Re: Performance issue with 8.2.3 - "C" application
От | Karl Denninger |
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Тема | Re: Performance issue with 8.2.3 - "C" application |
Дата | |
Msg-id | 46A6BBB1.1020502@denninger.net обсуждение исходный текст |
Ответ на | Re: Performance issue with 8.2.3 - "C" application (Tom Lane <tgl@sss.pgh.pa.us>) |
Ответы |
Re: Performance issue with 8.2.3 - "C" application
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Список | pgsql-performance |
Yeah, the problem doesn't appear to be there. As I said, if I look at the PS of the system when its bogging, there aren't a whole bunch of processes stuck doing these, so while this does take a second or two to come back, that's not that bad.
Its GENERAL performance that just bites - the system is obviously out of CPU, but what I can't get a handle on is WHY. It does not appear to be accumulating large amounts of runtime in processes I can catch, but the load average is quite high.
This is why I'm wondering if what I'm taking here is a hit on the fork/exec inside the portmaster, in the setup internally in there, in the IPC between my process via libPQ, etc - and how I can profile what's going on.
Tom Lane wrote:
Its GENERAL performance that just bites - the system is obviously out of CPU, but what I can't get a handle on is WHY. It does not appear to be accumulating large amounts of runtime in processes I can catch, but the load average is quite high.
This is why I'm wondering if what I'm taking here is a hit on the fork/exec inside the portmaster, in the setup internally in there, in the IPC between my process via libPQ, etc - and how I can profile what's going on.
Karl Denninger (karl@denninger.net) http://www.denninger.net
Tom Lane wrote:
Karl Denninger <karl@denninger.net> writes:But.... here's the query that has a habit of taking the most time....select forum, * from post where toppost = 1 and (replied > (select lastview from forumlog where login='theuser' and forum=post.forum and number is null)) is not false AND (replied > (select lastview from forumlog where login='theuser' and forum=post.forum and number=post.number)) is not f alse order by pinned desc, replied desc offset 0 limit 20Did that ever perform well for you? It's the sub-selects that are likely to hurt ... in particular,-> Index Scan using post_top on post (cost=0.00..57266.37 rows=113 width=757) Index Cond: (toppost = 1) Filter: (((replied > (subplan)) IS NOT FALSE) AND ((replied > (subplan)) IS NOT FALSE))versusIndex Scan using post_top on post (cost=0.00..632.03 rows=1013 width=11) Index Cond: (toppost = 1)The planner thinks that the two subplan filter conditions will eliminate about 90% of the rows returned by the bare indexscan (IIRC this is purely a rule of thumb, not based on any statistics) and that testing them 1013 times will add over 50000 cost units to the basic indexscan. That part I believe --- correlated subqueries are expensive. regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 5: don't forget to increase your free space map settings %SPAMBLOCK-SYS: Matched [hub.org+], message ok
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