Re: Performance implications of 8K pread()s

On Wed, Jul 12, 2023 at 1:11 AM Dimitrios Apostolou <jimis@gmx.net> wrote:
> So would it make sense for postgres to perform reads in bigger blocks? Is it
> easy-ish to implement (where would one look for that)? Or must the I/O unit be
> tied to postgres' page size?

FYI as of last week we can do a little bit of that on the master branch:

postgres=# select count(*) from t;

preadv(46, ..., 8, 256237568) = 131072
preadv(46, ..., 5, 256368640) = 131072
preadv(46, ..., 8, 256499712) = 131072
preadv(46, ..., 5, 256630784) = 131072

postgres=# set io_combine_limit = '256k';
postgres=# select count(*) from t;

preadv(47, ..., 5, 613728256) = 262144
preadv(47, ..., 5, 613990400) = 262144
preadv(47, ..., 5, 614252544) = 262144
preadv(47, ..., 5, 614514688) = 262144

Here's hoping the commits implementing this stick, for the PostgreSQL
17 release.  It's just the beginning though, we can only do this for
full table scans so far (plus a couple of other obscure places).
Hopefully in the coming year we'll get the "streaming I/O" mechanism
that powers this hooked up to lots more places... index scans and
other stuff.  And writing.  Then eventually pushing the I/O into the
background.  Your questions actually triggered us to talk about why we
couldn't switch a few things around in our project and get the I/O
combining piece done sooner.  Thanks!