Date: Thu, 14 Jan 2016 18:55:51 +0100
From: Andres Freund <andres@anarazel.de>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>,
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 <d.vasilyev@postgrespro.ru>,
 "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: Performance degradation in commit ac1d794
Message-ID: <20160114175551.GM10941@awork2.anarazel.de>
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On 2016-01-14 18:14:21 +0100, Andres Freund wrote:
> On 2016-01-14 12:07:23 -0500, Robert Haas wrote:
> > > Do we want to provide a backward compatible API for all this? I'm fine
> > > either way.
> >
> > How would that work?
> 
> I'm thinking of something like;
> 
> int WaitOnLatchSet(LatchEventSet *set, int wakeEvents, long timeout);
> 
> int
> WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,long timeout)
> {
>         LatchEventSet set;
> 
>         LatchEventSetInit(&set, latch);
> 
>         if (sock != PGINVALID_SOCKET)
>            LatchEventSetAddSock(&set, sock);
> 
>         return WaitOnLatchSet(set, wakeEvents, timeout);
> }
> 
> I think we'll need to continue having wakeEvents and timeout parameters
> for WaitOnLatchSet, we quite frequently want to wait socket
> readability/writability, not wait on the socket, or have/not have
> timeouts.

Hm. If we really want to support multiple sockets at some point the
above WaitOnLatchSet signature isn't going to fly, because it won't
support figuring out which fd the event this triggered on.

So it seems we'd need to return something like
struct LatchEvent
{
        enum LatchEventType {WL_LATCH_EVENT;WL_TIMEOUT;WL_SOCKET_EVENT; WL_POSTMASTER_EVENT;...} event_type;
        int mask;
        pgsocket event_sock;
};

that'd also allow to extend this to return multiple events if we want
that at some point. Alternatively we could add a pgsocket* argument, but
that doesn't really seem much better.

Not super happy about the above proposal.

Andres


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