Coordination between memory-saving mechanisms in computers that run virtual machines

1. A method, comprising:

running in a computer a hypervisor that allocates physical memory pages of the computer to a Virtual Machine (VM);

running in the VM a guest Operating System (OS), a virtual memory and a virtual storage device, wherein the guest OS maps the allocated physical memory pages to respective virtual memory pages, retains virtual memory pages that are frequently-accessed by the VM in the virtual memory, and swaps-out virtual memory pages that are rarely-accessed by the VM to the virtual storage; and

in the hypervisor, selecting one or more of the physical memory pages allocated to the VM, and preventing the corresponding virtual memory pages from being swapped-out by the guest OS, by marking the corresponding virtual memory pages in the guest OS as accessed thus causing the guest OS to regard the corresponding virtual memory pages as frequently-accessed.

2. The method according to claim 1 , wherein marking the virtual memory pages as accessed comprises causing the guest OS to regard at least one virtual memory page as frequently-accessed even though the virtual memory page is rarely-accessed.

3. The method according to claim 1 , wherein selecting the physical memory pages comprises selecting at least one physical memory page that was deduplicated by the hypervisor.

4. The method according to claim 1 , wherein selecting the physical memory pages comprises selecting at least one physical memory page that was swapped-out by the hypervisor from a physical memory of the computer to a physical non-volatile storage device of the computer.

5. The method according to claim 1 , wherein marking the virtual memory pages as accessed comprises modifying, by the hypervisor, page-access information maintained by the guest OS.

6. The method according to claim 1 , wherein marking the virtual memory pages as accessed comprises reporting the selected physical memory pages to a driver running in the VM, and marking the virtual memory pages as accessed by the driver.

7. The method according to claim 6 , wherein marking the virtual memory pages as accessed comprises modifying, by the driver, page-access information maintained by the guest OS.

8. The method according to claim 6 , wherein marking the virtual memory pages as accessed comprises accessing the virtual memory pages by the driver.

9. A computer, comprising:

a physical memory; and

a Central Processing Unit (CPU), which is configured to run a hypervisor that allocates physical memory pages of the physical memory to a Virtual Machine (VM), to run in the VM a guest Operating System (OS), a virtual memory and a virtual storage device, wherein the guest OS maps the allocated physical memory pages to respective virtual memory pages, retains virtual memory pages that are frequently-accessed by the VM in the virtual memory, and swaps-out virtual memory pages that are rarely-accessed by the VM to the virtual storage, to select by the hypervisor one or more of the physical memory pages allocated to the VM, and to prevent the corresponding virtual memory pages from being swapped-out by the guest OS, by marking the corresponding virtual memory pages in the guest OS as accessed thus causing the guest OS to regard the corresponding virtual memory pages as frequently-accessed.

10. The computer according to claim 9 , wherein the CPU is configured to cause the guest OS to regard at least one virtual memory page as frequently-accessed even though the virtual memory page is rarely-accessed.

11. The computer according to claim 9 , wherein the CPU is configured to select the physical memory pages by selecting at least one physical memory page that was deduplicated by the hypervisor.

12. The computer according to claim 9 , wherein the CPU is configured to select the physical memory pages by selecting at least one physical memory page that was swapped-out by the hypervisor from a physical memory of the computer to a physical non-volatile storage device of the computer.

13. The computer according to claim 9 , wherein the CPU is configured to marking the virtual memory pages as accessed by modifying, by the hypervisor, page-access information maintained by the guest OS.

14. The computer according to claim 9 , wherein the CPU is configured to mark the virtual memory pages as accessed by reporting the selected physical memory pages to a driver running in the VM, and marking the virtual memory pages as accessed by the driver.

15. The computer according to claim 14 , wherein the CPU is configured to mark the virtual memory pages as accessed by modifying, by the driver, page-access information maintained by the guest OS.

16. The computer according to claim 14 , wherein the CPU is configured to mark the virtual memory pages as accessed by accessing the virtual memory pages by the driver.

17. A computer software product, the product comprising a tangible non-transitory computer-readable medium in which program instructions are stored, which instructions, when read by a Central Processing Unit (CPU) of a computer, cause the processor to run a hypervisor that allocates physical memory pages of a physical memory of the computer to a Virtual Machine (VM), to run in the VM a guest Operating System (OS), a virtual memory and a virtual storage device, wherein the guest OS maps the allocated physical memory pages to respective virtual memory pages, retains virtual memory pages that are frequently-accessed by the VM in the virtual memory, and swaps-out virtual memory pages that are rarely-accessed by the VM to the virtual storage, to select by the hypervisor one or more of the physical memory pages allocated to the VM, and to prevent the corresponding virtual memory pages from being swapped-out by the guest OS, by marking the corresponding virtual memory pages in the guest OS as accessed thus causing the guest OS to regard the corresponding virtual memory pages as frequently-accessed.