Re: pgsql: Generational memory allocator

On 26 November 2017 at 08:46, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> I wrote:
>> Instead I propose that we should make sure that the palloc request size
>> for XLogReaderState->main_data is always maxalign'd. The existing
>> behavior in DecodeXLogRecord of palloc'ing it only just barely big
>> enough for the current record seems pretty brain-dead performance-wise
>> even without this consideration. Generally, if we need to enlarge
>> that buffer, we should enlarge it significantly, IMO.
>
> I've confirmed that the attached is sufficient to stop the valgrind crash
> on my machine. But as I said, I think we should be more aggressive at
> resizing the buffer, to reduce resize cycles. I'm inclined to start out
> with a buffer size of 128 or 256 or so bytes and double it when needed.
> Anybody have a feeling for a typical size for the "main data" part
> of a WAL record?

We reuse the buffer and only pfree/palloc when we need to enlarge the
buffer, so not sure we need to do the doubling thing and it probably
doesn't matter what the typical size is.

So I think we're just good to go with your patch.

--
Simon Riggs http://www.2ndQuadrant.com/
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