement SOA so that frequently
accessed data can be kept in Java's memory on the services instead of
_always_ having to query the database. These are just a few of the
techniques available to fix problems like this.

> > > > > * Also, ensure log_lock_waits is turned on for the server and check
> > > > > > PostgreSQL's logs for messages about long lock waits.
> > > > >
> > > > > There is also nothing
> > > >
> > > > This is even worse than the previous statement. What did you _do_ to
> > > > determine this?
> > > >
> > > > Since you're unable to diagnose and fix the problem on your own, the
> > > > possibility exists that the problem is with your diagnostic steps. If
> > > > you don't share the details of those steps, I have no way to help you
> > > > with them.
> > >
> > > See the log in
> > >
> > https://github.com/andervalbh/serv-gis01/blob/master/postgresql-2015-06-05_000000.log
> >
> > Apologies, my above comment was intended to be in regards to the IO load.
> >
> > However, that log shows that you have massive lock contention. In the early
> > part of the log a process waits over 5 minutes for a share lock:
> > 2015-06-05 05:51:32 BRT [44236822] - [10.57.1.26] [14-1]
> > db=citgis,user=opegis LOG: process 44236822 acquired ShareLock on
> > transaction 3090377758 after 374146.817 ms
> >
> > Without seeing the source of the PL/pgSQL functions that seem to be
> > causing the problem, I can only suggest that you review those functions
> > for excessive locking and inefficient access. I'm going to guess, with
> > the large number of AccessExclusiveLocks in the log, that the functions
> > are doing explicit locking, and not doing so in an efficient manner.
>
> This function PL / pgSQL has been fixed, but see that during the period in
> which the SQL presenting slowness problem (between 9:26:00 and 9:27:00)
> there is no block occurrences.
>
> > > > > > * Based on the connection behavior you describe, I'm guessing it's
> > a
> > > > > > Tomcat app using some sort of conection pool. Whatever it is,
> > tune
> > > > > > your connection pool settings so that the max size of the pool
> > > > doesn't
> > > > > > exceed the available PostgreSQL connections. At least that will
> > > > prevent
> > > > > > errors from happening when the problem occurs.
> > > > >
> > > > > Yes it is a tomcat application using connection pooling and will be
> > set
> > > > to
> > > > > not exceed the limit of PostgreSQL connections, but the central
> > point is
> > > > > that the number of connections has increased due to what I am calling
> > > > > "momentary stoppages"
> > > >
> > > > I understand that. I made the suggestion as a temporary fix to reduce
> > > > the impact until you are able to fix the actual cause.
> > > >
> > > > > > * The query you you keep cancelling below, run just EXPLAIN on it
> > (not
> > > > > > EXPLAIN ANALYZE) which will certainly finish and give you a plan
> > > > > > that can be reviewed to help determine what the problem is.
> > > > >
> > > > > Explain plan in http://explain.depesz.com/s/bWw
> > > >
> > > > I'm confused. The query at that URL only took 10ms to complete, which
> > > > doesn't seem at all unreasonable.
> > >
> > > I agree with you and that's why I ask why the delay in the execution at
> > > certain times
> > > as demonstrated above.
> > >
> > > > > * On the OS, monitor iostat or something else to see if you're
> > > > > > saturating disk capacity when the problem happens.
> > > > >
> > > > > The OS is also being monitored and there occurs no saturation.
> > > >
> > > > Again, without any details, it's possible that there is a problem
> > > > in your monitoring.
