On 5/13/26 18:50, Gregory Price wrote: > When doing aggressive overcommit of VMs on a single host, a pull > model of stat retrieval is problematic if a guest becomes some form > of unresponsive. In particular, it's difficult to discern the > difference between a hung guest and a slow guest - and why the > guest is experiencing that. > > Add VIRTIO_BALLOON_F_STATS_PUSH feature that allows the host to > configure the guest to push stats on a timer instead of the default > pull model. > > The host sets stats_push_interval_ms in the balloon config space: > 0 = disabled (pull-only, default) > N > 0 = guest pushes stats every N milliseconds > > The push mode reuses the existing stats VQ, same buffer format, > same tags. The host can change the interval at runtime by updating > the config field. > > Push mode provides two advantages over pull: > 1. Guest liveness detection: in pull mode, the host cannot > distinguish a slow guest from a hung guest without implementing > its own timeout tracking. In push mode, the absence of expected > stats buffers is an implicit liveness signal; if the guest > fails to push within the expected interval, the host can > conclude it is unresponsive. > 2. Latency-sensitive consumers (e.g., memory pressure response > loops) receive fresh stats at a guaranteed cadence without > the host needing to poll. > > STATS_PUSH requires STATS_VQ; the driver clears STATS_PUSH during > feature validation if STATS_VQ is absent. When push mode is active, > the pull callback is suppressed to avoid racing on buffer submission. > > The pull model remains available and is the default.
I don't quite see the big benefit here, really: either it's a timer in the hypervisor or a timer in the VM. A slow VM will, in either model, delay the update of stats. If you need some "liveness detection", is virtio-balloon stats updates really the right mechanism? I don't quite understand the "Latency-sensitive consumers" problem. If the VM is slow, it is slow and will mess with latency-sensitive consumers in either way? -- Cheers, David
