odd postgresql performance (excessive lseek)
| От | Jon Nelson |
|---|---|
| Тема | odd postgresql performance (excessive lseek) |
| Дата | |
| Msg-id | AANLkTikS8ptnCVJK+6u-Z3Xh8WMKdgq+T+trdxt3xgYB@mail.gmail.com обсуждение исходный текст |
| Ответы |
Re: odd postgresql performance (excessive lseek)
|
| Список | pgsql-performance |
postgres 8.4.4 on openSUSE 11.3 (2.6.36rc7, x86_64). I was watching a fairly large query run and observed that the disk light went out. I checked 'top' and postgres was using 100% CPU so I strace'd the running process. This is what I saw: lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(87, 0, SEEK_END) = 585531392 lseek(94, 270680064, SEEK_SET) = 270680064 read(94, "<elided>"..., 8192) = 8192 and I observed that pattern quite a bit. I know lseek is cheap, but a superfluous systemcall is a superfluous systemcall, and over a short period amounted to 37% (according to strace) of the time spent in the system. What's with the excess calls to lseek? The query plan was a nested loop anti-join (on purpose). -- Jon
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