В Ср, 07/09/2022 в 16:13 +1200, David Rowley пишет: > On Tue, 6 Sept 2022 at 01:41, David Rowley <dgrowle...@gmail.com> wrote: > > > > On Fri, 2 Sept 2022 at 20:11, David Rowley <dgrowle...@gmail.com> wrote: > > > > > > On Thu, 1 Sept 2022 at 12:46, Tom Lane <t...@sss.pgh.pa.us> wrote: > > > > > > > > David Rowley <dgrowle...@gmail.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
