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Rafal Wojtczuk 2010-09-23 16:10:05 +00:00
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Qmemman.md
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@ -10,11 +10,11 @@ qmemman, Qubes memory manager
Rationale
---------
Traditionally, Xen VMs are assigned a fixed amount of memory. It is not the optimal solution, as some VMs may require more memory, while others underutilize memory. Thus, there is a need for solution capable of shifting free memory from VM to another VM.
Traditionally, Xen VMs are assigned a fixed amount of memory. It is not the optimal solution, as some VMs may require more memory than assigned initially, while others underutilize memory. Thus, there is a need for solution capable of shifting free memory from VM to another VM.
The [​tmem](http://oss.oracle.com/projects/tmem/) project provides a "pseudo-RAM" that is assigned on per-need basis. However this solution has some disadvantages:
- It does not provide real RAM, just an interface to copy memory to/from fast, RAM-based storage. It is perfect for swap, but not ideal for many tasks.
- It does not provide real RAM, just an interface to copy memory to/from fast, RAM-based storage. It is perfect for swap, good for file cache, but not ideal for many tasks.
- It is deeply integrated with the Linux kernel. When Qubes will support Windows guests natively, we would have to port *tmem* to Windows, which may be challenging.
Therefore, in Qubes another solution is used. There is the *qmemman* dom0 daemon. All VMs report their memory usage (via xenstore) to *qmemman*, and it makes decisions on whether to balance memory across domains. The actual mechanism to add/remove memory from a domain (*xc.domain\_set\_target\_mem*) is already supported by both PV Linux guests and Windows guests (the latter via PV drivers).