Comments on: Memory

By: 犯错了 (vm_nr_hugepages) « 弹冠相庆

犯错了 (vm_nr_hugepages) « 弹冠相庆 — Sat, 23 Oct 2010 05:59:01 +0000

[...] Memory [...]

By: Jonathan Lewis

Jonathan Lewis — Wed, 30 Jun 2010 08:47:05 +0000

Marcus,

Questions relating to bits around the edges (O/S, discs, networks) are where I start to pursue specialists in the technology when I see the need. In this case I can’t give you answers that I am certain of, but:

(a) I believe it will be in process memory if you’re using small pages, but if you’re on RHEL and switch to huge pages I think you also get a shared memory map with locked memory pages which means there is only one (much smaller) map and it’s probably in system (or at least “public”) space.

(b) I think you’re probably correct, although some people might say that this is more “damage limitation” than an advantage (particularly on 32-bit Windows) a shared map would be a relatively small benefit compared to the limitation that requires the sum of all process memory to be inside the 32-bit limit.

By: Marcus Moennig

Marcus Moennig — Sun, 27 Jun 2010 07:40:16 +0000

I have two questions here:

1.) Where is the page table allocated? In the system space of the process memory?

2.) Is this an advantage for the Windows OS, since on this OS there is only one Oracle process and sessions get their own thread within it, so only one page table is used for all session threads?

By: Log Buffer #192, A Carnival of The Vanities for DBAs | The Pythian Blog

Log Buffer #192, A Carnival of The Vanities for DBAs | The Pythian Blog — Fri, 25 Jun 2010 15:17:50 +0000

[...] an OakTable Network member, Jonathan Lewis, posted about the experience of one client migrating to 64 bit and increasing their SGA to see performance going down. The moral [...]

By: Wayne Lang

Wayne Lang — Thu, 24 Jun 2010 16:29:36 +0000

On Linux, Oracle allocates all sga memory up to sga_max_size when starts an instance, even sga_target may be lower.

By: Jonathan Lewis

Jonathan Lewis — Thu, 24 Jun 2010 16:03:44 +0000

Ken,
You’re probably already familiar with the options to make use of more memory – the /3 switch to allow a larger user address space in windows, and the “paging/windowing” options that allow the db_block_buffers (I think you have to use the old-style parameter if you do this) to use high memory pages by creating a window in low memory that uses address switching to take advantage of high memory.

They can help in the short term – but neither is a good solution to the problems of supporting a large number of users.

By: Jonathan Lewis

Jonathan Lewis — Thu, 24 Jun 2010 15:57:34 +0000

Alex,

Excellent point about the incompatibility between HugePages and 11g’s Automatic Memory Management (AMM) – thanks for highlighting it.

By: Øyvind Isene

Øyvind Isene — Thu, 24 Jun 2010 08:22:17 +0000

Also make sure that the memory size as specified by sga_max_size can be reserved within the memory reserved for huge pages. In tests we did (10.2.0.4 on RHEL 5) we confirmed that huge pages where hardly used (<100MB) and the entire SGA being allocated outside the memory reserved for huge pages when sga_max_size was slightly higher than size reserved for huge pages. Makes sense, however we thought that since sga_target was within the memory reserved for huge pages, Oracle would start allocating from them. But it didn't until we lowered sga_max_size.

By: Grégory

Grégory — Thu, 24 Jun 2010 07:11:13 +0000

Hi Ken,

I upgraded some of my customers from Oracle 9i or Oracle 10GR2 from 32 bits to 64 bits without problems.
Problem related to ORA-04031 disappear, performance increase due to more memory for buffer cache ….
On Windows, large pages mut not be used because OS has problem to relase correctly memory ( during shutdown db ).

I used export / import with Oracle 9i. With Oracle 10GR2, I just invalided all objects and recompiled them.
You can also use rman backup.

Regards,
Gregory

By: Ken

Ken — Thu, 24 Jun 2010 05:47:29 +0000

We are also facing the same issue here, we have Oracle 10gR2 on windows server 2003 (32 bit). At server 12 GB ram is installed but due to OS dependency only 2 Gb ram is availabe to oracle, recently we have faced lot of Ora-04031 (Unable to allocate XXX bytes of memory….) error. After setting different values of SGA from last 3 days i have’t recieved this error, current value of sga is 1400 MB but still database performance is slow.

I have suggested upgradation of OS and database to 64 bit, i.e switching to Windows server 2003 (64 bit) and 10gR2 (64 bit) versions, and will share my experience when perform this activity (which is planned in couple of weeks).

Regards,
Ken