[pgpool-general: 2474] Re: PGPool tuning

Justin Cooper jcooper at vendwatchtelematics.com
Thu Jan 23 12:47:07 JST 2014


On Wed, Jan 22, 2014 at 9:46 PM, Justin Cooper <
jcooper at vendwatchtelematics.com> wrote:

> Oh, it's entirely our application framework's fault!
>
> It shouldn't be waiting for a session to become free if it is also
> possible for sessions to be running for 5 minutes...
>
>
Or, if it's going to wait, it should close the DB connection to pgpool
while it waits!



> Our fix is to make the "pinger" not require a session, since it is only
> looking for a lockfile and doesn't need session data to do that.
>
>
> Thanks again for your helpful suggestions, Tatsuo!
>
> Justin
>
>
>
> On Wed, Jan 22, 2014 at 7:21 PM, Tatsuo Ishii <ishii at postgresql.org>wrote:
>
>> Justin,
>>
>> Thank you for the follow up.
>>
>> It would be nice if we could avoid something like "ping" client to
>> fill up all available sockets of pgpool-II. Any idea anyone?
>>
>> Best regards,
>> --
>> Tatsuo Ishii
>> SRA OSS, Inc. Japan
>> English: http://www.sraoss.co.jp/index_en.php
>> Japanese: http://www.sraoss.co.jp
>>
>> > Just to follow up on this, it turns out it was a problem in our
>> > application...
>> >
>> > We had a long running job that was tying up the user's session, paired
>> with
>> > a second browser window that was making a "ping" type call every 10
>> > seconds.  These pings were stacking up and after 300s, they would tie up
>> > all of PGPool's available sockets to apache.
>> >
>> >
>> > Thanks for the help.  I did a lot of testing with PGPool and thought it
>> was
>> > the source of the problem, but the more I tested the more I became
>> > convinced that PGPool is actually working great!
>> >
>> >
>> > Cheers,
>> > Justin
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> > On Thu, Jan 16, 2014 at 10:24 PM, Tatsuo Ishii <ishii at postgresql.org>
>> wrote:
>> >
>> >> When the lock up happens, what "select * from pg_stat_activity"
>> >> and "select * from pg_locks" show?
>> >>
>> >> Best regards,
>> >> --
>> >> Tatsuo Ishii
>> >> SRA OSS, Inc. Japan
>> >> English: http://www.sraoss.co.jp/index_en.php
>> >> Japanese: http://www.sraoss.co.jp
>> >>
>> >> > Thank you, Tatsuo.
>> >> >
>> >> > We are still experiencing the problem once or twice per day.  I am
>> making
>> >> > incremental changes on our live cluster after testing them on the
>> test
>> >> > cluster.  So far we have done the following:
>> >> >
>> >> > -Comment out unused 2nd backend in pgpool.conf
>> >> > -Add a connect_timeout of 10 seconds to the pg_connect() connection
>> >> string
>> >> > in the PHP application
>> >> > -set sysctl net.core.somaxconn = 1024
>> >> >
>> >> > We just did the last step today so we will see if there is any
>> impact.
>> >> >
>> >> > When the fault happens, there is work being done in the database, yet
>> >> > "select * from pg_stat_activity;" shows only a few running queries
>> at the
>> >> > time.  To me, this says that Apache+PHP still has the connection
>> open to
>> >> > pgpool.
>> >> >
>> >> > I'll be sure to post back if we figure it out!
>> >> >
>> >> > Justin
>> >> >
>> >> >
>> >> >
>> >> >
>> >> >
>> >> >
>> >> >
>> >> > On Mon, Jan 13, 2014 at 7:55 PM, Tatsuo Ishii <ishii at postgresql.org>
>> >> wrote:
>> >> >
>> >> >> Thanks for posting detailed analythis. It looks really interesting.
>> >> >> I need more time to understanding full details.
>> >> >>
>> >> >> In the mean time I wonder if you care about listen queue
>> >> >> setting. Currently pgpool listens up to num_init_children*2 (which
>> 64,
>> >> >> in your case). However Apache connects to pgpool up to 256, which is
>> >> >> way too low compared with 64. Also Linux allows max the listen queue
>> >> >> to up 128 by default on most systems. You can check it by looking
>> at:
>> >> >>
>> >> >> $ sysctl net.core.somaxconn
>> >> >> net.core.somaxconn = 128
>> >> >>
>> >> >> 128 is too low compared with 256, of course.
