Hackers,

I've been searching the archives, but I can't find the thread from last month 
where we discussed the problem with Linux memory overcommits in kernel 2.4.x.

Can someone point me to the right thread?   I think maybe the subject line was 
something deceptive ....

-- 
-Josh Berkus
 Aglio Database Solutions
 San Francisco


Wed, 20 Aug 2003 11:02:37 -0700
Josh Berkus <josh@agliodbs.com>
Can't find thread on Linux memory overcommit

http://archives.postgresql.org/pgsql-hackers/2003-07/msg00608.php

Subject is "reprise on Linux overcommit handling"  - is that too 
deceptive? :-)

andrew

Josh Berkus wrote:

>Hackers,
>
>I've been searching the archives, but I can't find the thread from last month 
>where we discussed the problem with Linux memory overcommits in kernel 2.4.x.
>
>Can someone point me to the right thread?   I think maybe the subject line was 
>something deceptive ....
>
>  
>



Wed, 20 Aug 2003 14:27:43 -0400
Andrew Dunstan <andrew@dunslane.net>
Re: Can't find thread on Linux memory overcommit
On 8/20/2003 1:02 PM, Josh Berkus wrote:

>Hackers,
>
>I've been searching the archives, but I can't find the thread from last month 
>where we discussed the problem with Linux memory overcommits in kernel 2.4.x.
>
>Can someone point me to the right thread?   I think maybe the subject line was 
>something deceptive ....
>
>  
>
Re: [HACKERS] Pre-allocation of shared memory ...
On 6/11/2003


Wed, 20 Aug 2003 13:35:35 -0500
Thomas Swan <tswan@idigx.com>
Re: Can't find thread on Linux memory overcommit
On Wednesday 20 August 2003 23:57, Andrew Dunstan wrote:
> http://archives.postgresql.org/pgsql-hackers/2003-07/msg00608.php
>
> Subject is "reprise on Linux overcommit handling"  - is that too
> deceptive? :-)

I did little searching on this and found..

http://www.ussg.iu.edu/hypermail/linux/kernel/0306.3/1647.html

Can anybody comment on how much diference 2.6 would make to this situation of 
OOM kiiler "feature"?

If there could be any cross-OS comparisons, that would be great as well. A 
summary would really help.

 Shridhar



Thu, 21 Aug 2003 13:09:25 +0530
Shridhar Daithankar <shridhar_daithankar@persistent.co.in>
Re: Can't find thread on Linux memory overcommit
I see btw that no change has been made to the docs. That's bad IMNSHO. 
The situation with RH is unchanged with today's kernel errata patch, 
too. I propose to submit a doc patch with the following wording, unless 
someone objects or improves it:


-----------------------

Linux kernel version 2.4.* has poor default memory overcommit behavior, 
which can result in the postmaster being killed by the kernel due to 
memory demands by another process if the system runs out of memory. To 
avoid this situation, run postgres on a machine where you can be sure 
that other processes will not run the machine out of memory. If your 
kernel supports strict and/or parnoid modes of overcommit handling, you 
can also relieve this problem by altering the system's default 
behaviour. This can be determined by examining the function 
vm_enough_memory in the file mm/mmap.c in the kernel source. If this 
file reveals that strict and/or paranoid modes are supported by your 
kernel, turn one of these modes on by using

     sysctl -w vm.overcommit_memory=2

for strict mode or

    sysctl -w vm.overcommit_memory=3

for paranoid mode.

Warning: using these settings in a kernel which does not support these 
modes will almost certainly increase the danger of the kernel killing 
the postmaster, rather than reducing it. If in any doubt, consult a 
kernel expert or your kernel vendor.

These modes are expected to be supported in all 2.6 and later kernels. 
Some vendor 2.4 kernels may also support these modes. However, it is 
known that some vendor documents suggest that they support them while 
examination of the kernel source reveals that they do not.

-----------------


The kernel docs on these modes state this:

Gotchas
-------

The C language stack growth does an implicit mremap. If you want absolute
guarantees and run close to the edge you MUST mmap your stack for the
largest size you think you will need. For typical stack usage is does
not matter much but its a corner case if you really really care


Does this affect Pg?

andrew

Andrew Dunstan wrote:

>
> http://archives.postgresql.org/pgsql-hackers/2003-07/msg00608.php
>
> Subject is "reprise on Linux overcommit handling"  - is that too 
> deceptive? :-)
>
> andrew
>
> Josh Berkus wrote:
>
>> Hackers,
>>
>> I've been searching the archives, but I can't find the thread from 
>> last month where we discussed the problem with Linux memory 
>> overcommits in kernel 2.4.x.
>>
>> Can someone point me to the right thread?   I think maybe the subject 
>> line was something deceptive ....
>>
>>  
>>
>
>
>
> ---------------------------(end of broadcast)---------------------------
> TIP 7: don't forget to increase your free space map settings
>



