Question on User Data Space with 64 bit applications

[Bill Costigan]

I've been reading a number of documents describing how HPUX allocates memory to applications.

The documents talk about how the user data and stack must fit into a single memory quadrant. In some cases the limit to the quadrant is said to be 4TB in other cases it can be no more than 1/4 of the actual RAM on the system.

Which is correct? If a system has 8GB of memory can a 64 bit application define and use data structures greater than 2GBs?

Thanks.

[Dennis Handly · Accepted solution]

You should be able to have data structures greater than 4 Gb, provided they aren't in the stack. You will have to adjust maxdsiz_64bit and possibly add swapspace.

[Steven E. Protter]

Shalom,

Seems you are quoting 32 bit rules. It is possible on the system you have specified to have a memory area of greater than 2 GB.

Though I'm cautious as to the need for something that big. One would think that application programming could be made more efficient.

SEP

[whiteknight]

Bill,


There is a very good article on

Memory allocation Quadrant
http://www2.itrc.hp.com/service/cki/docDisplay.do?docLocale=en&docId=emr_na-c00958840-4


Enjoy reading.
WK
Please assign points.

[Bill Hassell]

For 64bit programs, the limits are extremely large and like all programs, the space is virtual which includes RAM and swap. I have run a 64bit process on a 2GB RAM machine that successfully asked for 32 GB of local data space. (I had 40 GB of swap space).

That said, you must change the very low kernel limits for maxdsiz and maxdsiz_64, the suggestion being maxdsiz=2GB and maxdsiz_64=16GB, assuming that you may need up to 16 GB of local memory. Of course, once you exceed available RAM, processes will be deactivated and then portions of RAM paged out to make room for the very large process(es). This can impact performance massively, from 50:1 to 200:1 or more. If your system has 8GB of RAM your 64 bit program can easily address a lot more than 8 GB up to the limit of swap space.

[Bill Costigan]

Points for all. Thanks.
I was as little supprised at the flurry of responses. This posting was from 2004. I forgot it was even out there until I saw all your responses.

Better late than never.

[TwoProc]

I think Dennis was heading up the "cold case" department of the forum community today. :-)

[Dennis Handly]

>John: I think Dennis was heading up the "cold case" department of the forum community today.

Yep. These were the ones that I could answer. There are lots still left.