Thread: Postgresql-9.1 CentOS7 effective_cache_size issue

Hi All,

I've been performance tuning a new database server for the past couple of weeks with very mixed results, I've read every guide to tuning I can locate on Google aswell as Gregory Smiths - Postgresql 9.0 High Performance book.

The server is a HP DL385P gen8, dual processor AMD Opteron 6386SE, 16core 2.8Ghz (32 cores total). 128GB DDR3 1600mhz, 8 x 16GB sticks. 4 x 300GB 6G SAS 10K in a RAID1+0 configuration.

We are using CentOS7.1 minimal with Postgresql-9.2.13.

I'm seeing good IOPS, memory throughput is good, the server is benchmarking very well in comparison to it's predecessor.

I have left most of the configuration as defaults and tuned the following parameters;


shared_buffers = 8GB
max_prepared_transactions = 5
work_mem = 32MB
max_stack_depth = 7MB
max_files_per_process = 1000000
wal_level = hot_standby
max_wal_senders = 3
wal_keep_segments = 128
wal_buffers=64MB
checkpoint_segments = 64
maintenance_work_mem=2GB

## note this is commented out
#effective_cache_size = 40GB

# increased logging levels for PGBADGER
track_activities = on
track_counts = on
track_functions = all
log_parser_stats = off
log_planner_stats = off
log_executor_stats = off
log_statement_stats = off
log_min_duration_statement = 0
log_line_prefix = '%t [%p]: [%l-1] user=%u,db=%d '
log_checkpoints = on
log_connections = on
log_disconnections = on
log_lock_waits = on
log_temp_files = 0
log_destination = 'stderr'
logging_collector = on
log_directory = 'pg_log'
log_filename = 'postgresql-%Y-%m-%d.log'
log_rotation_age = 0
client_min_messages = notice
log_min_messages = warning
log_min_error_statement = error
log_min_duration_statement = 0
log_checkpoints = on
log_connections = on
log_disconnections = on
log_duration = off
log_error_verbosity = verbose
log_hostname = on
log_line_prefix = '%t [%p]: [%l-1] db=%d,user=%u '
log_lock_waits = on
log_statement = 'none'
log_temp_files = 0

From my readings online I have configured shmmax and shmall in /etc/sysctl.conf, the suggested settings were 50% of RAM dedicated to shared_buffers.

kernel.shmmax = 67462680576     # roughly 62GB allowing 60GB for PGSQL.
kernel.shmall = 16470381        # shmmax/16 the same ratio as default values and my previous server.

the shmmax and shamall can be reduced, this was my starting point.

Now, when I make changes with work_mem and shared_buffers I am seeing performance increases / decreases as I would expect.

When I set effective_cache_size to anything other than the default (comment out my setting) my TPS takes a huge nose dive, from 37TPS down to 5TPS.


wal_buf wal_seg effective_cache_size    shared_buffers  work_mem
64MB    64      defaults                8GB             64MB

pgbench - my data on my database
TPS             total transactions
37.324716       11224
34.353093       10337
19.832292       6003
10.010148       3120
5.859798        2073


changing effective_cache_size (tested from 1GB to 80GB) causes these benchmark results
wal_buf wal_seg effective_cache_size    shared_buffers  work_mem
64MB    64      *****                   8GB             64MB

pgbench - my data on my database
TPS     total transactions
5.86    1,770
3.78    1,168
1.34    430
0.66    258
0.37    512


looking at vmstat, free, top and ipcs I'm not seeing anything unusual, nothing is being swapped to disk, cache is not flooding and I am only consuming about 8GB of RAM no matter what configuration changes I make.

Are there known issues with Postgresql-9.2.13 and Centos7? I found one article where a guy had the same kind of issues with memory consumption.

http://postgresql.nabble.com/PostgreSQL-9-3-abd-CentOS-7-memory-usage-td5822755.html

Can anybody point me in the right direction?! am I making some fundamental mistakes with my configuration?

Any assistance would be great, I'm pushing to get this box into production later this week!

Thank you in advance,

Michael


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