maxdsiz_64bit value

[Alex Lavrov.]

Hello,

current value of this kernel parameter on my system is 0X80000000 (2GB), but I see that I have many processes that use above 60% of this size, so as I know it should be increased.

According to my logs, the "biggest" segment size is 1.2GB, so I'm thinking about increasing the size up to 4GB.

My swap utilization is on 0% for some reason.

Just wanted to make sure that my calculations are right and I'm on the right way.

Thanx.

[AISSAOUI Nourreddine]

Hi Alex,

I think there is no problem to increase maxdsiz_64bit to 4GB. You need to increase accordingly the amount of device swap to a minimum of 4GB, for reservation purpose. In the matter, the swap reservation is more relevant than "utilization".

Nourreddine

[Steven E. Protter]

I am wondering if there is a specific symptom prompting you to make this change.

Once I get a system running the way it should with acceptable response time, I think twice before making changes.

If a performance analysis and/or user complaint is prompting it, I see no reason not to increase this parameter. Mine is more toward the 4 GB range, I believe.

If the system is running well, and there are no new applications on the horizon, likely to make demands, consider not making the changes.

Swap utilization of 0% is wonderful. That means you have enough memory to runn all your applications without actually paging.

Let me distinguish. Swap should be getting reserved. If thats not happening, make sure the swap area is properly configured. But if its being reserved and not used, pat yourself on the back, you're doing a good job on capacity planning.

SEP

[Alex Lavrov.]

I saw in System Healthcheck recommendations, that I should increase this parameter, that's why I raised this issue.

As for the swap, here is a swapinfo output:
Kb Kb Kb PCT START/ Kb
TYPE AVAIL USED FREE USED LIMIT RESERVE PRI NAME
dev 4194304 0 4194304 0% 0 - 1 /dev/vg00/lvol3
dev 35553280 0 35553280 0% 0 - 1 /dev/vg_swap1/lvol1
reserve - 7626088 -7626088
memory 12519108 2128456 10390652 17%


As I can see there is a resercation.

[Bill Hassell · Accepted solution]

maxdsiz and maxdsiz_64 are simple fences (high limits). The only requirement is that macdsiz_64 be larger than maxdisz. If not, then both 32 and 64 bit programs are bound by maxdsiz. The only function that these values have is to prevent runaway or badly written programs from using all virtual memory (RAM and swap). NOTE: most programs do handle memory limitations very gracefully. Rather than stating that a request for additional RAM has been denied, they usually crash with an ENOMEM or errno 12 message.

So if maxdsiz_64 is 2Gb then a program that is trying to get more than that amount will not run, ideally complaining that there are memory limits preventing the program from working as designed. Very, very few programs accomodate limited memory automatically, that is, they do not adjust to the current memory limits, they just terminate.

So you can set maxdsiz_64 to 500Gb if you like -- it doesn't affect the system or memory usage unless a 64bit program needs a lot more than 2Gb.

[Alex Lavrov.]

Thanx all for the replies!