8.3.9 - latency spikes with Linux (and tuning for consistently low latency) - Mailing list pgsql-performance
From | Marinos Yannikos |
---|---|
Subject | 8.3.9 - latency spikes with Linux (and tuning for consistently low latency) |
Date | |
Msg-id | 4BC742C8.2050301@geizhals.at Whole thread Raw |
Responses |
Re: 8.3.9 - latency spikes with Linux (and tuning for consistently low latency)
(Tom Lane <tgl@sss.pgh.pa.us>)
Re: 8.3.9 - latency spikes with Linux (and tuning for consistently low latency) (Greg Smith <greg@2ndquadrant.com>) |
List | pgsql-performance |
Hi, we are seeing latency spikes in the 2-3 second range (sometimes 8-10s) for queries that usually take 3-4ms on our systems and I am running out of things to try to get rid of them. Perhaps someone here has more ideas - here's a description of the systems and what I've tried with no impact at all: 2 x 6-core Opterons (2431) 32GB RAM 2 SATA disks (WD1500HLFS) in software RAID-1 Linux 2.6.26 64 bit (Debian kernel) PostgreSQL 8.3.9 (Debian package) FS mounted with option noatime vm.dirty_ratio = 80 3 DB clusters, 2 of which are actively used, all on the same RAID-1 FS fsync=off shared_buffers=5GB (database size is ~4.7GB on disk right now) temp_buffers=50MB work_mem=500MB wal_buffers=256MB (*) checkpoint_segments=256 (*) commit_delay=100000 (*) autovacuum=off (*) (*) added while testing, no change w.r.t. the spikes seen at all The databases have moderate read load (no burst load, typical web backend) and somewhat regular write load (updates in batches, always single-row update/delete/inserts using the primary key, 90% updates, a few 100s to 1000s rows together, without explicit transactions/locking). This is how long the queries take (seen from the client): Thu Apr 15 18:16:14 CEST 2010 real 0m0.004s Thu Apr 15 18:16:15 CEST 2010 real 0m0.004s Thu Apr 15 18:16:16 CEST 2010 real 0m0.003s Thu Apr 15 18:16:17 CEST 2010 real 0m0.005s Thu Apr 15 18:16:18 CEST 2010 real 0m0.068s Thu Apr 15 18:16:19 CEST 2010 real 0m0.004s Thu Apr 15 18:16:20 CEST 2010 real 0m0.005s Thu Apr 15 18:16:21 CEST 2010 real 0m0.235s Thu Apr 15 18:16:22 CEST 2010 real 0m0.005s Thu Apr 15 18:16:23 CEST 2010 real 0m3.006s <== ! Thu Apr 15 18:16:27 CEST 2010 real 0m0.004s Thu Apr 15 18:16:28 CEST 2010 real 0m0.084s Thu Apr 15 18:16:29 CEST 2010 real 0m0.003s Thu Apr 15 18:16:30 CEST 2010 real 0m0.005s Thu Apr 15 18:16:32 CEST 2010 real 0m0.038s Thu Apr 15 18:16:33 CEST 2010 real 0m0.005s Thu Apr 15 18:16:34 CEST 2010 real 0m0.005s The spikes aren't periodic, i.e. not every 10,20,30 seconds or 5 minutes etc, they seem completely random... PostgreSQL also reports (due to log_min_duration_statement=1000) small bursts of queries that take much longer than they should: [nothing for a few minutes] 2010-04-15 16:50:03 CEST LOG: duration: 8995.934 ms statement: select ... 2010-04-15 16:50:04 CEST LOG: duration: 3383.780 ms statement: select ... 2010-04-15 16:50:04 CEST LOG: duration: 3328.523 ms statement: select ... 2010-04-15 16:50:05 CEST LOG: duration: 1120.108 ms statement: select ... 2010-04-15 16:50:05 CEST LOG: duration: 1079.879 ms statement: select ... [nothing for a few minutes] (explain analyze yields 5-17ms for the above queries) Things I've tried apart from the PostgreSQL parameters above: - switching from ext3 with default journal settings to data=writeback - switching to ext2 - vm.dirty_background_ratio set to 1, 10, 20, 60 - vm.dirty_expire_centisecs set to 3000 (default), 8640000 (1 day) - fsync on - some inofficial Debian 2.6.32 kernel and ext3 with data=writeback (because of http://lwn.net/Articles/328363/ although it seems to address fsync latency and not read latency) - running irqbalance All these had no visible impact on the latency spikes. I can also exclude faulty hardware with some certainty (since we have 12 identical systems with this problem). I am suspecting some strange software RAID or kernel problem, unless the default bgwriter settings can actually cause selects to get stuck for so long when there are too many dirty buffers (I hope not). Unless I'm missing something, I only have a non-RAID setup or ramdisks (tmpfs), or SSDs left to try to get rid of these, so any suggestion will be greatly appreciated. Generally, I'd be very interested in hearing how people tune their databases and their hardware/Linux for consistently low query latency (esp. when everything should fit in memory). Regards, Marinos
pgsql-performance by date: