questions related to lockable memory in HP-UX

[Tim_ct]

I found 30% of my RAM is "lockable" in dmesg and being confused. After reading some articles on the web and found lockable memory have some relation with pseudo-swap, but do not have a clear picture.

Can anyone told the relationship between pseudo-swap and lockable memory?

and what facter determine a process use the lockable or unlockable area?

And I have the following understanding, plz correct me if I am wrong!

1. Device swap have higher priority to be used comparing with pseduo-swap

2. When there is memory pressure, pseudo swap usage will drop as process are paging out to device swap

3. Lockable memory are always the same after it is configured?

am I right?


Thanks!

[Don Morris_1 · Accepted solution]

Only 30%? That's really, really (and I mean *really*) low. Usually around 75% to 90% of memory is lockable.

In order of your asking -- pseudo-swap and lockable are not directly related concepts. Pseudo-swap just allows for pages of memory to be reserved from a fake swap pool (not disk or FS swap). If the system actually ever needs to swap out pages -- it will first page out pages to actual swap. Once all of that is paged out, the pages remaining will reflect the reservations from pseudo-swap. As such -- you can view such reservations as "swapping to themselves", which is what locking a page in memory is as well. So -- as a result, when a page is locked in memory, the system will reserve from pseudo-swap if the virtual object hasn't reserved any swap yet. As a result if pseudo-swap is configured, the total lockable memory on the system is constrained by the amount of pseudo-swap.

"Real" swap will be used for reservation over pseudo-swap in most cases. (Obviously, when you're actually swapping there's no real preference going on... if an object has pages of actual swap reserved, it will go allocate them and get the pages pushed out -- and if it doesn't (no swap or pseudo-swap), it won't). The exception is when the pages are being locked before swap is reserved (pseudo-swap preferred here since the pages will stay in memory as mentioned above).

No, pseudo-swap usage doesn't get affected by paging. Pseudo-swap is all about reservation, actual paging is allocation of already reserved swap.

Lockable memory is only changed when memory is removed or added to the system. Prior to 11.31, that's boot only obviously. Post, it is on boot or memory OL* operations.

Getting back to the 30% -- what release are you running? If prior to 11.31, did you change unlockable_mem? What's the whole set of Class information details given at boot (System, Kernel and User Mb Physical, Swap and Lockable)?

What you may not realize about lockable is that kernel dynamic memory allocations are always locked from the point of view of the system -- and as such, if only 30% of your memory is lockable, your kernel is going to be restricted as well... and the kernel running out of memory is not a good thing. You may need to retune your system -- but first we have to find out how you got lockable so low.

[whiteknight]

Tim

read this too
http://docs.hp.com/en/5965-4641/ch01s03.html

WK

[Tim_ct]

Sorry, I missread the values in dmesg, the lockable memory is actually about 73% of total physical memory.

Don Morris, thank you very much for your detail explaination and correct my misunderstanding, and thank you whiteknight, the link is very useful.

So nice to talk to you guys here!

[Tim_ct]

solved