Date: Mon, 18 Nov 2019 16:10:33 +0100
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Ekaterina Amez <ekaterina.amez@zunibal.com>
Cc: pgsql-performance@lists.postgresql.org
Subject: Re: Out of memory error on automatic vacuum
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On Mon, Nov 18, 2019 at 03:46:03PM +0100, Ekaterina Amez wrote:
>
>El 18/11/19 a las 15:16, Tomas Vondra escribió:
>>
>>Not sure I understand. Whenever PostgreSQL process needs memory it
>>requests it from the kernel by calling malloc(), and the amount of
>>availabe RAM is limited.  So when kernel can't provide more memory,
>>it returns NULL.
>>
>Understood.
>
>>
>>If it finished a couple of minutes before, it's unlikely to be the
>>related. But hard to say, without knowing the details.
>
>Yeah, I thought the same but for me is too much coincidence and is 
>suspicious (or at least a thing to have in mind).
>
>
>>No, that's just one of the memory contexts (they form a tree), using
>>only 1kB of memory. What matters is the "grand total"
>>
>>Grand total: 1009356 bytes in 130 blocks; 436888 free (72 chunks);
>>572468 used
>>
>>which is ~1MB.
>>
>OK, in my lack of knowledge I was understanding "memory context" as 
>the whole message.
>
>
>>Another thing you might do is adding a swap (if you don't have one
>>already), as a safety.
>>
>>
>Excuse my ignorance but... swap? You mean some mechanism that prevents 
>server to be unavailable by having a second instance running but not 
>accesible and changing from one to the other when the main fails? 
>(It's the best way I find to describe it, as I don't usually 
>speak/write english).
>

swap = space on disk, so that OS can page out unused data from RAM when
there's memory pressure

It's a basic sysadmin knowledge, I think. Search for mkswap.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services