Re: Performance monitor signal handler

* Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> [010316 10:06] wrote:
> Jan Wieck <JanWieck(at)Yahoo(dot)com> writes:
> > Uh - not much time to spend if the statistics should at least
> > be half accurate. And it would become worse in SMP systems.
> > So that was a nifty idea, but I think it'd cause much more
> > statistic losses than I assumed at first.
>
> > Back to drawing board. Maybe a SYS-V message queue can serve?
>
> That would be the same as a pipe: backends would block if the collector
> stopped accepting data. I do like the "auto discard" aspect of this
> UDP-socket approach.
>
> I think Philip had the right idea: each backend should send totals,
> not deltas, in its messages. Then, it doesn't matter (much) if the
> collector loses some messages --- that just means that sometimes it
> has a slightly out-of-date idea about how much work some backends have
> done. It should be easy to design the software so that that just makes
> a small, transient error in the currently displayed statistics.

MSGSND(3) FreeBSD Library Functions Manual MSGSND(3)

ERRORS
msgsnd() will fail if:

[EAGAIN] There was no space for this message either on the
queue, or in the whole system, and IPC_NOWAIT was set
in msgflg.

--
-Alfred Perlstein - [bright(at)wintelcom(dot)net|alfred(at)freebsd(dot)org]