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 # Resource Model
+
+The resource model for gVisor does not assume a fixed number of threads of
+execution (i.e. vCPUs) or amount of physical memory. Where possible, decisions
+about underlying physical resources are delegated to the host system, where
+optimizations can be made with global information. This delegation allows the
+sandbox to be highly dynamic in terms of resource usage: spanning a large number
+of cores and large amount of memory when busy, and yielding those resources back
+to the host when not.
+
+Some of the details here may depend on the [platform](../platforms/), but in
+general this page describes the resource model used by gVisor. If you're not
+familiar with the terms here, uou may want to start with the [Overview](../).
+
+## Processes
+
+Much like a Virtual Machine (VM), a gVisor sandbox appears as an opaque process
+on the system. Processes within the sandbox do not manifest as processes on the
+host system, and process-level interactions within the sandbox requires entering
+the sandbox (e.g. via a [Docker exec][exec]).
+
+## Networking
+
+Similarly to processes, the sandbox attaches a network endpoint to the system,
+but runs it's own network stack. All network resources, other than packets in
+flight, exist only inside the sandbox, bound by relevant resource limits.
+
+You can interact with network endpoints exposed by the sandbox, just as you
+would any other container, but network introspection similarly requires entering
+the sandbox.
+
+## Files
+
+Files may be backed by different implementations. For host-native files (where a
+file descriptor is available), the Gofer may return a file descriptor to the
+Sentry via [SCM_RIGHTS][scmrights][^1].
+
+These files may be read from and written to through standard system calls, and
+also mapped into the associated application's address space. This allows the
+same host memory to be shared across multiple sandboxes, although this mechanism
+does not preclude the use of side-channels (see the [security
+model](../security/)).
+
+Note that some file systems exist only within the context of the sandbox. For
+example, in many cases a `tmpfs` mount will be available at `/tmp` or
+`/dev/shm`, which allocates memory directly from the sandbox memory file (see
+below). Ultimately, these will be accounted against relevant limits in a similar
+way as the host native case.
+
+## Threads
+
+The Sentry models individual task threads with [goroutines][goroutine]. As a
+result, each task thread is a lightweight [green thread][greenthread], and may
+not correspond to an underlying host thread.
+
+However, application execution is modelled as a blocking system call with the
+Sentry. This means that additional host threads may be created, *depending on
+the number of active application threads*. In practice, a busy application will
+converge on the number of active threads, and the host will be able to make
+scheduling decisions about all application threads.
+
+## Time
+
+Time in the sandbox is provided by the Sentry, through its own [vDSO][vdso] and
+timekeeping implementation. This is divorced from the host time, and no state is
+shared with the host, although the time will be initialized with the host clock.
+
+The Sentry runs timers to note the passage of time, much like a kernel running
+on hardware (though the timers are software timers, in this case). These timers
+provide updates to the vDSO, the time returned through system calls, and the
+time recorded for usage or limit tracking (e.g. [RLIMIT_CPU][rlimit]).
+
+When all application threads are idle, the Sentry disables timers until an event
+occurs that wakes either the Sentry or an application thread, similar to a
+[tickless kernel][tickless]. This allows the Sentry to achieve near zero CPU
+usage for idle applications.
+
+## Memory
+
+The Sentry implements its own memory management, including demand-paging and a
+Sentry internal page cache for files that cannot be used natively. A single
+[memfd][memfd] backs all application memory.
+
+### Address spaces
+
+The creation of address spaces is platform-specific. For some platforms,
+additional "stub" processes may be created on the host in order to support
+additional address spaces. These stubs are subject to various limits applied at
+the sandbox level (e.g. PID limits).
+
+### Physical memory
+
+The host is able to manage physical memory using regular means (e.g. tracking
+working sets, reclaiming and swapping under pressure). The Sentry lazily
+populates host mappings for applications, and allow the host to demand-page
+those regions, which is critical for the functioning of those mechanisms.
+
+In order to avoid excessive overhead, the Sentry does not demand-page individual
+pages. Instead, it selects appropriate regions based on heuristics. There is a
+trade-off here: the Sentry is unable to trivially determine which pages are
+active and which are not. Even if pages were individually faulted, the host may
+select pages to be reclaimed or swapped without the Sentry's knowledge.
+
+Therefore, memory usage statistics within the sandbox (e.g. via `proc`) are
+approximations. The Sentry maintains an internal breakdown of memory usage, and
+can collect accurate information but only through a relatively expensive API
+call. In any case, it would likely be considered unwise to share precise
+information about how the host is managing memory with the sandbox.
+
+Finally, when an application marks a region of memory as no longer needed, for
+example via a call to [madvise][madvise], the Sentry *releases this memory back
+to the host*. There can be performance penalties for this, since it may be
+cheaper in many cases to retain the memory and use it to satisfy some other
+request. However, releasing it immediately to the host allows the host to more
+effectively multiplex resources and apply an efficient global policy.
+
+## Limits
+
+All Sentry threads and Sentry memory are subject to a container cgroup. However,
+application usage will not appear as anonymous memory usage, and will instead be
+accounted to the `memfd`. All anonymous memory will correspond to Sentry usage,
+and host memory charged to the container will work as standard.
+
+The cgroups can be monitored for standard signals: pressure indicators,
+threshold notifiers, etc. and can also be adjusted dynamically. Note that the
+Sentry itself may listen for pressure signals in its containing cgroup, in order
+to purge internal caches.
+
+[goroutine]: https://tour.golang.org/concurrency/1
+[greenthread]: https://en.wikipedia.org/wiki/Green_threads
+[scheduler]: https://morsmachine.dk/go-scheduler
+[vdso]: https://en.wikipedia.org/wiki/VDSO
+[rlimit]: http://man7.org/linux/man-pages/man2/getrlimit.2.html
+[tickless]: https://en.wikipedia.org/wiki/Tickless_kernel
+[memfd]: http://man7.org/linux/man-pages/man2/memfd_create.2.html
+[scmrights]: http://man7.org/linux/man-pages/man7/unix.7.html
+[madvise]: http://man7.org/linux/man-pages/man2/madvise.2.html
+[exec]: https://docs.docker.com/engine/reference/commandline/exec/
+
+[^1]: Unless host networking is enabled, the Sentry is not able to create or open host file descriptors itself, it can only receive them in this way from the Gofer.