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

  • From: Chris Taylor <christopherdtaylor1994@xxxxxxxxx>
  • To: Mark Bobak <Mark.Bobak@xxxxxxxxxxxx>
  • Date: Wed, 16 Apr 2014 10:59:22 -0500

Is there a difference between the architectures of the machines that have
/dev/shm allotments and the machines that do not?  I see some references in
the Oracle Docs about /dev/shm on x86 boxes so I was curious if 32-bit
behaves differently than 64-bit boxes in this regard?


On Wed, Apr 16, 2014 at 10:47 AM, Mark Bobak <Mark.Bobak@xxxxxxxxxxxx>wrote:

>  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)
>   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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