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

  • From: Tanel Poder <tanel@xxxxxxxxxxxxxx>
  • To: Mark Bobak <Mark.Bobak@xxxxxxxxxxxx>
  • Date: Fri, 18 Apr 2014 02:42:54 -0500

If you are wondering why the heck Oracle needs such files at all - it was
already hinted in a different reply - the JOX files are related to
in-database JVM JIT compilation (JOX is the java module name prefix in
Oracle kernel).

So it's not SGA, it's just how Oracle executes the JIT compiled stuff. The
reason for these files is that if you compile something to native code and
want to let the CPU to natively execute that binary code, you will need to
map that code into a memory segment in that process, with proper permission
bits (usually r-x for that shared mem segment).

So the Java JIT compiler in a process will generate the binary code, save
it into a (JOX..) file and mmap it into its own address space for
execution, somewhat like a dynamic library...

Btw, another (simple) reason why /dev/shm files may show up when you've
disabled it for your database instance is the ASM instance... but in this
case you'd see /dev/shm/ora_+ASM_* files not the JOX ones...

*Tanel Poder*
Enkitec (The Exadata Experts)
Services <> | Training<>
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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.

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