Steven,

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I think it was Joe who mentioned the assists. To be honest while I think for CMS, VM is the “bees knees”, when you try to run MVS in a VM the wheels can fall off the car because of the need to maintain shadow page tables.

So in VM/370 all guests run real problem state and even when running V=R they don’t have access to real page zero. All interrupts get passed to CP not the Guest. CP then has to either simulate them or reflect them to the OS.

For an OS like CMS which runs DAT off this isn’t a huge problem. When running multiple users who “IPL CMS” things are improved because each user does share most of the CMS nucleus as a “Discontigous Saved Segment”, a piece of read only memory that usually sits at the top of memory and so tends to stay in real store. Again this usually isn’t relevant for Hercules because we typically only have one user but the CMS in all the releases available for download are built like that.

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On the other hand MVS wants to do its own paging, so every time MVS changes the page tables, an interrupt is generated and VM has to update its shadow page tables. There are several ways to reduce this overhead.

If you are only running one copy of MVS you can dedicate some low store to it, and so run it as V=R. ?but if you have multiple machines obviously only one can be v=r. The assists are really useful here. Some releases of “OS” also have handshake features that allows the OS to pass information to CP and reduce the overhead.

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Someone mentioned poor performance with MUSIC, well I suspect what killed MUSIC under VM was the paging overhead…

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… on the other hand some releases of DOS reputedly ran better under VM because the OS paging could be disabled, and the CP paging was much better…

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For those interested GC20-1821 has more info…

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but the some of the facilities described in there are SEPP or BSEPP which we don’t have///

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Dave

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