Squeezed design

Squeezed is a single host memory ballooning daemon. It helps by:

  1. allowing VM memory to be adjusted dynamically without having to reboot; and

  2. avoiding wasting memory by keeping everything fully utilised, while retaining the ability to take memory back to start new VMs.

Squeezed currently includes a simple Ballooning policy which serves as a useful default. The policy is written with respect to an abstract Xen memory model, which is based on a number of assumptions about the environment, for example that most domains have co-operative balloon drivers. In theory the policy could be replaced later with something more sophisticated (for example see xenballoond).

The Toolstack interface is used by Xenopsd to free memory for starting new VMs. Although the only known client is Xenopsd, the interface can in theory be used by other clients. Multiple clients can safely use the interface at the same time.

The internal structure consists of a single-thread event loop. To see how it works end-to-end, consult the example.

No software is ever perfect; to understand the flaws in Squeezed, please consult the list of issues.

Environmental assumptions

  1. The Squeezed daemon runs within a Xen domain 0 and communicates to xenstored via a Unix domain socket. Therefore Squeezed is granted full access to xenstore, enabling it to modify every domain’s memory/target.

  2. The Squeezed daemon calls setmaxmem in order to cap the amount of memory a domain can use. This relies on a patch to xen which allows maxmem to be set lower than totpages See Section maxmem for more information.

  3. The Squeezed daemon assumes that only domains which write control/feature-balloon into xenstore can respond to ballooning requests. It will not ask any other domains to balloon.

  4. The Squeezed daemon assumes that the memory used by a domain is: (i) that listed in domain_getinfo as totpages; (ii) shadow as given by shadow_allocation_get; and (iii) a small (few KiB) of miscellaneous Xen structures (e.g. for domains, vcpus) which are invisible.

  5. The Squeezed daemon assumes that a domain which is created with a particular memory/target (and startmem, to within rounding error) will reach a stable value of totpages before writing control/feature-balloon. The daemon writes this value to memory/memory-offset for future reference.

-   The Squeezed daemon
    does not know or care exactly what causes the difference between
    `totpages` and `memory/target` and it does *not*
    expect it to remain constant across Xen releases. It
    only expects the value to remain constant over the lifetime of a
  1. The Squeezed daemon assumes that the balloon driver has hit its target when difference between memory/target and totpages equals the memory-offset value.
-   Corrollary: to make a domain with a responsive balloon driver
    currenty using `totpages` allocate or free
    it suffices to
    set `memory/target` to
    and wait for the
    balloon driver to finish. See Section [memory model](#the-memory-model) for more
  1. The Squeezed daemon must maintain a “slush fund” of memory (currently 9MiB) which it must prevent any domain from allocating. Since (i) some Xen operations (such as domain creation) require memory within a physical address range (e.g. less than 4GiB) and (ii) since Xen preferentially allocates memory outside these ranges, it follows that by preventing guests from allocating all host memory (even transiently) we guarantee that memory from within these special ranges is always available. Squeezed operates in two phases: first causing memory to be freed; and second causing memory to be allocated.

  2. The Squeezed daemon assumes that it may set memory/target to any value within range: memory/dynamic-max to memory/dynamic-min

  3. The Squeezed daemon assumes that the probability of a domain booting successfully may be increased by setting memory/target closer to memory/static-max.

  4. The Squeezed daemon assumes that, if a balloon driver has not made any visible progress after 5 seconds, it is effectively inactive. Active domains will be expected to pick up the slack.

Toolstack interface

The toolstack interface introduces the concept of a reservation. A reservation is: an amount of host free memory tagged with an associated reservation id. Note this is an internal Squeezed concept and Xen is completely unaware of it. When the daemon is moving memory between domains, it always aims to keep

host free memory >= s + sum_i(reservation_i)

where s is the size of the “slush fund” (currently 9MiB) and reservation_t is the amount corresponding to the *i*th reservation.

As an aside: Earlier versions of Squeezed always associated memory with a Xen domain. Unfortunately this required domains to be created before memory was freed which was problematic because domain creation requires small amounts of contiguous frames. Rather than implement some form of memory defragmentation, Squeezed and Xenopsd were modified to free memory before creating a domain. This necessitated making memory reservations first-class stand-alone entities.

Once a reservation is made (and the corresponding memory is freed), it can be transferred to a domain created by a toolstack. This associates the reservation with that domain so that, if the domain is destroyed, the reservation is also freed. Note that Squeezed is careful not to count both a domain’s reservation and its totpages during e.g. domain building: instead it considers the domain’s allocation to be the maximum of reservation and totpages.

The size of a reservation may either be specified exactly by the caller or the caller may provide a memory range. If a range is provided the daemon will allocate at least as much as the minimum value provided and as much as possible up to the maximum. By allocating as much memory as possible to the domain, the probability of a successful boot is increased.

