[pgpool-general: 2443] Re: PGPool tuning

Tatsuo Ishii ishii at postgresql.org
Fri Jan 17 13:24:02 JST 2014


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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