[PERFORM] Getting Slow
About six months ago, our normally fast postgres server started having performance issues. Queries that should have been instant were taking up to 20 seconds to complete (like selects on the primary key of a table). Running the same query 4 times in a row would yield dramatically different results... 1.001 seconds, 5 seconds, 22 seconds, 0.01 seconds, to complete. At the time we upgraded the hardware and the performance problems went away. But I did not feel like we had solved the underlying problem. Now, six months later, the same thing is happening... and I'm kind of glad because now, I'd like to find out what the real issue is. I'm just starting to diagnose it so I don't know a lot yet, but what I do know, I'll share with you here in the hopes of starting off on the right track. I've already described the main symptom. Here are some other random observations: - The server log shows frequent archived transaction log file entries. Usually once every 10 minutes or so, but sometimes 2 or 3 per minute. - The server box seems otherwise to be responsive. CPU sits at about 90% idle. - When queries are especially slow, the server shows a big spike in read/write activity. - This morning I did a VACUUM ANALYZE. It seemed to help for 30 minutes or so, but then it was back to being slowish. I'd hate to schedule these because it feels more like a band-aid. For a long time we've been doing just fine with autovacuum, so why start scheduling vacuums now? Here's info about our configuration. Any advise/pointers would be much appreciated. Thanks! Computer: Mac Pro Dual Core Intel Operating System: Mac OS 10.4.7 Client Memory: 4GB RAM Data Drives: 3 drives in a software RAID (internal) Log/Backup Drive: 1 (the startup disk, internal) Postgres Version: 8.1.4 Data Size: 5.1 GB # of Tables: 60 Size of Tables: Most are under 100,000 records. A few are in the millions. Largest is 7058497. Average Number of Simultaneous Client Connections: 250 max_connections = 500 shared_buffers = 1 work_mem = 2048 max_stack_depth = 6000 effective_cache_size = 3 fsync = on wal_sync_method = fsync archive_command = 'cp -i %p /Users/postgres/officelink/wal_archive/%f /dev/null' max_fsm_pages = 15 stats_start_collector = on stats_row_level = on log_min_duration_statement = 2000 log_line_prefix = '%t %h ' superuser_reserved_connections = 3 autovacuum = on autovacuum_naptime = 60 autovacuum_vacuum_threshold = 150 autovacuum_vacuum_scale_factor = 0.0001 autovacuum_analyze_scale_factor = 0.0001 sudo pico /etc/rc sysctl -w kern.sysv.shmmax=4294967296 sysctl -w kern.sysv.shmall=1048576 sudo pico /etc/sysctl.conf kern.maxproc=2048 kern.maxprocperuid=800 kern.maxfiles=4 kern.maxfilesperproc=3 Processes: 470 total, 2 running, 4 stuck, 464 sleeping... 587 threads 13:34:50 Load Avg: 0.45, 0.34, 0.33 CPU usage: 5.1% user, 5.1% sys, 89.7% idle SharedLibs: num = 157, resident = 26.9M code, 3.29M data, 5.44M LinkEdit MemRegions: num = 15307, resident = 555M + 25.5M private, 282M shared PhysMem: 938M wired, 934M active, 2.13G inactive, 3.96G used, 43.1M free VM: 116G + 90.1M 1213436(0) pageins, 263418(0) pageouts PID COMMAND %CPU TIME #TH #PRTS #MREGS RPRVT RSHRD RSIZE VSIZE 29804 postgres 0.0% 0:03.24 1 927 1.27M 245M 175M 276M 29720 postgres 0.0% 0:01.89 1 927 1.25M 245M 125M 276M 29714 postgres 0.0% 0:03.70 11027 1.30M 245M 215M 276M 29711 postgres 0.0% 0:01.38 11027 1.21M 245M 107M 276M 29707 postgres 0.0% 0:01.27 1 927 1.16M 245M 78.2M 276M 29578 postgres 0.0% 0:01.33 1 927 1.16M 245M 67.8M 276M 29556 postgres 0.0% 0:00.39 1 927 1.09M 245M 91.8M 276M 29494 postgres 0.0% 0:00.19 1 927 1.05M 245M 26.5M 276M 29464 postgres 0.0% 0:01.98 1 927 1.16M 245M 88.8M 276M 29425 postgres 0.0% 0:01.61 1 927 1.17M 245M 112M 276M 29406 postgres 0.0% 0:01.42 1 927 1.15M 245M 118M 276M 29405 postgres 0.0% 0:00.13 1 926 924K 245M 17.9M 276M 29401 postgres 0.0% 0:00.98 11027 1.13M 245M 84.4M 276M 29400 postgres 0.0% 0:00.90 11027 1.14M 245M 78.4M 276M 29394 postgres 0.0% 0:01.56 11027 1.17M 245M 111M 276M
Re: [PERFORM] Getting Slow
