Performance impact of Pseudo Swap

[James R. Ferguson]

Hi (again) Trevor:

Your comment that "Unfortunately due to non-disclosure agreement I can not disclose further content or the origin of the document" certainly piques my academic interest.

I sincerely hope that you will seek release of this information, and will repost more of the document and/or any ensuing discussion you may have "with someone who has knowledge of how pseudo swap is handled internally within the kernel."

Not to do this, leaves this thread quite open in my opinion.

With my regards!

...JRF...

[Trevor Dyson]

Hi James,

The rest of the document in question adds nothing to this particular discussion so you are not missing out on anything there.

Rest assured that if I find out any further information that either proves or disproves that pseudo swap negatively impacts performance (assuming that paging is not occuring) then I will share this information.

I have given you a few more points to make up for my meanness earlier on ;-)

I have just noticed that this has been posted into the database forum when I intended it to go to general - oops!.

[Carsten Krege]

James, this buged me, too. :)

I searched the HP intranet and found an HP internal performance cookbook where it says:

"swapmem_on

This trick to enable pseudo swap is used to increase the amount of reservable virtual memory. It's only useful when you can?t configure as much swap as you need . For example, say you have more phyical memory installed than you have disk available to use as swap: in this case if ppseudo swap is not turned on, you?ll never be able to use all the memory you have installed. The problem is, managing pseudo-swap takes up some memory itself, and can slow performance! We recommend you set this to 0 unless you have a boatload of memory and not enough disk available for allocating to swap."


In my opinion the statement that pseudo swap can "slow performance" refers exactly to the scenario that Stefan describes above. Since the priority of device swap is higher than of pseudo swap, a system that has been configured in a sensible way should never run into such a situation, where device and filesystem swap is exhausted and (pseudo swapped) pages have to be locked in memory.

Of course pseudo swap uses also some memory, but I'm very sure that this is neglectable for large memory systems where pseudo swap becomes really interesting.

Carsten

[James R. Ferguson]

Hi Carsten:

GREAT! Thank you for your tenacity. That is a grand contribution from you. As usual, you are most helpful and insightful!

Trevor, thanks too! I think you should put the rabbit next to Carsten's last post to flag this thread as having some excellent information.

With my regards, Jim.

...JRF...