Is non-insignifcant a mutated way of saying significant? The i/o code in vm is like much of CP highly optimized. Overhead has been reduced greatly since the XA introduction of SIE emulation. VM has low overhead due to: 1. avoidance. Let SIE handle it. 2. highly optimized code paths. I have measured a lot of z/vm systems running linux and CP overhead invariably comes in at under 3%. This is running in lpars with workloads ranging from dozens, scores, to the low hundreds of linux guests. I consider this excellent. Wouldn't everyone except diehard Windows proponents? And as I look at the quote I don't agree with it, so, no z/VM is not similar to VMWARE. We don't impose significante i/o overheads. I don't know what the statement means by memory access. When in memory a virtual machine runs with no overhead, the segment and page tables are used by the hardware. Paging subsystem of course supports memory overcommitment. How to define most processor access? Most but not all instructions are provided by SIE. What isn't is simulated. The overhead is low for most simulation paths. Other instructions like DIAGNOSE are designed to exit to CP. David Kreuter
________________________________ From: The IBM z/VM Operating System on behalf of Alan Ackerman Sent: Thu 10/30/2008 7:25 PM To: [email protected] Subject: [IBMVM] I/O Overhead - z/VM versus VMWARE I got asked: "Does z/VM impose non-insignificant overhead? Is it similar to VMware,= in which virtual I/O imposes significant overhead, but most processor and = memory access runs at close to native physical speed?" I don't know anything about VMWARE so I could not answer the question. = I know that CCW Translation in VM costs significant cycles. I think FCP disks < dedicated DASD < fullpack minidisks < small minidisks= . I would HOPE that the zSeries, with so much of virtualization built into = the hardware, would have lower costs than VMWARE, but I don't really kn= ow. Any takers? Are there any web sites that give performance comparisons VM versus VMWAR= E? Alan Ackerman = Alan (dot) Ackerman (at) Bank of America (dot) com
