Memory Management

[sjmp]

Question pertaining to memory overcommitment and memory sharing. Can someone help me identify what VMware means when discussing memory sharing and shared memory pages - the doc says the system can support higher levels of overcommitment depending on workload characteristics - a workload of many nearly identical machines."

How do we define "nearly identical machines"? Based on what? Not sure if / how to distinguish the opportunity to exploit shared memory pages.

How does one gage the amount of overcommitment to allocate to the vm's?

Memory Shares - is this a setting or is it part of the code? If one machine has more shares than they can be left with higher limits, as ESX will allocate resources to them as a priority?

[AndreTheGiant]

Think as TSP (Transparen Sharing Paging) as memory deduplicator.

When can work well? When several VM have the same OS, same path, same programs.

Note that share is also use in resource management (there are CPU shares and Mem shares), but in this case is related to a priority to access the resources.

Andre

[sjmp]

Thanks Andre.

Still trying to gage "nearly identical machines".

Same OS - yes

Same Path - yes

But applications vary. One is a DC, DNS, Printer server. the other is a file server. Does this eliminate them as candidates for shared memory paging?

And if two machines are identical but one has a secondary drive with data (file server) does the second drive config eliminate them as "nearly identical"

Is there any documentation that goes more in depth on this subject so I can differentiate?

Thanks SJMP

[AndreTheGiant]

Same path is not necessary. Is the memory rapresentation that is important.

Cause TSP it work on memory pages, you can have some process (or also some shared library) with a lot identical pages (at least the pages of the text segment of each process/library that is identical).

Andre

[mcowger]

https://www.vmware.com/files/pdf/perf-vsphere-memory_management.pdf

The algorithm for TPS is described in there.

Pretty much no config different will prevent TPS - if the memory pages hashes to the same value, it will get shared.

--Matt

VCP, vExpert, Unix Geek, Storage Nerd