Re: shared-memory based stats collector

From: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
To: andres(at)anarazel(dot)de
Cc: ah(at)cybertec(dot)at, 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: 2018-09-26 00:55:09
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Hello. Thank you for the comments.

At Thu, 20 Sep 2018 10:37:24 -0700, Andres Freund <andres(at)anarazel(dot)de> wrote in <20180920173724(dot)5w2n2nwkxtyi4azw(at)alap3(dot)anarazel(dot)de>
> 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.

Ok, I just intended to avoid reading many bytes from a file and
thought that writer-side can be resolved later.

Currently locks on the shared stats table is acquired by dshash
mechanism in a partition-wise manner. The number of the
partitions is currently fixed to 2^7 = 128, but writes for the
same table confilicts each other regardless of the number of
partitions. As the first step, I'm going to add
conditional-locking capability to dsahsh_find_or_insert and each
backend holds a queue of its pending updates.


Kyotaro Horiguchi
NTT Open Source Software Center

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