On Thu, Jun 07, 2007 at 01:48:43PM -0400, Joe Lester wrote: of a table). Running the same query 4 times in a row would yield dramatically different results... 1.001 seconds, 5 seconds, 22 seconds, 0.01 seconds, to complete. - When queries are especially slow, the server shows a big spike in read/write activity. My bet is that you're maxing your disk subsystem somehow. The problem with being I/O bound is that it doesn't matter how great you do on average: if you have too much I/O traffic, it looks like you're stopped. Softraid can be expensive -- first thing I'd look at is to see whether you are in fact hitting 100% of your I/O capacity and, if so, what your options are for getting more room there. A -- Andrew Sullivan | [EMAIL PROTECTED] The year's penultimate month is not in truth a good way of saying November. --H.W. Fowler ---(end of broadcast)--- TIP 7: You can help support the PostgreSQL project by donating at http://www.postgresql.org/about/donate
Re: [PERFORM] Getting Slow
On Thu, Jun 07, 2007 at 01:48:43PM -0400, Joe Lester wrote: - The server log shows frequent archived transaction log file entries. Usually once every 10 minutes or so, but sometimes 2 or 3 per minute. Sounds like you've got a lot of writes going. You might want more power in your I/O? Operating System: Mac OS 10.4.7 Client Is there a particular reason for this? It's not known to be the best server OS around -- it's hard to say that an OS change would do anything for your problem, but it looks like an unusual choice. /* Steinar */ -- Homepage: http://www.sesse.net/ ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [PERFORM] Getting Slow
Joe Lester wrote: max_fsm_pages = 15 This may be a bit too low -- it's just a little more than 1 GB, which means it might fail to keep track of all your tables (or it may not, if you don't have many updates). autovacuum_naptime = 60 autovacuum_vacuum_threshold = 150 autovacuum_vacuum_scale_factor = 0.0001 autovacuum_analyze_scale_factor = 0.0001 The scale factors seems awfully low. How about 0.01 instead and see if you avoid vacuuming all your tables with every iteration ... have you noticed how much work autovacuum is really doing? It may be too much. Also if autovacuum is eating all your I/O you may want to look into throttling it back a bit by setting autovacuum_vacuum_cost_delay to a non-zero value. -- Alvaro Herrerahttp://www.advogato.org/person/alvherre La tristeza es un muro entre dos jardines (Khalil Gibran) ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [PERFORM] Getting Slow
On Thu, 7 Jun 2007, Joe Lester wrote: Memory: 4GB RAM shared_buffers = 1 work_mem = 2048 effective_cache_size = 3 With these parameters, your server has 80MB dedicated to its internal caching, is making query decisions assuming the operating system only has 240MB of memory available for its caching, and is only allowing individual clients to have a tiny amount of memory to work with before they have to swap things to disk. You're not giving it anywhere close to enough memory to effectively work with a 5GB database, and your later reports show you're barely using 1/2 the RAM in this system usefully. Multiply all these parameters by 10X, restart your server, and then you'll be in the right ballpark for a system with 4GB of RAM. There might be some other tuning work left after that, but these values are so far off that until you fix them it's hard to say what else needs to be done. See http://www.westnet.com/~gsmith/content/postgresql/pg-5minute.htm for more information on this topic. -- * Greg Smith [EMAIL PROTECTED] http://www.gregsmith.com Baltimore, MD ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [PERFORM] Getting Slow
Alvaro Herrera [EMAIL PROTECTED] writes: Also if autovacuum is eating all your I/O you may want to look into throttling it back a bit by setting autovacuum_vacuum_cost_delay to a non-zero value. BTW, why is it that autovacuum_cost_delay isn't enabled by default? I can hardly believe that anyone will want to run it without that. *Especially* not with multiple workers configured by default. regards, tom lane ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [PERFORM] Getting Slow
Tom Lane wrote: Alvaro Herrera [EMAIL PROTECTED] writes: Also if autovacuum is eating all your I/O you may want to look into throttling it back a bit by setting autovacuum_vacuum_cost_delay to a non-zero value. BTW, why is it that autovacuum_cost_delay isn't enabled by default? I can hardly believe that anyone will want to run it without that. *Especially* not with multiple workers configured by default. Just because we haven't agreed a value. Default autovacuum parameters is something we should definitely discuss for 8.3. -- Alvaro Herrerahttp://www.CommandPrompt.com/ PostgreSQL Replication, Consulting, Custom Development, 24x7 support ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