>> >> >>
>> >> >> If the allowed listen queue length (backlog) is too low, lots of
>> retry
>> >> >> happens in kernel's TCP layer.
>> >> >>
>> >> >> Best regards,
>> >> >> --
>> >> >> Tatsuo Ishii
>> >> >> SRA OSS, Inc. Japan
>> >> >> English: http://www.sraoss.co.jp/index_en.php
>> >> >> Japanese: http://www.sraoss.co.jp
>> >> >>
>> >> >> > Greetings!
>> >> >> >
>> >> >> > We are having an issue with PGPool and I wanted to post my
>> analysis to
>> >> >> this
>> >> >> > list to see if: A). My analysis seems correct to you all and B).
>> To
>> >> see
>> >> >> if
>> >> >> > you folks might have any advice on tuning.
>> >> >> >
>> >> >> >
>> >> >> > For the last month plus, we have been experiencing an intermittent
>> >> fault
>> >> >> > state on our production cluster.  When the fault occurs, any
>> request
>> >> to
>> >> >> the
>> >> >> > Apache+PHP web server will either time out connecting, or will
>> connect
>> >> >> but
>> >> >> > return with a "Could not connect to DB" message from PHP.  I've
>> done
>> >> some
>> >> >> > analysis on the problem and this is what I've found.
>> >> >> >
>> >> >> > First let me describe the cluster as it is configured today.  We
>> have
>> >> one
>> >> >> > web front end running Apache+PHP, which has a MaxClients setting
>> of
>> >> 256,
>> >> >> > meaning that it's possible to have 256 concurrently running
>> processes.
>> >> >>  The
>> >> >> > PHP application is configured to connect to PGPool 3.2.1 for its
>> >> database
>> >> >> > connection.  PGPool is configured with max_init_children of 32 and
>> >> >> max_pool
>> >> >> > of 8.  The application runs on 10-12 different databases, all
>> with the
>> >> >> same
>> >> >> > Postgres username+password.
>> >> >> >
>> >> >> > When the fault occurs, it looks like this: Apache has 256 running
>> >> >> processes
>> >> >> > and load on the web front end drops to near 0.  PGPool has all 32
>> >> sockets
>> >> >> > that face Apache filled, and all 256 sockets that face Postgres
>> >> filled.
>> >> >> >  Postgres has 256 connections and its load goes to near 0.  If you
>> >> try to
>> >> >> > connect to PGPool from the command line, it will time out in
>> >> connecting,
>> >> >> or
>> >> >> > sometimes partially connect and then receive a connection closed
>> >> message.
>> >> >> >
>> >> >> > Using our test cluster, I ran some tests that give me high
>> confidence
>> >> >> that
>> >> >> > PGPool is actually working correctly, as are Apache and Postgres,
>> and
>> >> >> that
>> >> >> > the fundamental problem is just a badly tuned configuration.
>>  This is
>> >> the
>> >> >> > test that shows that best:
>> >> >> >
>> >> >> >
>> >> >> >    1. Stop Apache, restart PGPool
>> >> >> >    2. Start up 100 psql command line clients to connect to PGPool
>> >> with a
>> >> >> >    single database
>> >> >> >    3. The first 32 psql clients connect and work fine
>> >> >> >    4. The 33rd psql client blocks waiting to connect (it will
>> time out
>> >> >> >    after 30 seconds, but in this test we don't wait that long)
>> >> >> >    5. fg the psql client #1, then exit the client, freeing up one
>> of
>> >> >> >    PGPool's connections
>> >> >> >    6. One of the 68 blocking psql clients now gets through and
>> can run
>> >> >> >    queries
>> >> >> >    7. Any of the 32 connected psql clients can get through as well
>> >> >> >
>> >> >> > This shows that PGPool is working as expected.
>> >> >> >
>> >> >> > Now we try a test that is more like the real world:
>> >> >> >
>> >> >> >    1. Restart PGPool
>> >> >> >    2. Start up 10-20 psql command line clients.  These are
>> simulating
>> >> >> long
>> >> >> >    running php processes.
>> >> >> >    3. Start siege web testing tool with 100-200 concurrent
>> requests to
>> >> >> >    Apache.