> > > >
> > > >
> > > See the monitoring spreadsheet generated from nmon in
> > >
> > https://github.com/andervalbh/serv-gis01/blob/master/transfacil07_150605_0000.nmon.rar
> > >
> > >
> > > >
> > > > > > The Server is a IBM P720 128G RAM PostgreSQL 9.2.9 on
> > > > > > > powerpc-ibm-aix7.1.0.0, compiled by gcc (GCC) 4.6.4, 64-bit
> > > > > > >
> > > > > > > Evolution of the number of connections for a period
> > > > > > >
> > > > > > > snap_datetime | #connections
> > > > > > > ----------------------------------------------+-------------
> > > > > > > 2015-06-05 09:25:00.954731-03 | 74
> > > > > > > 2015-06-05 09:26:00.249187-03 | 205
> > > > > > > 2015-06-05 09:27:00.826874-03 | 207
> > > > > > > 2015-06-05 09:28:00.374666-03 | 73
> > > > > > > 2015-06-05 09:29:00.690696-03 | 75
> > > > > > >
> > > > > > >
> > > > > > > Occurrence of the problem
> > > > > > >
> > > > > > > citgis=# select max(dvi_sub.dvi_cod_dados_via_iti) as
> > > > > > dvi_cod_dados_via_iti,
> > > > > > > dvi_sub.via_cod_viagem
> > > > > > > from gis_dvi_dados_viagem_itinerario dvi_sub,
> > > > gis_via_viagem
> > > > > > > via_sub, gis_iti_itinerario iti_sub
> > > > > > > where dvi_sub.via_cod_viagem =
> > via_sub.via_cod_viagem
> > > > > > > and via_sub.via_status_viagem = 'A'
> > > > > > > and via_sub.via_dt_hora_ini > now() - interval '9
> > > > hours'
> > > > > > > and iti_sub.lin_cod_linha = 389
> > > > > > > and iti_sub.iti_sentido_itinerario = 'I'
> > > > > > > and iti_sub.iti_cod_itinerario_linha =
> > > > > > > via_sub.iti_cod_itinerario_linha
> > > > > > > and dvi_sub.vlo_cod_localizacao is not null
> > > > > > > group by dvi_sub.via_cod_viagem,
> > > > > > > iti_sub.iti_cod_itinerario_linha, via_sub.equ_cod_eqpto
> > > > > > > ;
> > > > > > > Cancel request sent
> > > > > > > ERROR: canceling statement due to user request
> > > > > > > Time: 2555.557 ms
> > > > > > > citgis=# select max(dvi_sub.dvi_cod_dados_via_iti) as
> > > > > > dvi_cod_dados_via_iti,
> > > > > > > dvi_sub.via_cod_viagem
> > > > > > > from gis_dvi_dados_viagem_itinerario dvi_sub,
> > > > gis_via_viagem
> > > > > > > via_sub, gis_iti_itinerario iti_sub
> > > > > > > where dvi_sub.via_cod_viagem =
> > via_sub.via_cod_viagem
> > > > > > > and via_sub.via_status_viagem = 'A'
> > > > > > > and via_sub.via_dt_hora_ini > now() - interval '9
> > > > hours'
> > > > > > > and iti_sub.lin_cod_linha = 389
> > > > > > > and iti_sub.iti_sentido_itinerario = 'I'
> > > > > > > and iti_sub.iti_cod_itinerario_linha =
> > > > > > > via_sub.iti_cod_itinerario_linha
> > > > > > > and dvi_sub.vlo_cod_localizacao is not null
> > > > > > > group by dvi_sub.via_cod_viagem,
> > > > > > > iti_sub.iti_cod_itinerario_linha, via_sub.equ_cod_eqpto
> > > > > > > ;
> > > > > > > Cancel request sent
> > > > > > > ERROR: canceling statement due to user request
> > > > > > > Time: 2192.572 ms
> > > > > > > citgis=# explain analyze select
> > max(dvi_sub.dvi_cod_dados_via_iti) as
> > > > > > > dvi_cod_dados_via_iti,
> > > > > > > dvi_sub.via_cod_viagem
> > > > > > > from gis_dvi_dados_viagem_itinerario dvi_sub,
> > > > gis_via_viagem
> > > > > > > via_sub, gis_iti_itinerario iti_sub
> > > > > > > where dvi_sub.via_cod_viagem =
> > via_sub.via_cod_viagem
> > > > > > > and via_sub.via_status_viagem = 'A'
> > > > > > > and via_sub.via_dt_hora_ini > now() - interval '9
> > > > hours'
> > > > > > > and iti_sub.lin_cod_linha = 389
> > > > > > > and iti_sub.iti_sentido_itinerario = 'I'
> > > > > > > and iti_sub.iti_cod_itinerario_linha =
> > > > > > > via_sub.iti_cod_itinerario_linha
> > > > > > > and dvi_sub.vlo_cod_localizacao is not null
> > > > > > > group by dvi_sub.via_cod_viagem,
> > > > > > > iti_sub.iti_cod_itinerario_linha, via_sub.equ_cod_eqpto
> > > > > > > ;
> > > > > > > Cancel request sent
> > > > > > > ERROR: canceling statement due to user request
> > > > > > > Time: 10164.015 ms
