Re: Reducing the chunk header sizes on all memory context types

В Ср, 07/09/2022 в 16:13 +1200, David Rowley пишет:
> On Tue, 6 Sept 2022 at 01:41, David Rowley <dgrowleyml(at)gmail(dot)com> wrote:
> >
> > On Fri, 2 Sept 2022 at 20:11, David Rowley <dgrowleyml(at)gmail(dot)com> wrote:
> > >
> > > On Thu, 1 Sept 2022 at 12:46, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > > >
> > > > David Rowley <dgrowleyml(at)gmail(dot)com> writes:
> > > > > Maybe we should just consider always making room for a sentinel for
> > > > > chunks that are on dedicated blocks. At most that's an extra 8 bytes
> > > > > in some allocation that's either over 1024 or 8192 (depending on
> > > > > maxBlockSize).
> > > >
> > > > Agreed, if we're not doing that already then we should.
> > >
> > > Here's a patch to that effect.
> >
> > If there are no objections, then I plan to push that patch soon.
>
> I've now pushed the patch which adds the sentinel space in more cases.
>
> The final analysis I did on the stats gathered during make
> installcheck show that we'll now allocate about 19MBs more over the
> entire installcheck run out of about 26GBs total allocations.
>
> That analysis looks something like:
>
> Before:
>
> SELECT CASE
>          WHEN pow2_size > 0
>               AND pow2_size = size THEN 'No'
>          WHEN pow2_size = 0
>               AND size = maxalign_size THEN 'No'
>          ELSE 'Yes'
>        END                    AS has_sentinel,
>        Count(*)               AS n_allocations,
>        Sum(CASE
>              WHEN pow2_size > 0 THEN pow2_size
>              ELSE maxalign_size
>            END) / 1024 / 1024 mega_bytes_alloc
> FROM   memstats
> GROUP  BY 1;
> has_sentinel | n_allocations | mega_bytes_alloc
> --------------+---------------+------------------
>  No           |      26445855 |            21556
>  Yes          |      37602052 |             5044
>
> After:
>
> SELECT CASE
>          WHEN pow2_size > 0
>               AND pow2_size = size THEN 'No'
>          WHEN pow2_size = 0
>               AND size = maxalign_size THEN 'Yes' -- this part changed
>          ELSE 'Yes'
>        END                    AS has_sentinel,
>        Count(*)               AS n_allocations,
>        Sum(CASE
>              WHEN pow2_size > 0 THEN pow2_size
>              WHEN size = maxalign_size THEN maxalign_size + 8
>              ELSE maxalign_size
>            END) / 1024 / 1024 mega_bytes_alloc
> FROM   memstats
> GROUP  BY 1;
> has_sentinel | n_allocations | mega_bytes_alloc
> --------------+---------------+------------------
>  No           |      23980527 |             2177
>  Yes          |      40067380 |            24442
>
> That amounts to previously having about 58.7% of allocations having a
> sentinel up to 62.6% currently, during the installcheck run.
>
> It seems a pretty large portion of allocation request sizes are
> power-of-2 sized and use AllocSet.

19MB over 26GB is almost nothing. If it is only for enable-casserts
builds, then it is perfectly acceptable.

regards
Yura