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Subject[RFC] memory pressure detection in VMs using PSI mechanism for dynamically inflating/deflating VM memory
Hello all,

We’re from the Linux memory team here at Qualcomm. We are currently
devising a VM memory resizing feature where we dynamically inflate or
deflate the Linux VM based on ongoing memory demands in the VM. We
wanted to propose few details about this userspace daemon in form of RFC
and wanted to know the upstream’s opinion. Here are few details –

1. This will be a native userspace daemon that will be running only in
the Linux VM which will use virtio-mem driver that uses memory hotplug
to add/remove memory. The VM (aka Secondary VM, SVM) will request for
memory from the host which is Primary VM, PVM via the backend hypervisor
which takes care of cross-VM communication.

2. This will be guest driver. This daemon will use PSI mechanism to
monitor memory pressure to keep track of memory demands in the system.
It will register to few memory pressure events and make an educated
guess on when demand for memory in system is increasing.

3. Currently, min PSI window size is 500ms, so PSI monitor sampling
period will be 50ms. In order to get quick response time from PSI, we’ve
reduced the min window size to 50ms so that as small as 5ms increase in
memory pressure can be reported to userspace by PSI.

/* PSI trigger definitions */
-#define WINDOW_MIN_US 500000   /* Min window size is 500ms */
+#define WINDOW_MIN_US 50000    /* Min window size is 50ms */

4. Detecting increase in memory demand – when a certain usecase starts
in VM that does memory allocations, it will stall causing PSI mechanism
to generate a memory pressure event to userspace. To simply put, when
pressure increases certain set threshold, it can make educated guess
that a memory requiring usecase has ran and VM system needs memory to be

5. Detecting decrease in memory pressure – the reverse part where we
give back memory to PVM when memory is no longer needed is bit tricky.
We look for pressure decay and see if PSI averages (avg10, avg60,
avg300) go down, and along with other memory stats (such as free memory
etc) we make an educated guess that usecase has ended and memory has
been free’ed by the usecase, and this memory can be given back to PVM
when its no longer needed.

6. I’m skimming much on the logic and intelligence but the daemon relies
on PSI mechanism to know when memory demand is going up and down, and
communicates with virtio-mem driver for hot-plugging/unplugging memory.
We also factor in the latency involved with roundtrips between SVM<->PVM
so we size the memory chuck that needs to be plugged-in accordingly.

7. The whole purpose of daemon using PSI mechanism is to make this si
guest driven rather than host driven, which currently is the case mostly
with virtio-mem users. The memory pressure and usage monitoring happens
inside the SVM and the SVM makes the decisions to request for memory
from PVM. This avoids any intervention such as admin in PVM to monitor
and control the knobs. We have also set max limit of how much SVMs can
grow interms of memory, so that a rouge VM would not abuse this scheme.

This daemon is currently in just Beta stage now and we have basic
functionality running. We are yet to add more flesh to this scheme to
make sure any potential risks or security concerns are taken care as well.

We would happy to know your opinions on such a scheme.

Thanks and Regards,

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