On Aug 12, 2004, at 12:33 PM, Coffin Michael C wrote:
Hi David,
I've always been under the impression that the best configuration for Linux/390 guest swap is to make the virtual machine storage as large as it SHOULD need under normal operating conditions, and give the guest practically NO swap. Since Linux wants to "exploit" his memory and swap space as if he were a "stand alone" machine - the intent is to not give him more than he absolutely needs to prevent him from doing "clever things" with it, no?
I disagree. I would make virtual storage only as large as necessary to accommodate a reasonable workload with very little memory in use by buffers and cache. Then enough high-priority swap-on-VDISK to handle usual (workload-specific) fluctuations in memory usage without having to go to real DASD, and then enough low-priority swap on real DASD to handle extraordinary, infrequent memory spikes (if you need those handled, rather than just having the application get a failure when it tries to allocate however hoggishly much it asked for).
Response time to all remains excellent (well, except for 'piggy' things like Java on Linux/390 - which we've pretty much given up on).
It's probably not mostly Java's fault. It's possible to write reasonably efficient code in Java. It's just that no one who uses Java *does*, because the language encourages you to use superfluous classes for everything (the language culture does too--don't underestimate the effect of linguistic culture in programming), and since it's garbage-collected you don't even need to think about memory usage when you're working. I'm not a big fan of malloc() and free()--it's really easy to make mistakes when the programmer has to manually manage memory allocation--but at least making it the application developer's *job* means that your developers can't just sweep the problem under someone else's rug, which is what Java encourages.
Adam
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