Re: [HACKERS] Moving relation extension locks out of heavyweight lock manager

On Thu, Apr 26, 2018 at 3:30 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Tue, Apr 10, 2018 at 9:08 PM, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>> Never mind. There was a lot of items especially at the last CommitFest.
>>
>>> In terms of moving forward, I'd still like to hear what
>>> Andres has to say about the comments I made on March 1st.
>>
>> Yeah, agreed.
>
> $ ping -n andres.freund
> Request timeout for icmp_seq 0
> Request timeout for icmp_seq 1
> Request timeout for icmp_seq 2
> Request timeout for icmp_seq 3
> Request timeout for icmp_seq 4
> ^C
> --- andres.freund ping statistics ---
> 6 packets transmitted, 0 packets received, 100.0% packet loss
>
> Meanwhile, https://www.postgresql.org/message-id/4c171ffe-e3ee-acc5-9066-a40d52bc5ae9@postgrespro.ru
> shows that this patch has some benefits for other cases, which is a
> point in favor IMHO.

Thank you for sharing. That's good to know.

Andres pointed out the performance degradation due to hash collision
when multiple loading. I think the point is that it happens at where
users don't know. Therefore even if we make N_RELEXTLOCK_ENTS
configurable parameter, since users don't know the hash collision they
don't know when they should tune it.

So it's just an idea but how about adding an SQL-callable function
that returns the estimated number of lock waiters of the given
relation? Since user knows how many processes are loading to the
relation, if a returned value by the function is greater than the
expected value user can know hash collision and will be able to start
to consider to increase N_RELEXTLOCK_ENTS.

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center