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

From: Cunningham, Mike <mcunningham_at_thedoctors.com>
Date: Wed, 16 Apr 2014 14:56:51 -0700
Message-ID: <C8FDF1081BF81B418F937FE98B7B3CC77D8BCA_at_NPEXCHMB102.tdc.internal>

Thanks guys. This is very educational (and enjoyable) for the OP and the rest. Any day something new can be learned is a great day.

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

From: Ls Cheng [mailto:exriscer_at_gmail.com] Sent: Wednesday, April 16, 2014 10:54 AM To: Cunningham, Mike
Cc: Mark Bobak; Chris.Ruel_at_lfg.com; oracle-l_at_freelists.org Subject: Re: Question about hugepages, shared memory, and /dev/shm....

Hi
sga_target is compatible with hugepages, no matter 10g or 11g. I have been using it since 10g

Regards

On Wed, Apr 16, 2014 at 7:49 PM, Cunningham, Mike <mcunningham_at_thedoctors.com<mailto:mcunningham_at_thedoctors.com>> wrote: Hi Mark, thanks for explanation. Now I’m really confused and once again faced with questioning my understanding. I could have sworn I read that 10g AMM was not compatible with huge pages, but I can’t find my notes with a quick glance. Looks like I have another reading assignment.

Michael Cunningham
Senior Database Administrator
The Doctors' Company
707.226.0221<tel:707.226.0221> - desk
707.337.0184<tel:707.337.0184> - cell

From: Mark Bobak [mailto:Mark.Bobak_at_proquest.com<mailto:Mark.Bobak_at_proquest.com>] Sent: Wednesday, April 16, 2014 10:20 AM

To: Cunningham, Mike; Chris.Ruel_at_lfg.com<mailto:Chris.Ruel_at_lfg.com>; oracle-l_at_freelists.org<mailto:oracle-l_at_freelists.org> Subject: Re: Question about hugepages, shared memory, and /dev/shm....

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_at_thedoctors.com<mailto:mcunningham_at_thedoctors.com>> Date: Wednesday, April 16, 2014 at 1:13 PM To: "Chris.Ruel_at_lfg.com<mailto:Chris.Ruel_at_lfg.com>" <Chris.Ruel_at_lfg.com<mailto:Chris.Ruel_at_lfg.com>>, Mark Bobak <Mark.Bobak_at_ProQuest.com<mailto:Mark.Bobak_at_ProQuest.com>>, "oracle-l_at_freelists.org<mailto:oracle-l_at_freelists.org>" <oracle-l_at_freelists.org<mailto:oracle-l_at_freelists.org>> 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<tel:707.226.0221> - desk
707.337.0184<tel:707.337.0184> - cell

From: oracle-l-bounce_at_freelists.org<mailto:oracle-l-bounce_at_freelists.org> [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Ruel, Chris Sent: Wednesday, April 16, 2014 8:59 AM To: Mark.Bobak_at_proquest.com<mailto:Mark.Bobak_at_proquest.com>; oracle-l_at_freelists.org<mailto:oracle-l_at_freelists.org> 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_at_lfg.com<mailto:cruel_at_lfg.com> * Desk:317.759.2172<tel:317.759.2172> * Cell 317.523.8482<tel:317.523.8482>

From:oracle-l-bounce_at_freelists.org<mailto:oracle-l-bounce_at_freelists.org> [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Mark Bobak Sent: Wednesday, April 16, 2014 11:47 AM To: oracle-l_at_freelists.org<mailto:oracle-l_at_freelists.org> 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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