Re: BufferAlloc: don't take two simultaneous locks

At Fri, 11 Mar 2022 15:30:30 +0900 (JST), Kyotaro Horiguchi <horikyota.ntt@gmail.com> wrote in 
> 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.
> 
> -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.

While I looked buf_table part, I came up with additional comments.

BufTableInsert(BufferTag *tagPtr, uint32 hashcode, int buf_id)
{
        hash_search_with_hash_value(SharedBufHash,
                                    HASH_ASSIGN,
...
BufTableDelete(BufferTag *tagPtr, uint32 hashcode, bool reuse)

BufTableDelete considers both reuse and !reuse cases but
BufTableInsert doesn't and always does HASH_ASSIGN.  That looks
odd. We should use HASH_ENTER here.  Thus I think it is more
reasonable that HASH_ENTRY uses the stashed entry if exists and
needed, or returns it to freelist if exists but not needed.

What do you think about this?

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center