Re: pgsql: Generational memory allocator

On 27 November 2017 at 04:46, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Simon Riggs <simon@2ndquadrant.com> writes:
>> On 26 November 2017 at 08:46, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> 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.
>
> Well, I'm concerned about the possibility of a lot of palloc thrashing
> if the first bunch of records it reads happen to have steadily increasing
> sizes.  However, rather than doubling, it might be sufficient to set a
> robust minimum on the first allocation, ie use something along the lines
> of Max(1024, MAXALIGN(state->main_data_len)).

Agreed.

I was just researching what that number should be... and I was
thinking that we should use the maximum normal tuple size, which I
think is

TOAST_TUPLE_THRESHOLD +
SizeOfXLogRecord +
SizeOfXLogRecordDataHeaderLong

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