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

  • From: Carlos Sierra <carlos.sierra.usa@xxxxxxxxx>
  • To: mcunningham@xxxxxxxxxxxxxx
  • Date: Wed, 16 Apr 2014 13:17:00 -0400

From 361323.1
HugePages and Oracle 11g Automatic Memory Management (AMM)

The AMM and HugePages are not compatible. One needs to disable AMM on 11g to be 
able to use HugePages. See Document 749851.1 for further information.


Cheers,

Carlos Sierra

blog: carlos-sierra.net
twitter: @csierra_usa

Life's Good!

On Apr 16, 2014, at 1:13 PM, Cunningham, Mike <mcunningham@xxxxxxxxxxxxxx> 
wrote:

> 
> 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] On 
> Behalf Of Ruel, Chris
> Sent: Wednesday, April 16, 2014 8:59 AM
> To: Mark.Bobak@xxxxxxxxxxxx; 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 * Desk:317.759.2172 * Cell 317.523.8482
>  
> From: 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
> 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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