Re: Reducing WaitEventSet syscall churn

Hello.

At Tue, 10 Mar 2020 08:19:24 +0900 (JST), Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com> wrote in
me> I'l continue reviewing in later mail.
me>
me> > 0005: "libpq: Add PQsocketChangeCount to advertise socket changes."
me> ....

At Thu, 27 Feb 2020 12:17:45 +1300, Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote
in
> On Sat, Feb 8, 2020 at 10:15 AM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
> 0005: "libpq: Add PQsocketChangeCount to advertise socket changes."
>
> To support a long lived WES, libpq needs a way tell us when the socket
> changes underneath our feet. This is the simplest thing I could think
> of; better ideas welcome.

I think at least windows is the reason for not just detecting the
change of the value of fd. Instead of counting disconnection, we
could use event libpq-event.

QregisterEventProc returns false for all of bad-parameter,
already-registered, out-of-memory and proc-rejection. I don't think it
is usable interface so the attached 0005 patch fixes that. (but I
found it not necessarily needed after making 0007, but I included it
as a proposal separate from this patch set. It's not including the
corresponding doc fix.).

> 0006: "Reuse a WaitEventSet in libpqwalreceiver.c."
>
> Rather than having all users of libpqwalreceiver.c deal with the
> complicated details of wait set management, have libpqwalreceiver
> expose a waiting interface that understands socket changes.

Looks reasonable. The attached 0006 and 0007 are a possible
replacement if we use libpq-event.

> Unfortunately, I couldn't figure out how to use CommonWaitSet for this
> (ie adding and removing sockets to that as required), due to
> complications with the bookkeeping required to provide the fd_closed
> flag to RemoveWaitEvent(). So it creates its own internal long lived
> WaitEventSet.

Agreed since they are used different way. But with the attached closed
connection is marked as wes_socket_position = -1.

> 0007: "Use a WaitEventSet for postgres_fdw."

Continues..

The attached are:
0001-0004 Not changed
0005 Fix interface of PQregisterEventProc
0006 Add new libpq event for this use.
0007 Another version of "0006 Reuse a WaitEventSet in
libpqwalreceiver.c" based on libpq event.
0008-0011 Not changed (old 0007-0010, blindly appended)

passed the regression (includeing TAP recovery test) up to here.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center