Re: Hugepages - benefits / drawbacks

From: Krishna Manoharan <>
Date: Mon, 6 Oct 2008 17:51:52 -0700
Message-ID: <>

Please note the below about Huge pages.

  1. Huge pages are used for SGA only. Don't reserve more than you intend to allocate to SGA (SGA_MAX_SIZE). Otherwise, you are going to see heavy swapping.
  2. Huge pages cannot be paged out. They are locked in memory (similar to ISM on Solaris) and even if un-used, not freed back to the OS. No other application other than Oracle can use this memory.
  3. Since PGA cannot use Huge Pages (on Solaris PGA uses ISM and will not get paged out to disk), make sure that enough free memory is available on the system for PGA + OS requirements + other apps.
  4. The formula you want to use

Physical Memory > shmall > (shmmax = nr_hugepages)

Physical Memory - nr_hugepages = PGA + OS + Other apps

5. Huge pages are more efficient because of the bigger pagesize (2M) and also help reduce TLB misses (assuming you are having a system with more than 16GB Memory).

6. Huge Pages do not give you a bigger heap size (_realfree_heap_pagesize_hint is not applicable).

Krishna Manoharan
On Mon, Oct 6, 2008 at 3:37 PM, Nuno Souto <> wrote:

> Yong Huang wrote,on my timestamp of 7/10/2008 3:43 AM:
>> The previous DBA setting up the server and database didn't
> > use huge pages. I don't know if he was unaware of it or was
> > afraid of the big page size.
> My guess is "unaware". Most folks aren't even aware of
> the problem hugepages address. That sort of knowledge is
> frowned upon by the point-and-click tuition brigade,
> nowadays.
> And in some cases it's taken care of silently, which
> is IMO the best way to approach this. Make it part
> of default settings if the h/w addressing is 64-bit
> class: that way no one can forget to "click" it on.
> --
> Cheers
> Nuno Souto
> in sunny Sydney, Australia
> --

Received on Mon Oct 06 2008 - 19:51:52 CDT

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