Re: BufferAlloc: don't take two simultaneous locks

В Пт, 11/03/2022 в 15:30 +0900, Kyotaro Horiguchi пишет:
> At Thu, 03 Mar 2022 01:35:57 +0300, Yura Sokolov <y(dot)sokolov(at)postgrespro(dot)ru> wrote in
> > В Вт, 01/03/2022 в 10:24 +0300, Yura Sokolov пишет:
> > > Ok, here is v4.
> >
> > And here is v5.
> >
> > First, there was compilation error in Assert in dynahash.c .
> > Excuse me for not checking before sending previous version.
> >
> > Second, I add third commit that reduces HASHHDR allocation
> > size for non-partitioned dynahash:
> > - moved freeList to last position
> > - alloc and memset offset(HASHHDR, freeList[1]) for
> > non-partitioned hash tables.
> > I didn't benchmarked it, but I will be surprised if it
> > matters much in performance sence.
> >
> > Third, I put all three commits into single file to not
> > confuse commitfest application.
>
> Thanks! I looked into dynahash part.
>
> struct HASHHDR
> {
> - /*
> - * The freelist can become a point of contention in high-concurrency hash
>
> Why did you move around the freeList?
>
>
> - long nentries; /* number of entries in associated buckets */
> + long nfree; /* number of free entries in the list */
> + long nalloced; /* number of entries initially allocated for
>
> Why do we need nfree? HASH_ASSING should do the same thing with
> HASH_REMOVE. Maybe the reason is the code tries to put the detached
> bucket to different free list, but we can just remember the
> freelist_idx for the detached bucket as we do for hashp. I think that
> should largely reduce the footprint of this patch.

If we keep nentries, then we need to fix nentries in both old
"freeList" partition and new one. It is two freeList[partition]->mutex
lock+unlock pairs.

But count of free elements doesn't change, so if we change nentries
to nfree, then no need to fix freeList[partition]->nfree counters,
no need to lock+unlock.

>
> -static void hdefault(HTAB *hashp);
> +static void hdefault(HTAB *hashp, bool partitioned);
>
> That optimization may work even a bit, but it is not irrelevant to
> this patch?
>
> + case HASH_REUSE:
> + if (currBucket != NULL)
> + {
> + /* check there is no unfinished HASH_REUSE+HASH_ASSIGN pair */
> + Assert(DynaHashReuse.hashp == NULL);
> + Assert(DynaHashReuse.element == NULL);
>
> I think all cases in the switch(action) other than HASH_ASSIGN needs
> this assertion and no need for checking both, maybe only for element
> would be enough.

Agree.