[Windows] Docker VMM: com.docker.sailor.exe holds ~15 GB host memory with a 4 GiB VM and does not return it under sustained memory pressure #724

Bug description

With Docker Desktop 4.93.0 (240920) on Windows using the Docker VMM backend (engine linux/sailor), the VMM process com.docker.sailor.exe holds ~15 GB of host working set, while the Docker VM is configured with only 4 GiB of RAM and the containers inside use ~1.5 GiB.

The memory is never returned to the host, even though the VMM itself detects sustained host memory pressure and continuously tries to reclaim. Its reclaim action only signals the guest to drop page caches, which frees almost nothing (the guest already has ~1.9 GiB free and only ~0.8 GiB of page cache). Only restarting Docker Desktop releases the memory; it then grows back to ~15 GB over several hours of normal use.

The docs state Docker VMM "Returns idle memory to the host when containers aren't active" (https://docs.docker.com/desktop/features/vmm/), so this looks like a bug in the host-side memory reclaim on Windows.

Snapshot (2026-10-04 11:18 local; VMM process started 2026-10-04 01:32)

com.docker.sailor.exe:

• Working set: 14,541 MB (observed range over the day: 14,379–16,919 MB)
• Working set - private: 3,079 MB
• Private bytes (commit): 4,454 MB
• The remainder (~11.5 GB) is shareable working set and is not guest RAM: the VM reports 4,167,532 kB total with MemFree ~1.9 GB and Cached ~0.8 GB.

Host (32 GB RAM, NVMe SSD):

• Available memory: ~358 MB
• Commit: 51.3 / 63.8 GB
• Pagefile usage: ~5.9 GB
• Sustained hard page faults: ~2,170 pages/s (\Memory\Pages Input/sec), while disks are otherwise idle (6.6% disk time)

VM / containers:

• VM: 4 GiB RAM (MemoryMiB=4096), 16 vCPU
• clickhouse/clickhouse-server: ~1.0 GiB
• postgres:17-alpine: ~0.5 GiB
• Containers are idle most of the time

Evidence from Docker's own host log

%LOCALAPPDATA%\Docker\log\host\monitor.log* contains 3,676 lines between 2026-10-03 16:07 and 2026-10-04 11:18 local time (roughly one every ~10 s while pressure is active, alternating info/warning):

[2026-10-03T08:07:02.632509100Z] {"component":"sailor","level":"warning","msg":"host memory pressure: signalled guest drop-caches","target":"drop_caches_host","time":"2026-10-03T16:07:02.632509100+08:00"}
[2026-10-04T03:18:21.429501900Z] {"component":"sailor","level":"info","msg":"host memory pressure: signalled guest drop-caches","target":"drop_caches_host","time":"2026-10-04T11:18:21.428746200+08:00"}

The VMM sees the pressure and keeps reclaiming, but the reclaim only reaches guest page cache while ~11.5 GB of host-side shareable working set is left untouched.

Impact

Host available memory drops to a few hundred MB and the machine hard-faults continuously, noticeably slowing down daily work. docker desktop restart fixes it temporarily. There is no user-facing setting to bound this host-side memory.

Reproduction steps

1. Windows 10 x64 (build 26300), Docker Desktop 4.93.0, VMM backend selected (Settings > General > Virtual Machine Manager), VM memory at the 4 GiB minimum, 16 CPUs.
2. Start a couple of long-running containers (e.g. ClickHouse + Postgres) and use Docker normally for a few hours, on a host under moderate memory pressure (IDEs + browser using most of 32 GB).
3. Watch com.docker.sailor.exe in Task Manager (Details) and %LOCALAPPDATA%\Docker\log\host\monitor.log.
4. The process working set grows to ~15 GB (≈4x the configured VM RAM) and does not shrink despite sustained host memory pressure; the log fills with "host memory pressure: signalled guest drop-caches".
5. docker desktop restart releases the memory; it regrows over hours.

Platform

Windows

Version information

docker version:
Client:
 Version:           29.8.1
 API version:       1.56
 Go version:        go1.26.8
 Git commit:        4a63305
 Built:             Tue Sep 15 16:28:39 2026
 OS/Arch:           windows/amd64
 Context:           desktop-linux

Server: Docker Desktop 4.93.0 (240920)
 Engine:
  Version:          29.8.1
  API version:      1.56 (minimum version 1.40)
  Go version:       go1.26.8
  Git commit:       464cd50
  Built:            Tue Sep 15 16:25:38 2026
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          v2.3.5
 runc:
  Version:          v1.5.1

docker info (server, trimmed):
 Server Version: 29.8.1
 Storage Driver: overlayfs
  driver-type: io.containerd.snapshotter.v1
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Cgroup Version: 2
 Kernel Version: 7.0.14-linuxkit   [engine: linux/sailor — Docker VMM Beta]
 Operating System: Docker Desktop
 OSType: linux
 Architecture: x86_64
 CPUs: 16
 Total Memory: 3.879GiB
 Name: docker-desktop
 Docker Root Dir: /var/lib/docker

Host:
 Windows 10 Pro for Workstations, version 26H2, build 26300.9550
 32 GB RAM, NVMe SSD, Windows Hypervisor Platform enabled