Re: [HACKERS] A Better External Sort?

Ron,

> That 11MBps was your =bulk load= speed.  If just loading a table
> is this slow, then there are issues with basic physical IO, not just
> IO during sort operations.

Oh, yeah.  Well, that's separate from sort.  See multiple posts on this
list from the GreenPlum team, the COPY patch for 8.1, etc.  We've been
concerned about I/O for a while.

Realistically, you can't do better than about 25MB/s on a single-threaded
I/O on current Linux machines, because your bottleneck isn't the actual
disk I/O.   It's CPU.   Databases which "go faster" than this are all, to
my knowledge, using multi-threaded disk I/O.

(and I'd be thrilled to get a consistent 25mb/s on PostgreSQL, but that's
another thread ... )

> As I said, the obvious candidates are inefficient physical layout
> and/or flawed IO code.

Yeah, that's what I thought too.   But try sorting an 10GB table, and
you'll see: disk I/O is practically idle, while CPU averages 90%+.   We're
CPU-bound, because sort is being really inefficient about something. I
just don't know what yet.

If we move that CPU-binding to a higher level of performance, then we can
start looking at things like async I/O, O_Direct, pre-allocation etc. that
will give us incremental improvements.   But what we need now is a 5-10x
improvement and that's somewhere in the algorithms or the code.

--
--Josh

Josh Berkus
Aglio Database Solutions
San Francisco