Large page support in the Linux Kernel?

Large page support in the Linux Kernel?

[Buddy Lumpkin]

I was wondering if there is going to be large page support for the linux
kernel in the near future. I am sure most people know the benefit this has
for large databases like Oracle, ultimately larger pages means the TLB slots
refer to a much larger area of memory increasing the chance of a TLB hit and
avoiding the need to walk the page tables in the MMU.

I have spotted a few previous threads where fragmentation and the complexity
in making these pages such that they can be evicted were offered as reasons
to defer this feature. I think this is a mistake. 

In Solaris, the first large page support was added to the shared memory
segment driver and called Intimate Shared Memory. ISM uses 4MB pages, and is
a segment that is locked in memory (non-pagable). Solaris hasn't done
anything to address fragmentation as far as I know until recently in Solaris
9 (ISM has been available since 2.6, maybe prior). 

I have seen many cases where a DBA shuts down an Oracle instance, but cannot
restart it because significant memory use has taken place since the instance
was shut down (filesystem backed pages read into the page cache, malloc,
free, etc..), and in these cases, there was plenty of available memory on
the system to handle the request for the ISM segment. In most Oracle
installations, the SGA is allocated with an ISM segment that stays resident
for entire duration that the Oracle instance is running. This is hardly a
case where fragmentation is going to cause an issue since normal 4k or 8k
pages are allocated easily in the presence of this segment.

To a kernel programmer, the fragmentation and paging issues may seem like a
big issue, but any DBA I know would happily make this tradeoff on their
production databases for the additional performance that large pages offers.

If fragmentation isn't a big deal, and pages can be locked in memory, are
large pages still a difficult feature to add to the linux kernel?

Lastly, I know I have mentioned Oracle and Solaris a lot. Please don't flame
me for this, I think the points I am trying to make are reasonable.

Regards,

--Buddy

Re: Large page support in the Linux Kernel?

[Paul Jackson]

> I was wondering if there is going to be large page support for the linux
> kernel in the near future.

Do a search in this lkml for "hugetlb",  and read the linux file
Documentation/vm/hugetlbpage.txt.  Is that what you're looking for?

-- 
                          I won't rest till it's the best ...
                          Programmer, Linux Scalability
                          Paul Jackson <pj@sgi.com> 1.650.933.1373

RE: Large page support in the Linux Kernel?

[Buddy Lumpkin]

Yup, this is it. Thanks :)

--Buddy

-----Original Message-----
From: linux-kernel-owner@vger.kernel.org
[mailto:linux-kernel-owner@vger.kernel.org] On Behalf Of Paul Jackson
Sent: Saturday, May 01, 2004 3:36 AM
To: Buddy Lumpkin
Cc: linux-kernel@vger.kernel.org
Subject: Re: Large page support in the Linux Kernel?

> I was wondering if there is going to be large page support for the linux
> kernel in the near future.

Do a search in this lkml for "hugetlb",  and read the linux file
Documentation/vm/hugetlbpage.txt.  Is that what you're looking for?

-- 
                          I won't rest till it's the best ...
                          Programmer, Linux Scalability
                          Paul Jackson <pj@sgi.com> 1.650.933.1373
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Re: Large page support in the Linux Kernel?

[Tom Sightler]

On Sat, 2004-05-01 at 05:28, Buddy Lumpkin wrote:
> Lastly, I know I have mentioned Oracle and Solaris a lot. Please don't flame
> me for this, I think the points I am trying to make are reasonable.

The only reason to be flamed is that you didn't seem to do much research
before posting.  RHEL 2.1 has supported a feature called "BigPages"
specifically used for Oracle for quite a while.  Good documentation on
how to set this up can be found from Redhat, Oracle, and a quick Google
search.

See
http://www.puschitz.com/TuningLinuxForOracle.shtml#UsingLargeMemoryPages
for instructions on how to set it up.  Basically you pass a kernel
parameter to tell the system how much memory to allocate as big pages
and the kernel reserves this memory for it's use.

RHEL 3.0 and current 2.6 kernel support a newer variant called Hugetlb
which seems similar from a user perspective but I'm not sure of the
implementation details.  It more dynamic, you can decrease or increase
memory allocated to Hugetlb's via /proc athough, you can't always grow
it (the kernel has to be able to allocate contiguous segments to grow
the system).  I'm not aware of any instruction in the community about
how to set this up, but Oracle's Metalink provides complete
instructions.  I'm already using it on my five production Oracle
instances and have had no problems with it.

Later,
Tom

Re: Large page support in the Linux Kernel?

[Rik van Riel]

On Sat, 1 May 2004, Buddy Lumpkin wrote:

> I was wondering if there is going to be large page support for the linux
> kernel in the near future.

I'm sorry to disappoint you, but your request is a few years
too late.  RHEL2.1, RHEL3, UL and the 2.6 kernel have had
large page support for SHM segments for quite a while now.

-- 
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it." - Brian W. Kernighan