> > > > > > > citgis=# select now();
> > > > > > > now
> > > > > > > -------------------------------
> > > > > > > 2015-06-05 09:27:22.006072-03
> > > > > > > (1 row)
> > > > > > >
> > > > > > > Time: 0.152 ms
> > > > > > > citgis=# select max(dvi_sub.dvi_cod_dados_via_iti) as
> > > > > > dvi_cod_dados_via_iti,
> > > > > > > dvi_sub.via_cod_viagem
> > > > > > > from gis_dvi_dados_viagem_itinerario dvi_sub,
> > > > gis_via_viagem
> > > > > > > via_sub, gis_iti_itinerario iti_sub
> > > > > > > where dvi_sub.via_cod_viagem =
> > via_sub.via_cod_viagem
> > > > > > > and via_sub.via_status_viagem = 'A'
> > > > > > > and via_sub.via_dt_hora_ini > now() - interval '9
> > > > hours'
> > > > > > > and iti_sub.lin_cod_linha = 389
> > > > > > > and iti_sub.iti_sentido_itinerario = 'I'
> > > > > > > and iti_sub.iti_cod_itinerario_linha =
> > > > > > > via_sub.iti_cod_itinerario_linha
> > > > > > > and dvi_sub.vlo_cod_localizacao is not null
> > > > > > > group by dvi_sub.via_cod_viagem,
> > > > > > > iti_sub.iti_cod_itinerario_linha, via_sub.equ_cod_eqpto
> > > > > > > ;
> > > > > > > dvi_cod_dados_via_iti | via_cod_viagem
> > > > > > > -----------------------+----------------
> > > > > > > 1059964443 | 7989813
> > > > > > > 1060072723 | 7990876
> > > > > > > (2 rows)
> > > > > > >
> > > > > > > Time: 5565.175 ms
> > > > > > > citgis=# select max(dvi_sub.dvi_cod_dados_via_iti) as
> > > > > > dvi_cod_dados_via_iti,
> > > > > > > dvi_sub.via_cod_viagem
> > > > > > > from gis_dvi_dados_viagem_itinerario dvi_sub,
> > > > gis_via_viagem
> > > > > > > via_sub, gis_iti_itinerario iti_sub
> > > > > > > where dvi_sub.via_cod_viagem =
> > via_sub.via_cod_viagem
> > > > > > > and via_sub.via_status_viagem = 'A'
> > > > > > > and via_sub.via_dt_hora_ini > now() - interval '9
> > > > hours'
> > > > > > > and iti_sub.lin_cod_linha = 389
> > > > > > > and iti_sub.iti_sentido_itinerario = 'I'
> > > > > > > and iti_sub.iti_cod_itinerario_linha =
> > > > > > > via_sub.iti_cod_itinerario_linha
> > > > > > > and dvi_sub.vlo_cod_localizacao is not null
> > > > > > > group by dvi_sub.via_cod_viagem,
> > > > > > > iti_sub.iti_cod_itinerario_linha, via_sub.equ_cod_eqpto
> > > > > > > ;
> > > > > > > dvi_cod_dados_via_iti | via_cod_viagem
> > > > > > > -----------------------+----------------
> > > > > > > 1059964445 | 7989813
> > > > > > > 1060072725 | 7990876
> > > > > > > (2 rows)
> > > > > > >
> > > > > > > Time: 27.944 ms
> > > > > > > citgis=# select max(dvi_sub.dvi_cod_dados_via_iti) as
> > > > > > dvi_cod_dados_via_iti,
> > > > > > > dvi_sub.via_cod_viagem
> > > > > > > from gis_dvi_dados_viagem_itinerario dvi_sub,
> > > > gis_via_viagem
> > > > > > > via_sub, gis_iti_itinerario iti_sub
> > > > > > > where dvi_sub.via_cod_viagem =
> > via_sub.via_cod_viagem
> > > > > > > and via_sub.via_status_viagem = 'A'
> > > > > > > and via_sub.via_dt_hora_ini > now() - interval '9
> > > > hours'
> > > > > > > and iti_sub.lin_cod_linha = 389
> > > > > > > and iti_sub.iti_sentido_itinerario = 'I'
> > > > > > > and iti_sub.iti_cod_itinerario_linha =
> > > > > > > via_sub.iti_cod_itinerario_linha
> > > > > > > and dvi_sub.vlo_cod_localizacao is not null
> > > > > > > group by dvi_sub.via_cod_viagem,
> > > > > > > iti_sub.iti_cod_itinerario_linha, via_sub.equ_cod_eqpto
> > > > > > > ;
> > > > > > > dvi_cod_dados_via_iti | via_cod_viagem
> > > > > > > -----------------------+----------------
> > > > > > > 1059964445 | 7989813
> > > > > > > 1060072727 | 7990876
> > > > > > > (2 rows)
> > > > > > >
> > > > > > > Time: 24.428 ms
> > > > > > >
> > > > > > > Greetings
> > > > > > >
> > > > > > > Anderson
> > > > > >
> > > > > >
> > > > > > --
> > > > > > Bill Moran <wmoran(at)potentialtech(dot)com>
> > > > > >
> > > >
> > > >
> > > > --
> > > > Bill Moran <wmoran(at)potentialtech(dot)com>
> > > >
> >
> >
> > --
> > Bill Moran <wmoran(at)potentialtech(dot)com>
> >

--
Bill Moran