Jim C. Nasby wrote: > On Mon, Apr 10, 2006 at 12:02:56PM -0700, Luke Lonergan wrote: > > Hannu, > > > > On 4/10/06 2:23 AM, "Hannu Krosing" <[EMAIL PROTECTED]> wrote: > > > > >> The cost of fetching a page from the OS is not really much of an > > >> overhead, > > > > > > Have you tested this ? > > > > I have - the overhead of fetching a page from Linux I/O cache to buffer > > cache is about an additional 20% over fetching it directly from buffer cache > > on PG 7.4. > > Is there any pratcical way to tell the difference between a page comming > from the OS cache and one comming from disk? Or maybe for a set of pages > an estimate on how many came from cache vs disk? There's some areas > where having this information would be very useful, such as for vacuum > delay. It would make tuning much easier, and it would also give us some > insight on how heavily loaded disks were, which would also be useful > info for vacuum to have (so we could adjust vacuum_cost_delay > dynamically based on load).
getrusage() returns: ! 0.000062 elapsed 0.000000 user 0.000062 system sec ! [0.000000 user 0.009859 sys total] ! 0/0 [19/2] filesystem blocks in/out ! 0/0 [0/0] page faults/reclaims, 0 [0] swaps ! 0 [0] signals rcvd, 0/0 [4/5] messages rcvd/sent ! 0/0 [23/6] voluntary/involuntary context switches but I don't see anything in there that would show kernel cache vs. disk I/O. In fact, there is usually little connection in the kernel between an I/O request and the process that requests it. -- Bruce Momjian http://candle.pha.pa.us EnterpriseDB http://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---------------------------(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