Thread: Out of memory error handling in frontend code

Hello,

One of our customers recently complained that his pg_dump stopped 
abruptly with the message "out of memory".

After some time, we understood that the 20 million of large objects were 
responsible for the huge memory usage (more than 10 GB) by pg_dump.

I think a more useful error message would help for such cases. Indeed, 
it's not always possible to ask the client to run pg_dump with 
"valgrind --tool=massif" on its server.

Now, I understand that we don't want to add too much to the frontend 
code, it would be a lot of pain for not much gain.

But I wonder if we could add some checks in a few strategic places, as 
in the attached patch.

The idea would be to print a more useful error message in most of the 
cases, and keep the basic "out of memory" for the remaining ones.

I haven't try to get the patch ready for review, I know that the format 
of the messages isn't right, I'd like to know what do you think of the 
idea, first.

Best regards,
Frédéric

> On 28 Sep 2023, at 10:14, Frédéric Yhuel <frederic.yhuel@dalibo.com> wrote:

> After some time, we understood that the 20 million of large objects were responsible for the huge memory usage (more
than10 GB) by pg_dump. 

This sounds like a known issue [0] which has been reported several times, and
one we should get around to fixing sometime.

> I think a more useful error message would help for such cases.

Knowing that this is case that pops up, I agree that we could do better around
the messaging here.

> I haven't try to get the patch ready for review, I know that the format of the messages isn't right, I'd like to know
whatdo you think of the idea, first. 

I don't think adding more details is a bad idea, but it shouldn't require any
knowledge about internals so I think messages like the one below needs to be
reworded to be more helpful.

+    if (loinfo == NULL)
+    {
+        pg_fatal("getLOs: out of memory");
+    }

--
Daniel Gustafsson

[0] https://www.postgresql.org/message-id/7da8823d83a2b66bdd917aa6cb2c5c2619d86011.camel@credativ.de

Hi Daniel,

Thank you for your answer.

On 9/28/23 14:02, Daniel Gustafsson wrote:
>> On 28 Sep 2023, at 10:14, Frédéric Yhuel <frederic.yhuel@dalibo.com> wrote:
> 
>> After some time, we understood that the 20 million of large objects were responsible for the huge memory usage (more
than10 GB) by pg_dump.
 
> 
> This sounds like a known issue [0] which has been reported several times, and
> one we should get around to fixing sometime.
>

Indeed, I saw some of these reports afterwards :)

>> I think a more useful error message would help for such cases.
> 
> Knowing that this is case that pops up, I agree that we could do better around
> the messaging here.
> 
>> I haven't try to get the patch ready for review, I know that the format of the messages isn't right, I'd like to
knowwhat do you think of the idea, first.
 
> 
> I don't think adding more details is a bad idea, but it shouldn't require any
> knowledge about internals so I think messages like the one below needs to be
> reworded to be more helpful.
> 
> +    if (loinfo == NULL)
> +    {
> +        pg_fatal("getLOs: out of memory");
> +    }
> 

OK, here is a second version of the patch.

I didn't try to fix the path getLOs -> AssignDumpId -> catalogid_insert 
-> [...] -> catalogid_allocate, but that's annoying because it amounts 
to 11% of the memory allocations from valgrind's output.

Best regards,
Frédéric