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VMware adds support for Intel's Page Modification Logging feature

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May 11, 2016

VMware confirmed earlier today that it will soon add support for Intel's Page Modification Logging feature in a future version of vSphere, according to the product's chief platform architect Richard Brunner.

Brunner said that VMware “worked closely with Intel on the definition of PML back in 2012.” The issue both were trying to solve is caused by fast multi-core CPUs, which lend themselves to the creation of many virtual CPUs.

When multiple vCPUS are in operation, “you have multiple threads in VCPUs all trying to do shared writes,” Brunner asserted.

PML debuted in this year's “Broadwell” Xeons and lets a hypervisor monitor all the memory pages that a guest VMs accesses, in order to better understand which pages have been used by which VM so that page and the data it contains can be protected so it won't be used by another VM.

Brunner explained it with a Halloween metaphor. Today, he said, the CPU knows that someone rang your doorbell on a search for tricks or treats. PML means you can name the kid who came for candy, remember what he took and stop them coming back for unjustified second helpings.

PML simply means that the hypervisor becomes more aware of memory worthy of protection or ready for re-use, with two outcomes.

First, reliability improves, which is why PML will be used for future fault-tolerance enhancements to vSphere.

Understanding which pages are worth keeping also helps when contemplating vMotion-– VMware's in-house term for moving workload from one machine and/or location to another.

vMotion, across several data centres, campuses or oceans is a big thing for VMware's resilience story.

And number two, PML can boost vSphere performance by a couple of points, further reducing the already-small overhead that virtualization imposes on workloads.

PML is “not currently in the product,” Brunner said, declining to say if it is in the current vSphere beta.

VMware's non-disclosure agreement works. There's now several participants in that test and they're all keeping quiet.

However, Brunner did say that PML will appear in a future vSphere edition. Another new virtualization-friendly feature in this year's Xeons is supervisor mode access protection (SMAP), a technology that makes it harder for the hypervisor to access a guest's memory.

And that's a good thing since it reduces the chances of an attack spreading from guest to host. vSphere's supported SMAP since version 6.0, indeed before SMAP appeared in silicon.

Citrix recently asserted that it was first to implement SMAP in Xen Server, a contention Brunner didn't take seriously.

Intel consults with partners to ensure its future silicon does useful things that will benefit software vendors, Intel itself and its end-users.

Indeed, Brunner added that VMware is already aware of some features that users won't see on sale until at least 2019 or 2020.

Source: VMware.

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