Re: pgsql: Generational memory allocator

On 27 November 2017 at 05:53, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Simon Riggs <simon@2ndquadrant.com> writes:
>> On 27 November 2017 at 04:46, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> 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
>
> Well, let's not overthink this, because anything under 8K is going to
> be rounded up to the next power of 2 anyway by aset.c.  Based on this
> point I'd say that BLCKSZ/2 or BLCKSZ/4 would be reasonable candidates
> for the minimum.

BLCKSZ/2 seems best then.

I guess that means palloc doesn't work well with BLCKSZ > 8192

-- 
Simon Riggs                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services