Re: checkpointer continuous flushing

From: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
To: Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>, Andres Freund <andres(at)anarazel(dot)de>
Cc: PostgreSQL Developers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: checkpointer continuous flushing
Date: 2015-08-08 17:49:03
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On 07/26/2015 06:01 PM, Fabien COELHO wrote:
> Attached is very minor v5 update which does a rebase & completes the
> cleanup of doing a full sort instead of a chuncked sort.

Some thoughts on this:

* I think we should drop the "flush" part of this for now. It's not as
clearly beneficial as the sorting part, and adds a great deal more code
complexity. And it's orthogonal to the sorting patch, so we can deal
with it separately.

* Is it really necessary to parallelize the I/O among tablespaces? I can
see the point, but I wonder if it makes any difference in practice.

* Is there ever any harm in sorting the buffers? The GUC is useful for
benchmarking, but could we leave it out of the final patch?

* Do we need to worry about exceeding the 1 GB allocation limit in
AllocateCheckpointBufferIds? It's enough got 2 TB of shared_buffers.
That's a lot, but it's not totally crazy these days that someone might
do that. At the very least, we need to lower the maximum of
shared_buffers so that you can't hit that limit.

I ripped out the "flushing" part, keeping only the sorting. I refactored
the logic in BufferSync() a bit. There's now a separate function,
nextCheckpointBuffer(), that returns the next buffer ID from the sorted
list. The tablespace-parallelization behaviour in encapsulated there,
keeping the code in BufferSync() much simpler. See attached. Needs some
minor cleanup and commenting still before committing, and I haven't done
any testing besides a simple "make check".

- Heikki

Attachment Content-Type Size
checkpoint-sort-heikki-1.patch application/x-patch 14.2 KB

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