Thu, 21 Aug 2003 09:19:24 -0400
Andrew Dunstan <andrew@dunslane.net>
Re: Can't find thread on Linux memory overcommit
On Thu, 21 Aug 2003, Andrew Dunstan wrote:

> Linux kernel version 2.4.* has poor default memory overcommit behavior, 
> which can result in the postmaster being killed by the kernel due to 
> memory demands by another process if the system runs out of memory. To 
> avoid this situation, run postgres on a machine where you can be sure 
> that other processes will not run the machine out of memory.

I would also note that the OOM killer logs its evil deeds:

printk(KERN_ERR "Out of Memory: Killed process %d (%s).\n", p->pid, p->comm);

So there's no need to wonder whether that's a source of trouble for your
PostgreSQL processes or not; just check the logs. I've had the OOM killer
go after large Perl processes and X, but never (yet) PostgreSQL, I'm happy
to say.

Jon

Thu, 21 Aug 2003 14:03:27 +0000 (UTC)
Jon Jensen <jon@endpoint.com>
Re: Can't find thread on Linux memory overcommit
Guys,

> So there's no need to wonder whether that's a source of trouble for your
> PostgreSQL processes or not; just check the logs. I've had the OOM killer
> go after large Perl processes and X, but never (yet) PostgreSQL, I'm happy
> to say.

Well, sadly, the reason I posted is that I (apparently) had a client's 
database fatally corrupted by this problem.  

Downloading Alan Cox's patch now ....

-- 
Josh Berkus
Aglio Database Solutions
San Francisco

Thu, 21 Aug 2003 09:16:29 -0700
Josh Berkus <josh@agliodbs.com>
Re: Can't find thread on Linux memory overcommit
Andrew,

> I see btw that no change has been made to the docs. That's bad IMNSHO.
> The situation with RH is unchanged with today's kernel errata patch,
> too. I propose to submit a doc patch with the following wording, unless
> someone objects or improves it:

First, off, I'm crossing this to PGSQL-DOCS, which is the correct list for doc 
patches.

Second, don't you think we should have some mention of the Alan Cox patch?   

Otherwise, I think your doc patch is good and needed before we go final.   
When we settle the second question, I'll submit a diff for you.

-- 
Josh Berkus
Aglio Database Solutions
San Francisco

Thu, 21 Aug 2003 09:19:48 -0700
Josh Berkus <josh@agliodbs.com>
Re: [HACKERS] Can't find thread on Linux memory overcommit
Josh Berkus <josh@agliodbs.com> writes:
> Well, sadly, the reason I posted is that I (apparently) had a client's 
> database fatally corrupted by this problem.  

"Fatally corrupted"?  That should not happen --- at worst, an OOM kill
should lose your current sessions.  We need more details.

			regards, tom lane

Thu, 21 Aug 2003 13:04:13 -0400
Tom Lane <tgl@sss.pgh.pa.us>
Re: Can't find thread on Linux memory overcommit 

I replied to Josh thus:

-----------------------

You need to be careful using Alan's patch. The reason RH stopped using 
this part of it in their errata kernels is that it had conflicts with 
other stuff, specifically the rmap stuff (he told me that himself in 
email).

I am very wary of advising people to use what is essentially an 
experimental patch in a production system. This should be a last resort 
- a better solution is to have better control over what is running on 
your db server, so you can ensure it never gets into an OOM situation. 
For mission critical apps I would advise running the postmaster on a 
dedicated machine, with no X or other nasty stuff running.

-------------------------

I do have a doc patch ready (with one sensible addition suggested by Jon 
Jensen).

andrew


Josh Berkus wrote:

>Andrew,
>
>  
>
>>I see btw that no change has been made to the docs. That's bad IMNSHO.
>>The situation with RH is unchanged with today's kernel errata patch,
>>too. I propose to submit a doc patch with the following wording, unless
>>someone objects or improves it:
>>    
>>
>
>First, off, I'm crossing this to PGSQL-DOCS, which is the correct list for doc 
>patches.
>
>Second, don't you think we should have some mention of the Alan Cox patch?   
>
>Otherwise, I think your doc patch is good and needed before we go final.   
>When we settle the second question, I'll submit a diff for you.
>
>  
>



Thu, 21 Aug 2003 13:08:47 -0400
Andrew Dunstan <andrew@dunslane.net>
Re: [HACKERS] Can't find thread on Linux memory overcommit
Tom,

> "Fatally corrupted"?  That should not happen --- at worst, an OOM kill
> should lose your current sessions.  We need more details.

