Thread: Suggestion for memory parameters

Hello All,

In a RDS postgres we are seeing some select queries when running and doing sorting on 50 million rows(as its having order by clause in it) , the significant portion of wait event is showing as "IO:BufFileWrite" and it runs for ~20minutes+.  

Going through the document in the link below, it states we should monitor the "FreeLocalStorage" metric and when monitoring that, I see it showing up to ~535GB as the max limit and when these queries run this goes down till 100GB. Note-  (it's a R7g8xl instance)

https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/apg-waits.iobuffile.html

We were thinking of bumping up the work_mem to a higher value in database level , which is currently having size 4MB default. But we will also have ~100 sessions running at same time and majority were from other applications which execute other single row "insert" queries and I hope that will not need high "work_mem" . And setting it at database level will consume 100 times that set work_mem value. So how to handle this situation?
 Or
 Is it fine to let it use "FreeLocalStorage" unless it goes till zero?

Also I am confused between the local storage (which is showing as 535GB) vs the memory/RAM which is 256GB for this instance class with ~128TB max storage space restriction, how are these storage different, (mainly the 535GB space which it's showing vs the 128TB storage space restriction)?  Appreciate your guidance.

select query looks something as below with no Joins but just single table fetch:-

Select....
from <table_name>
where
order by column1, column2 LIMIT $b1 OFFSET $B2 ;

On Thu, 26 Sept 2024 at 16:33, yudhi s <learnerdatabase99@gmail.com> wrote:

Hello All,

In a RDS postgres we are seeing some select queries when running and doing sorting on 50 million rows(as its having order by clause in it) , the significant portion of wait event is showing as "IO:BufFileWrite" and it runs for ~20minutes+.  

Going through the document in the link below, it states we should monitor the "FreeLocalStorage" metric and when monitoring that, I see it showing up to ~535GB as the max limit and when these queries run this goes down till 100GB. Note-  (it's a R7g8xl instance)

https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/apg-waits.iobuffile.html

We were thinking of bumping up the work_mem to a higher value in database level , which is currently having size 4MB default. But we will also have ~100 sessions running at same time and majority were from other applications which execute other single row "insert" queries and I hope that will not need high "work_mem" . And setting it at database level will consume 100 times that set work_mem value. So how to handle this situation?
 Or
 Is it fine to let it use "FreeLocalStorage" unless it goes till zero?

Also I am confused between the local storage (which is showing as 535GB) vs the memory/RAM which is 256GB for this instance class with ~128TB max storage space restriction, how are these storage different, (mainly the 535GB space which it's showing vs the 128TB storage space restriction)?  Appreciate your guidance.

select query looks something as below with no Joins but just single table fetch:-

Select....
from <table_name>
where
order by column1, column2 LIMIT $b1 OFFSET $B2 ;

My 2 cents 

I think you should set the work_mem on specific session level , if your sorting queries are only from specific handful of sessions, as because setting it up at database level will eat up your most of RAM(which you said is 256GB) and you said 100+ sessions getting spawned at any point in time.

> On Sep 26, 2024, at 7:03 AM, yudhi s <learnerdatabase99@gmail.com> wrote:
>
> In a RDS postgres ...

>  Is it fine to let it use "FreeLocalStorage" unless it goes till zero?

Hi Yudhi,
FreeLocalStorage and some of the other things you ask about are specific to AWS RDS, so you might have better luck getting answers on an RDS-specific mailing list. We also use RDS-hosted Postgres and so I completely understand how Postgres and RDS are intertwined.

We have had runaway queries exhaust FreeLocalStorage. It has been quite a while since that happened, so my memories are hazy, but I’m pretty sure that when we used all of FreeLocalStorage, the result was that Postgres restarted. It might be equivalent to using all memory and disk space on a standalone system. Once there’s no storage left, behavior is unpredictable but we can’t be surprised if things crash. Usually our runaway queries got killed before FreeLocalStorage filled up, but not always.

I second Veem’s suggestion to set work_mem on a per-session basis. Also note that the doc for work_mem says, “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."

https://www.postgresql.org/docs/current/runtime-config-resource.html#GUC-WORK-MEM

When I execute the query with explain (analyze, buffers),I see the section below in the plan having "sort method" information in three places each showing ~75MB size, which if combined is coming <250MB. So , does that mean it's enough to set the work_mem as ~250MB for these queries before they start?

 But yes somehow this query is finished in a few seconds when i execute using explain(analyze,buffers) while if i run it without using explain it runs for ~10minutes+. My expectation was that doing (explain analyze) should actually execute the query fully. Is my understanding correct here and if the disk spilling stats which I am seeing is accurate enough to go with?