Date: Fri, 1 Mar 2019 11:35:58 -0800
From: Shawn Debnath <sdn@amazon.com>
To: Robert Haas <robertmhaas@gmail.com>
CC: Andres Freund <andres@anarazel.de>, Thomas Munro <thomas.munro@gmail.com>,
        Thomas Munro <thomas.munro@enterprisedb.com>,
        Dmitry Dolgov
	<9erthalion6@gmail.com>,
        PostgreSQL-development
	<pgsql-hackers@postgresql.org>
Subject: Re: Refactoring the checkpointer's fsync request queue
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On Fri, Mar 01, 2019 at 01:15:21PM -0500, Robert Haas wrote:
> > >
> > > +typedef enum syncrequestowner
> > > +{
> > > +       SYNC_MD = 0             /* md smgr */
> > > +} syncrequestowner;
> > >
> > > I have a feeling that Andres wanted to see a single enum combining
> > > both the "operation" and the "owner", like SYNC_REQ_CANCEL_MD,
> > > SYNC_REQ_CANCEL_UNDO, ... but I actually like it better the way you
> > > have it.
> >
> > Obviously it's nicer looking this way, but OTOH, that means we have to 
> > send more data over the queue, because we can't easily combine the
> > request + "owner". I don't have too strong feelings about it though.
> 
> Yeah, I would lean toward combining those.

I disagree, at least with combining and retaining enums. Encoding all 
the possible request types with the current, planned and future SMGRs 
would cause a sheer explosion in the number of enum  values. Not to 
mention that you have multiple enum values for the same behavior - which 
just isn't clean. And handling of these enums in the code would be ugly 
too.

Do note that these are typedef'ed to uint8 currently. For a default 
config with 128 MB shared_buffers, we will use an extra 16kB (one extra 
byte to represent the owner).  I am hesitant to change this right now 
unless folks feel strongly about it.

If so, I would combine the type and owner by splitting it up in 4 bit 
chunks, allowing for 16 request types and 16 smgrs. This change would 
only apply for the in-memory queue. The code and functions would 
continue using the enums.

-- 
Shawn Debnath
Amazon Web Services (AWS)