>> >> >> >    4. At 100 clients, the response time from Apache slows down
>> and the
>> >> >> time
>> >> >> >    taken to service each request goes up to around 15s (from <
>> 1s).
>> >>  Psql
>> >> >> >    command line client can get through most of the time, but it
>> takes
>> >> >> some
>> >> >> >    time to connect as it is contending for one of the 32 slots to
>> >> PGPool
>> >> >> with
>> >> >> >    all of the Apache processes.
>> >> >> >    5. At 200 clients, response time goes up more and we start to
>> see
>> >> >> >    failures in Apache, as well as "Could not connect to DB"
>> responses.
>> >> >>  Psql
>> >> >> >    command line client often will timeout before it gets a
>> connection
>> >> to
>> >> >> >    PGPool.
>> >> >> >    6. Once lots of failures are happening at the 200 clients
>> level,
>> >> load
>> >> >> on
>> >> >> >    Postgres goes to near 0 as well as load on Apache.
>> >> >> >    7. Failure will also happen with 250 siege clients and no psql
>> >> command
>> >> >> >    line clients running.
>> >> >> >
>> >> >> >
>> >> >> > In step 4, I believe the response time from Apache goes up due to
>> >> PGPool
>> >> >> > having to spend so much time managing incoming connections from
>> >> Apache as
>> >> >> > well as managing connections to Postgres.  Database load is not
>> high
>> >> in
>> >> >> > this case, so the slowness is not due to Postgres being
>> overloaded.
>> >> >> >
>> >> >> > I believe that on the live cluster the load is even more severe as
>> >> there
>> >> >> > are more databases being used, and occasionally high load, long
>> >> running
>> >> >> > queries.
>> >> >> >
>> >> >> > It's also notable that restarting Apache has been our fix to get
>> >> >> everything
>> >> >> > running again.  I believe that this is because PGPool gets a
>> chance to
>> >> >> > catch up, which it does fairly quickly, and resumes with 32
>> available
>> >> >> > sockets for Apache.  If we do nothing, PGPool reaches a 10 minute
>> >> timeout
>> >> >> > specified in its config, and closes all 32 sockets, which causes
>> >> >> everything
>> >> >> > to resume working again.
>> >> >> >
>> >> >> >
>> >> >> > In the end, I believe the problem is that Apache is just sending
>> too
>> >> many
>> >> >> > requests to PGPool, and PGPool spends all of its time managing
>> >> >> connections,
>> >> >> > causing it to be slow at everything.  That slowness and contention
>> >> for 32
>> >> >> > slots among up to 256 Apache processes leads to connection
>> timeouts
>> >> (it
>> >> >> > should be noted that Apache seems to have no connect timeout
>> defined
>> >> and
>> >> >> > will wait for a connection until the PHP max execution time is
>> >> reached).
>> >> >> >  Once a threshold is reached, we enter a state where no Apache
>> >> process is
>> >> >> > able to connect to PGPool in enough time and we see the browser
>> >> requests
>> >> >> > either timing out entirely or returning the "Could not connect to
>> DB"
>> >> >> > message.
>> >> >> >
>> >> >> >
>> >> >> > The proposed solution to all of this is to adjust the
>> configuration of
>> >> >> > PGPool and Apache to ensure that we can never reach this
>> overwhelmed
>> >> >> state.
>> >> >> >  Specifically, we need to increase the number of PGPool processes
>> and
>> >> >> > decrease the maximum number of Apache processes.  We need to be
>> >> careful
>> >> >> as
>> >> >> > we do this, as there is surely an upper limit to how many PGPool
>> >> >> processes
>> >> >> > can be sustained and increasing that increases overhead on
>> Postgres
>> >> since
>> >> >> > it increases the number of persistent open connections between it
>> and
>> >> >> > PGPool.  The same for Apache, we need to lower MaxClients but not
>> so
>> >> low
>> >> >> > that it turns away requests that could have been handled.
>> >> >> >
>> >> >> >
>> >> >> > There are a few other adjustments that I believe will help that
>> I'll
>> >> >> > describe below.
