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From: Andrey Borodin <[email protected]>
To: Thomas Munro <[email protected]>
Cc: Gilles Darold <[email protected]>
Cc: Tomas Vondra <[email protected]>
Cc: Tomas Vondra <[email protected]>
Cc: Alexander Korotkov <[email protected]>
Cc: Anastasia Lubennikova <[email protected]>
Cc: Daniel Gustafsson <[email protected]>
Cc: Kyotaro Horiguchi <[email protected]>
Cc: pgsql-hackers <[email protected]>
Subject: Re: MultiXact\SLRU buffers configuration
Date: Fri, 26 Mar 2021 11:00:51 +0500
Message-ID: <[email protected]> (raw)
In-Reply-To: <CA+hUKGL0g+Nmf=2XePaXtw82BkW9CRzoxxXB4gqZOk7tHxNJXg@mail.gmail.com>
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Hi Thomas, Gilles, all!

Thanks for reviewing this!

> 25 марта 2021 г., в 02:31, Thomas Munro <[email protected]> написал(а):
> 
> I don't think we should put "slru" (the name of the buffer replacement
> algorithm, implementation detail) in the GUC names.
+1


> +        It defaults to 0, in this case CLOG size is taken as
> <varname>shared_buffers</varname> / 512.
> 
> We already know that increasing the number of CLOG buffers above the
> current number hurts as the linear search begins to dominate
Uh, my intent was to copy original approach of CLOG SLRU size, I just missed that Min(,) thing in shared_buffers logic.


> 26 марта 2021 г., в 08:46, Thomas Munro <[email protected]> написал(а):
> 
> Hi Andrey, all,
> 
> I propose some changes, and I'm attaching a new version:
> 
> I renamed the GUCs as clog_buffers etc (no "_slru_").  I fixed some
> copy/paste mistakes where the different GUCs were mixed up.  I made
> some changes to the .conf.sample.  I rewrote the documentation so that
> it states the correct unit and defaults, and refers to the
> subdirectories that are cached by these buffers instead of trying to
> give a new definition of each of the SLRUs.
> 
> Do you like those changes?
Yes!

> Some things I thought about but didn't change:
> 
> I'm not entirely sure if we should use the internal and historical
> names well known to hackers (CLOG), or the visible directory names (I
> mean, we could use pg_xact_buffers instead of clog_buffers).
While it is good idea to make notes about directory name, I think the real naming criteria is to help find this GUC when user encounters wait events in pg_stat_activity. I think there is no CLOG mentions in docs [0], only XactBuffer, XactSLRU et c.

> I'm not saying the 0002 patch is bug-free yet though, it's a bit finickity.
I think the idea of speeding up linear search is really really good for scaling SLRUs. It's not even about improving normal performance of the cluster, but it's important from preventing pathological degradation under certain circumstances. Bigger cache really saves SLAs :) I'll look into the patch more closely this weekend. Thank you!

Best regards, Andrey Borodin.

[0] https://www.postgresql.org/docs/current/monitoring-stats.html#MONITORING-PG-STAT-ACTIVITY-VIEW

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