Re: Dangerous hint in the PostgreSQL manual

Zitat von Andrew Sullivan <ajs(at)crankycanuck(dot)ca>:

> On Tue, Dec 11, 2007 at 03:08:36PM +0100, Listaccount wrote:
>> I would have not been surprised if the OOM-Killer would go around in
>> case of short memory but i was surprised to see fork failed with a
>> system having 1GB Memory available.
>
> You don't understand: the system _did not_ have 1G of memory available. It
> was all committed to applications that had asked for it. Just because they
> asked for it even though they were never going to use it doesn't mean that
> it isn't gone. It's used, as far as the kernel is concerned. The
> overcommit trick some OSes have implemented is a filthy hack to get around
> poor memory allocation discipline in applications.

For sure i understand the problem. The key is how you define
"available". But i agree with you that overcommit obfuscate careless
application design.

> The point of the PostgreSQL documentation is to tell you how best to run
> Postgres, safely and reliably. The only safe and reliable way to run on
> Linux is not to use overcommit. Turning it off ensures that the system
> can't run out of memory in this way.

Yes, but the documentation should at least warn if some setting
*could* lead to trouble you would not have otherwise.

> What I _would_ support in the docs is the following addition in 17.4.3,
> where this is discussed:
>
> . . .it will lower the chances significantly and will therefore
> lead to more robust system behavior. It may also cause fork() to fail
> when the machine appears to have available memory. This is done by
> selecting. . .
>
> Or something like that. This would warn potential users that they really do
> need to read their kernel docs.

On this one we can agree. Maybe we should mention the root-cause.

"It may also cause fork() to fail when the machine appears to have
available memory because of other applications doing careless memory
allocation"

Would be nice to save others from learning about this the hard way.

Regards

Andreas