>> >> >> >
>> >> >> > Apache MaxClients:
>> >> >> > This is how many concurrent Apache processes can run at once.  The
>> >> >> current
>> >> >> > setting of 256 is clearly more than the system can handle.  I
>> suggest
>> >> we
>> >> >> > drop it down to 128 to begin with and monitor the results.  I'd
>> like
>> >> to
>> >> >> > make this change before the others.
>> >> >> >
>> >> >> > Apache PHP DB connection timeout:
>> >> >> > I can see that it's waiting as long as 150s before returning with
>> >> 'Could
>> >> >> > not connect to DB' at times, which indicates that no timeout is
>> being
>> >> >> > specified.  This must be sent as part of the connection string,
>> like:
>> >> >> > "pgsql:host=127.0.0.1;port=5432;dbname=vw_bepensa;timeout=10".
>>  I'm
>> >> not
>> >> >> > sure at this point what a reasonable value would be, but I'm
>> thinking
>> >> 10
>> >> >> > seconds is a good start.
>> >> >> >
>> >> >> > PGPool backends:
>> >> >> > We currently have 2 backends specified in the config.  One has
>> >> >> > backend_weight of 1 and the other, that is not used, has
>> >> backedn_weight
>> >> >> of
>> >> >> > 0.  I have confirmed that whenever a client connects to PGPool and
>> >> >> requests
>> >> >> > a connection to a database, for example, PGPool opens a persistent
>> >> >> > connection to both backends.  We will comment out the backend that
>> >> >> > specifies the backup server, which should help PGPool a lot.
>> >> >> >
>> >> >> >
>> >> >> > PGPool max_init_children:
>> >> >> > This is the config parameter that specifies how many PGPool
>> processes
>> >> can
>> >> >> > run, and therefore how many sockets are available to Apache.
>> >>  Increasing
>> >> >> > this number by one increases the number of persistent connections
>> to
>> >> the
>> >> >> DB
>> >> >> > by max_pool, currently 8.  Postgres is currently configured to
>> only
>> >> allow
>> >> >> > 300 connections maximum, so that would need to be changed as well.
>> >>  More
>> >> >> > research and testing is needed to find the sweet spot.
>> >> >> >
>> >> >> > PGPool max_pool:
>> >> >> > This parameter specifies how many different DBs each PGPool
>> process
>> >> keeps
>> >> >> > in its cache of persistent connections to Postgres.  It is
>> currently
>> >> set
>> >> >> to
>> >> >> > 8, yet we have more than 8 different databases in production (I
>> see 12
>> >> >> > connected right now).  If a connection to a database is requested
>> of
>> >> >> PGPool
>> >> >> > by Apache, and the PGPool process servicing Apache's request does
>> not
>> >> >> have
>> >> >> > a connection to that database, it will drop one and use the slot
>> to
>> >> make
>> >> >> a
>> >> >> > new connection to the requested DB on Postgres.  If max_pool was
>> set
>> >> to
>> >> >> 12,
>> >> >> > this would stop happening and there would always be a persistent
>> >> >> connection
>> >> >> > to the db requested ready to go when requested by apache.
>>  Postgres
>> >> would
>> >> >> > ideally get no new db connections.  Increasing from 8 to 12 would
>> mean
>> >> >> that
>> >> >> > total connections to Postgres would be 32*12 = 384, which is above
>> >> >> > Postgres's connection limit.  So this parameter,
>> max_init_children,
>> >> and
>> >> >> > Postgres's connection limit must all be tuned to eachother, and
>> kept
>> >> low
>> >> >> > enough to not overwhelm Postgres.
>> >> >> >
>> >> >> >
>> >> >> > I suggest that we begin by commenting out the second backend in
>> >> >> > pgpool.conf, and lowering MaxClients on Apache to 128.  This
>> should
>> >> >> prevent
>> >> >> > PGPool being hammered past the point that it can handle.  If
>> PGPool
>> >> does
>> >> >> > fall behind, only 128 Apache connections will be hitting PGPool
>> and it
>> >> >> > seems to be able to handle that many in an orderly fashion.
>> >> >> >
>> >> >> > I also think adding a PHP connection timeout will help keep the
>> system
>> >> >> from
>> >> >> > grinding to a stop.
>> >> >> >
>> >> >> >
>> >> >> > Thank you for reading and any help or insight you can provide!
>> >> >> >
>> >> >> > Justin Cooper
>> >> >>
>> >>
>>
>
>
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