Joe and I are diagnosing.   Likely the files will come to you before the end 
of the day.

-- 
Josh Berkus
Aglio Database Solutions
San Francisco

Thu, 21 Aug 2003 10:09:27 -0700
Josh Berkus <josh@agliodbs.com>
Re: Can't find thread on Linux memory overcommit
On Thu, Aug 21, 2003 at 01:04:13PM -0400, Tom Lane wrote:
> Josh Berkus <josh@agliodbs.com> writes:
> > Well, sadly, the reason I posted is that I (apparently) had a client's 
> > database fatally corrupted by this problem.  
> 
> "Fatally corrupted"?  That should not happen --- at worst, an OOM kill
> should lose your current sessions.  We need more details.

Even if it kills the postmaster?


Kurt


Thu, 21 Aug 2003 19:11:39 +0200
Kurt Roeckx <Q@ping.be>
Re: Can't find thread on Linux memory overcommit
Alan,

> You need to be careful using Alan's patch. The reason RH stopped using
> this part of it in their errata kernels is that it had conflicts with
> other stuff, specifically the rmap stuff (he told me that himself in
> email).

Hmmm ... that leaves us without a workaround for this problem, then, yes?  
Because even parnoid-mode kernels you can discourage, but not prevent, 
overcommitting.

> For mission critical apps I would advise running the postmaster on a
> dedicated machine, with no X or other nasty stuff running.

Unfortunately, this is frequently not an option ... PostgreSQL is often 
together on a server with Apache, a JVM, and other server software.   As 
happened in the one case of possible (diagnosis pending) failure that we are 
looking into.  Of course, it could be something else ....

-- 
Josh Berkus
Aglio Database Solutions
San Francisco

Thu, 21 Aug 2003 10:13:26 -0700
Josh Berkus <josh@agliodbs.com>
Re: Can't find thread on Linux memory overcommit
Kurt Roeckx <Q@ping.be> writes:
> On Thu, Aug 21, 2003 at 01:04:13PM -0400, Tom Lane wrote:
>> "Fatally corrupted"?  That should not happen --- at worst, an OOM kill
>> should lose your current sessions.  We need more details.

> Even if it kills the postmaster?

Then you'd have to start a new postmaster.  But it still shouldn't
corrupt anything.

(Since the postmaster has small and essentially constant memory usage,
it's a pretty improbable target for the OOM killer, anyway.)

			regards, tom lane

Thu, 21 Aug 2003 13:14:51 -0400
Tom Lane <tgl@sss.pgh.pa.us>
Re: Can't find thread on Linux memory overcommit 
(er, that's "Andrew" :-)

It depends how important your data is to you. A modest server probably 
costs a few thousand dollars. How much does an expert to install a 
custom kernel cost? Probably about the same.

I believe paranoid mode is supposed to prevent any overcommiting that 
can't later be honoured when the process comes to map the memory, and 
strict mode is supposed to do the same in most normal circumstances. 
Maybe someone more expert in kernel hackery than I am can give a better 
answer.

BTW, Alan Cox is going on sabbatical from RH very soon - so there will 
be no more -ac patches. By the time he returns 2.6 should have been 
released and bedded down, with any luck.

andrew

Josh Berkus wrote:

>Alan,
>
>  
>
>>You need to be careful using Alan's patch. The reason RH stopped using
>>this part of it in their errata kernels is that it had conflicts with
>>other stuff, specifically the rmap stuff (he told me that himself in
>>email).
>>    
>>
>
>Hmmm ... that leaves us without a workaround for this problem, then, yes?  
>Because even parnoid-mode kernels you can discourage, but not prevent, 
>overcommitting.
>
>  
>
>>For mission critical apps I would advise running the postmaster on a
>>dedicated machine, with no X or other nasty stuff running.
>>    
>>
>
>Unfortunately, this is frequently not an option ... PostgreSQL is often 
>together on a server with Apache, a JVM, and other server software.   As 
>happened in the one case of possible (diagnosis pending) failure that we are 
>looking into.  Of course, it could be something else ....
>
>  
>



Thu, 21 Aug 2003 13:42:30 -0400
Andrew Dunstan <andrew@dunslane.net>
Re: Can't find thread on Linux memory overcommit
Folks,

> > Well, sadly, the reason I posted is that I (apparently) had a client's 
> > database fatally corrupted by this problem.  

OK, diagnosis progresses, it's not the Linux OOM problem, it's something else.  

-- 
-Josh Berkus
 Aglio Database Solutions
 San Francisco


Thu, 21 Aug 2003 12:22:21 -0700
Josh Berkus <josh@agliodbs.com>
Re: Can't find thread on Linux memory overcommit