Re: Question about hugepages, shared memory, and /dev/shm....

  • From: Mark Bobak <Mark.Bobak@xxxxxxxxxxxx>
  • To: "Cunningham, Mike" <mcunningham@xxxxxxxxxxxxxx>, "Chris.Ruel@xxxxxxx" <Chris.Ruel@xxxxxxx>, "oracle-l@xxxxxxxxxxxxx" <oracle-l@xxxxxxxxxxxxx>
  • Date: Wed, 16 Apr 2014 17:19:57 +0000

Hi Mike,

AMM (Automatic Memory Management), which is setting memory_target and letting 
Oracle manage both SGA and PGA from that chunk of memory, is only available in 
11g, and *not* compatible with hugepages.
ASMM (Automatic Shared Memory Mangement), which is setting sga_target and 
pga_aggregate_target, is available in 10g, and is compatible with hugepages.

My understanding was that /dev/shm segments and hugepages were mutually 
exclusive…but perhaps not?

Anyhow, I’m seeing ASMM + hugepages + segments in /dev/shm, which I thought 
wasn’t possible.

-Mark

From: <Cunningham>, Mike 
<mcunningham@xxxxxxxxxxxxxx<mailto:mcunningham@xxxxxxxxxxxxxx>>
Date: Wednesday, April 16, 2014 at 1:13 PM
To: "Chris.Ruel@xxxxxxx<mailto:Chris.Ruel@xxxxxxx>" 
<Chris.Ruel@xxxxxxx<mailto:Chris.Ruel@xxxxxxx>>, Mark Bobak 
<Mark.Bobak@xxxxxxxxxxxx<mailto:Mark.Bobak@xxxxxxxxxxxx>>, 
"oracle-l@xxxxxxxxxxxxx<mailto:oracle-l@xxxxxxxxxxxxx>" 
<oracle-l@xxxxxxxxxxxxx<mailto:oracle-l@xxxxxxxxxxxxx>>
Subject: RE: Question about hugepages, shared memory, and /dev/shm....

For what it’s worth.  While using AMM with huge pages the instance in my 
environment crashed on occasion during memory resizing.  That was on version 
10.2.0.3 with Linux 5.7 (x86_64).

I did not pay attention to the /dev/shm so I can’t offer anything there.  Also, 
learning from my past errors, I have never tried AMM in 11g with huge pages.

Michael Cunningham
Senior Database Administrator
The Doctors' Company
707.226.0221 - desk
707.337.0184 - cell

From: oracle-l-bounce@xxxxxxxxxxxxx<mailto:oracle-l-bounce@xxxxxxxxxxxxx> 
[mailto:oracle-l-bounce@xxxxxxxxxxxxx] On Behalf Of Ruel, Chris
Sent: Wednesday, April 16, 2014 8:59 AM
To: Mark.Bobak@xxxxxxxxxxxx<mailto:Mark.Bobak@xxxxxxxxxxxx>; 
oracle-l@xxxxxxxxxxxxx<mailto:oracle-l@xxxxxxxxxxxxx>
Subject: RE: Question about hugepages, shared memory, and /dev/shm....

I don’t know if you have come across this MOS doc yet but it’s pretty good at 
explaining some things relating to HP’s:


Oracle Support Document 361323.1 (HugePages on Linux: What It Is... and What It 
Is Not...) can be found at: 
https://support.oracle.com/epmos/faces/DocumentDisplay?id=361323.1
I would also add that depending on the version of Linux, you need to disable 
Transparent Huge pages for OEL6...at least we did.  Look at the referenced 
documents at the bottom and there is an article on this.  If you don’t disable 
THP’s, it can cause problems in RAC environments.

For your questions below, your understanding correct for #1 and #2.  I *think* 
you are correct on #3 and #4 but I have not dove in that far myself...have left 
it up to sysadmins to make sure it works...

Chris..




Chris Ruel * Oracle Database Administrator
cruel@xxxxxxx<mailto:cruel@xxxxxxx> * Desk:317.759.2172 * Cell 317.523.8482

From:oracle-l-bounce@xxxxxxxxxxxxx<mailto:oracle-l-bounce@xxxxxxxxxxxxx> 
[mailto:oracle-l-bounce@xxxxxxxxxxxxx] On Behalf Of Mark Bobak
Sent: Wednesday, April 16, 2014 11:47 AM
To: oracle-l@xxxxxxxxxxxxx<mailto:oracle-l@xxxxxxxxxxxxx>
Subject: Question about hugepages, shared memory, and /dev/shm....

Hi All,

So, I thought I really understood this stuff, but I’m a little baffled here, 
and I wonder if anyone can offer me a clue?

Here’s what I (think I) know:
1.)  AMM (setting memory_target) is *not* compatible with a hugepages 
configuration.  Any attempt to use hugepages will lock out the memory allocated 
to hugepages and AMM will only use non-hugepage memory allocations, the effect 
of which would be like removing the huge page allocated memory from the system.
2.)  ASMM (setting sga_target and pga_aggregate_target) and MMM (manually 
setting db_cache_size and pool sizes) *are* compatible with a hugepages 
configuration, and for any non-trivially sized SGA, hugepages is strongly 
recommended.
3.)  If hugepages are *not* configured, and AMM is used, memory segments will 
be mapped in /dev/shm.
4.)  If hugepages *are* used, no memory segments will be visible in /dev/shm.

So, that’s what I think is true about memory configuration and hugepages 
configuration.

That seems to be consistent throughout our environment, which mostly has ASMM 
or MMM and hugepages configuration.

However, and this is where my confusion comes in, we have several eBS 
environments, which seem to have a valid and active hugepages configuration, 
are using ASMM (not AMM), and *still* I can see memory segments allocated in 
/dev/shm??  Any idea how this is possible?

Here’s an example from our preprod environment:
(Content was too long for Oracle-L, so here’s a paste bin URL)

http://pastebin.com/7w2V2jEa


So, I’m a little baffled here.  I thought these were mutually exclusive 
features.



Note also that the timestamps on the /dev/shm segments is *after* instance 
startup time, so, I don’t think these are “orphan” memory segments….



Anyone out there can clue me in?



Thanks,



-Mark

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