From: | Andres Freund <andres(at)anarazel(dot)de> |
---|---|
To: | Antonin Houska <ah(at)cybertec(dot)at> |
Cc: | Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>, magnus(at)hagander(dot)net, robertmhaas(at)gmail(dot)com, tgl(at)sss(dot)pgh(dot)pa(dot)us, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: shared-memory based stats collector |
Date: | 2018-09-20 17:37:24 |
Message-ID: | 20180920173724.5w2n2nwkxtyi4azw@alap3.anarazel.de |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
Lists: | pgsql-hackers |
Hi,
On 2018-09-20 09:55:27 +0200, Antonin Houska wrote:
> I've spent some time reviewing this version.
>
> Design
> ------
>
> 1. Even with your patch the stats collector still uses an UDP socket to
> receive data. Now that the shared memory API is there, shouldn't the
> messages be sent via shared memory queue? [1] That would increase the
> reliability of message delivery.
>
> I can actually imagine backends inserting data into the shared hash tables
> themselves, but that might make them wait if the same entries are accessed
> by another backend. It should be much cheaper just to insert message into
> the queue and let the collector process it. In future version the collector
> can launch parallel workers so that writes by backends do not get blocked
> due to full queue.
I don't think either of these is right. I think it's crucial to get rid
of the UDP socket, but I think using a shmem queue is the wrong
approach. Not just because postgres' shm_mq is single-reader/writer, but
also because it's plainly unnecessary. Backends should attempt to
update the shared hashtable, but acquire the necessary lock
conditionally, and leave the pending updates of the shared hashtable to
a later time if they cannot acquire the lock.
Greetings,
Andres Freund
From | Date | Subject | |
---|---|---|---|
Next Message | John Naylor | 2018-09-20 18:30:18 | Re: generating bootstrap entries for array types |
Previous Message | Stephen Frost | 2018-09-20 17:34:27 | Re: Query is over 2x slower with jit=on |