Date: Thu, 3 Mar 2016 11:20:43 +0300
From: Aleksander Alekseev <a.alekseev@postgrespro.ru>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>, pgsql-hackers
 <pgsql-hackers@postgresql.org>, Anastasia Lubennikova
 <a.lubennikova@postgrespro.ru>
Subject: Re: Patch: fix lock contention for HASHHDR.mutex
Message-ID: <20160303112043.7d53782e@fujitsu>
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> Won't it always use the same freelist to remove and add the entry from
> freelist as for both cases it will calculate the freelist_idx in same
> way?

No. If "our" freelist is empty when we try to remove an item from it we
borrow item from another freelist. Then this borrowed item will be
returned to "our" freelist instead of original. Without some sort of
additional logic there is no way to figure out what freelist was
original.

Generally speaking negative nentries value is not something that
couldn't be solved. But I would like to remind that in this context we
are discussing a quick and dirty solution created for benchmark
purposes in a first place.

> You are not convinced, then lets leave it to committer unless
> somebody else wants to try that suggestion.

Agree. Frankly I'm tired of rewriting this patch over and over and over
again. So I would like to avoid rewriting it once again unless there is
a clear indication that this way we would gain something. Benchmarks
shows that this is not a case thus it's only a matter of taste and
intuition. We know from experience that both are bad advisers in
optimization matter.

I suggest we merge this patch already:

http://www.postgresql.org/message-id/CA+Tgmobtf9nH566_jjs=jrTyMq5HdQdaRF5j7o+AbdOWQHE_Ow@mail.gmail.com

... and get done with it. Clearly this patch is good enough ---
reviewed, tested, discussed, has proven performance extremum. When and
if dynahash will become a bottleneck again nothing will stop us from
optimizing it one more time for hardware we will have by then. Who knows
what this hardware will be like in 5-10 years? Don't we have better
things to do now than arguing about imaginary benchmarks and taste?

Lets see what is committer's opinion on this.


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