maxdsiz upper value

[Roberto Volsa]

Hello,
i would like to know what are the consequences of rising to much the maxdisiz kernel parameter (hpux 11i); it is now configured to 750 Mb.

thanks

[Jeff Schussele]

Hi Roberto,

The value is basically a fence to keep processes from running away & consuming too much memory.

From the man page...

What Are the Side Effects of Raising the Value?

Raising this tunable by definition allows larger data segments for every process. The maxdsiz and maxssiz tunables limit the amount of swap space that can be reserved or used by each process, but using more virtual address space does not translate directly into using more physical address space because virtual pages can be swapped out.

Note that if swap space on the machine is near capacity, raising this tunable increases the amount of reservable swap per process. This could exhaust the swap space on the system by allowing a process with a memory leak or a malicious program that uses huge amounts of memory to reserve too much swap space.

What Other Tunable Values Should Be Changed at the Same Time?

The maxssiz tunable should be considered because it too limits swap usage by process stack segment

Note that you need to know whether the process(es) is(are) 32-bit (maxdsiz) or 64-bit (maxdsiz_64bit) as one setting will not affect the other.

Rgds,
Jeff

[A. Clay Stephenson]

Assuming that you don't have rogue processes - none. Increasing maxdsiz, or the 64-bit version, maxdsiz_64bit, consumes not additional resources. The limits exist to "keep the horses in the corral". The danger with very large maxdsiz values is that a process can allocate dynamic memory to an extent that brings the system to its knees. This is most often a problem on development machines. Nowadays, 750MB is not considered a large maxdsiz value but there are cases where 100MB would be considered large - it depends. If you have processes which are legitimately hitting the maxdsiz limit then by all means increase it.

[harry d brown jr]

If an application goes haywire and starts to consume a lot of memory, then you could end up swapping, thus taking a performance hit.

But the bottom line is how much memory do you have, how much swap, and basically what do your applications need. I have had to pitch battle against SA's that didn't want to increase maxdsiz, yet the system had 10GB's of memory, and the application needed to use 1GB of it. I won of course!

live free or die
harry

[Patrick Wallek]

It entirely depends.

Here is an explanation of the maxdsiz parameter:

"Maximum process data storage segment space that can be used for statics and strings, as well as dynamic data space allocated by sbrk() and malloc() (32-bit processes)"

This was taken from http://docs.hp.com/hpux/onlinedocs/TKP-90202/TKP-90202.html

Basically this parameter is a fence. A process will NOT be able to allocate any more than what this parameters value is.