I'm not sure if you can artificially limit the frames available, but I
would expect that the effect of paging on a Java application would be
BAD. Very bad.
Java needs to perform regular garbage collection. Garbage collection is
likely to touch most of the frames in the address space. It is designed
to be fast, but obviously waiting for a page in will delay it. Waiting
for multiple page-ins (I'm not sure whether z/OS is smart enought to
recognise the pattern and bring pages back in before they are required)
will be very slow. If other pages from your application are paged out to
allow pages to be brought in it's even worse because they will probably
need to be paged straight back in again i.e. thrashing.
I wouldn't bother testing - I would just say that sufficient memory is
required to execute in real storage.
Another consequence of this behaviour is that you should not make the
Java heap too large. You want the heap small enough that garbage
collection happens often enough to keep those pages in storage. If you
have a massive heap that is not collected for a long time and z/OS pages
some of it out, the eventual garbage collection will be very slow.
Actually one way to test might be to make the Java heap larger than real
storage, then create and delete objects to consume it...
Andrew Rowley
Black Hill Software
On 5/08/2015 0:13, Vlcek, Vitezslav Vit wrote:
Is it possible to limit number of frames of real storage on job level? The
MEMLIMIT/REGION parameters limit virtual memory of whole address space however
I would like to limit the only real memory while the virtual memory remain as
it is.
The reason why I need it; I would like to test the performance of a (Java)
application in case of lack of frames of real storage. In case of shortage of
real storage a paging is expected. And I would like to simulate this situation
to see impact a performance of the Java application.
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