Re: work_mem

["Campbell, Lance" <lance@xxxxxxxxxxxx>]

It feels like there needs to be work_mem and work_mem_stack_size.  When work memory is needed a process “pops” a token off of a stack.  When it is done processing it “puts” the token back on the stack.  If the stack is empty then don’t allocate memory just write to disk for work_mem. 

 

This does two key things:

1) It allows for a real world understanding of how much memory is really needed on a day to day basis.  You can track how often a stack is empty.  You can also look at the number of temp files to see when work exceeds the work_mem allocation.  There is no “art” to setting these values.  You can use logical analysis to make choices.

 

2) This also prevents out of memory issues.  You are protecting yourself from extreme loads.

 

Lance

 

From: MichaelDBA <MichaelDBA@xxxxxxxxxxx>
Date: Friday, April 2, 2021 at 7:50 AM
To: SASIKUMAR Devaraj <sashikumard@xxxxxxxxx>
Cc: holger@xxxxxxxxxx <holger@xxxxxxxxxx>, pgsql-admin@xxxxxxxxxxxxxxxxxxxx <pgsql-admin@xxxxxxxxxxxxxxxxxxxx>
Subject: Re: work_mem

That is a common misconception.  It is not one work_mem buffer per SQL, but one work_mem buffer per required operation within that SQL.  So you can have manner work_mem buffers per SQL statement!

Right from the official docs:
work_mem sets the maximum amount of memory to be used by a query operation (such as a sort or hash table) before writing to temporary disk files. If this value is specified without units, it is taken as kilobytes... Note that for a complex query, several sort or hash operations might be running in parallel; each operation will be allowed to use as much memory as this value specifies before it starts to write data into temporary files. Also, several running sessions could be doing such operations concurrently. Therefore, the total memory used could be many times the value of work_mem; it is necessary to keep this fact in mind when choosing the value. Sort operations are used for ORDER BY, DISTINCT, and merge joins. Hash tables are used in hash joins, hash-based aggregation, and hash-based processing of IN subqueries.

Regards,
Michael Vitale

SASIKUMAR Devaraj wrote on 4/2/2021 8:45 AM:

Thanks Michael 

 

For example work_mem is 4M and if I had 300 connections connected to dB the total memory requirement is 1.2 Gb.

 So as per my understanding this 1.2 gb is not allocated as soon as 300 connections established, but it may vary from 0 to 1.2gb as per operations from client. Please confirm 

 

Regards

 

Sasi

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On Fri, Apr 2, 2021 at 6:09 PM, MichaelDBA

<MichaelDBA@xxxxxxxxxxx> wrote:

Memory is allocated dynamically per internal work_mem buffer requests.

SASIKUMAR Devaraj wrote on 4/2/2021 8:37 AM:

Hi All

 

As soon as client session is established work_mem will be allocated or only when sort happens for that particular session? Please advise how the internal behavior? This will help me to configure my database memory with high connections

 

Regards

 

Sasi

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On Wed, Mar 31, 2021 at 8:52 PM, Holger Jakobs

<holger@xxxxxxxxxx> wrote:

You can show any or all settings with the command SHOW.

show work_mem;
show all;

How many times the amount of work_mem has been allocated currently cannot be shown this way. Any sort operation running currently allocates it. So it is not identical with the number of current queries, because queries can allocate it several times.

Am 31. März 2021 16:04:07 MESZ schrieb "Campbell, Lance" <lance@xxxxxxxxxxxx>:

PostgreSQL 12

 

Is there a query that will tell us at any given time what amount of PostgreSQL memory is being used for work_mem?  

 

Thanks,

 

LANCE CAMPBELL

 

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