Re: Odd blocking (or massively latent) issue - even with EXPLAIN - Mailing list pgsql-performance

<p><font face="sans-serif" size="2">Hi</font><br /><br /><tt><font size="2">> <br />> We're seeing SELECT
statementsand even EXPLAIN (no ANAYLZE) <br />> statements hang indefinitely until *something* (we don't know
what)<br/>> releases some kind of resource or no longer becomes a massive bottle<br />> neck. These are the
symptoms.<br/></font></tt><br /><tt><font size="2">Is this in pgAdmin? Or psql on the console?</font></tt><br
/><tt><fontsize="2"><br />> However, the system seems healthy - no table ('heavyweight') locks are<br />> held by
anysession (this happens with only a few connected sessions),<br />> all indexes are used correctly, other
transactionsare writing data (we<br />> generally only have a few sessions running at a time - perhaps 10) etc.<br
/>>etc. In fact, we are writing (or bgwriter is), 2-3 hundred MB/s<br />> sometimes.<br /></font></tt><br
/><tt><fontsize="2">What is shown in "top" and "iostat" whilst the queries are running?</font></tt><br /><br
/><tt><fontsize="2">> <br />> We regularly run vacuum analyze at quiet periods - generally 1-2s daily.<br />>
<br/>> These sessions (that only read data) that are blocked can block from<br />> anything from between only 5
minutesto 10s of hours then miraculously<br />> complete successfully at once.<br />> <br /></font></tt><br
/><tt><fontsize="2">Are any "blockers" shown in pg_stat_activity?<br /></font></tt><br /><tt><font size="2">> <br
/>>checkpoint_segments = 128<br />> maintenance_work_mem = 256MB<br />> synchronous_commit = off<br />>
random_page_cost= 3.0<br />> wal_buffers = 16MB<br />> shared_buffers = 8192MB<br />>
checkpoint_completion_target= 0.9<br />> effective_cache_size = 18432MB<br />> work_mem = 32MB<br />>
effective_io_concurrency= 12<br />> max_stack_depth = 8MB<br />> log_autovacuum_min_duration = 0<br />>
log_lock_waits= on<br />> autovacuum_vacuum_scale_factor = 0.1<br />> autovacuum_naptime = 8<br />>
autovacuum_max_workers= 4<br /></font></tt><br /><tt><font size="2">Memory looks reasonably configured to me.
effective_cache_sizeis only an indication to the planner and is not actually allocated. <br /></font></tt><br
/><tt><fontsize="2">Is anything being written to the logfiles?</font></tt><br /><br /><tt><font
size="2">Cheers</font></tt><fontface="sans-serif">============================================= Romax Technology
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