Clients of the Squeezed provide a string name when they log in. All untransferred reservations made by a client are automatically deleted when a client logs in. This prevents memory leaks where a client crashes and loses track of its own reservation ids.

The interface looks like this:

string session_id login(
  string client_name

string reservation_id reserve_memory(
  string client_name,
  int kib

int amount, string reservation_id reserve_memory_range(
  string client_name,
  int min,
  int max

void delete_reservation(
  string client_name,
  string reservation_id

void transfer_reservation_to_domain(
  string client_name,
  string reservation_id,
  int domid

The Xenopsd code in pseudocode works as follows:

 r_id = reserve_memory_range("xenopsd", min, max);
    d = domain_create()
    transfer_reservation_to_domain("xenopsd", r_id, d)
    delete_reservation("xenopsd", r_id)

The interface is currently implemented using a trivial RPC protocol over a Unix domain socket in domain 0.

Ballooning policy

This section describes the very simple default policy currently built-into Squeezed.

Every domain has a pair of values written into xenstore: memory/dynamic-min and memory/dynamic-max with the following meanings:

  • memory/dynamic-min the lowest value that Squeezed is allowed to set memory/target. The administrator should make this as low as possible but high enough to ensure that the applications inside the domain actually work.
  • memory/dynamic-max the highest value that Squeezed is allowed to set memory/target. This can be used to dynamically cap the amount of memory a domain can use.

If all balloon drivers are responsive then Squeezed daemon allocates memory proportionally, so that each domain has the same value of: target-min/(max-min)


  • if memory is plentiful then all domains will have memory/target=memory/dynamic-max

  • if memory is scarce then all domains will have memory/target=memory/dynamic-min

Note that the values of memory/target suggested by the policy are ideal values. In many real-life situations (e.g. when a balloon driver fails to make progress and is declared inactive) the memory/target values will be different.

Note that, by default, domain 0 has memory/dynamic-min=memory/dynamic-max, effectively disabling ballooning. Clearly a more sophisticated policy would be required here since ballooning down domain 0 as extra domains are started would be counterproductive while backends and control interfaces remain in domain 0.

The memory model

Squeezed considers a ballooning-aware domain (i.e. one which has written the feature-balloon flag into xenstore) to be completely described by the parameters:

  • dynamic-min: policy value written to memory/dynamic-min in xenstore by a toolstack (see Section Ballooning policy)

  • dynamic-max: policy value written to memory/dynamic-max in xenstore by a toolstack (see Section Ballooning policy)

  • target: balloon driver target written to memory/target in xenstore by Squeezed.

  • totpages: instantaneous number of pages used by the domain as returned by the hypercall domain_getinfo

  • memory-offset: constant difference between target and totpages when the balloon driver believes no ballooning is necessary: where memory-offset = totpages - target when the balloon driver believes it has reached its target.

  • maxmem: upper limit on totpages: where totpages <= maxmem

For convenience we define a adjusted-target to be the target value necessary to cause a domain currently using totpages to maintain this value indefinitely so adjusted-target = totpages - memory-offset.

The Squeezed daemon believes that:

  • a domain should be ballooning iff adjusted-target <> target (unless it has become inactive)

  • a domain has hit its target iff adjusted-target = target (to within 1 page);

  • if a domain has target = x then, when ballooning is complete, it will have totpages = memory-offset + x; and therefore

  • to cause a domain to free y it sufficies to set target := totpages - memory-offset - y.

The Squeezed daemon considers non-ballooning aware domains (i.e. those which have not written feature-balloon) to be represented by pairs of:

  • totpages: instantaneous number of pages used by the domain as returned by domain_getinfo

  • reservation: memory initially freed for this domain by Squeezed after a transfer_reservation_to_domid call

Note that non-ballooning aware domains will always have startmem = target since the domain will not be instructed to balloon. Since a domain which is being built will have 0 <= totpages <= reservation, Squeezed computes unused(i)=reservation(i)-totpages and subtracts this from its model of the host’s free memory, ensuring that it doesn’t accidentally reallocate this memory for some other purpose.

The Squeezed daemon believes that:

  • all guest domains start out as non-ballooning aware domains where target=reservation=startmem$;

  • some guest domains become ballooning-aware during their boot sequence i.e. when they write feature-balloon

The Squeezed daemon considers a host to be represented by:

  • ballooning domains: a set of domains which Squeezed will instruct to balloon;

  • other domains: a set of booting domains and domains which have no balloon drivers (or whose balloon drivers have failed)

  • a "slush fund" of low memory required for Xen

  • physinfo.free_pages total amount of memory instantanously free (including both free_pages and scrub_pages)

  • reservations: batches of free memory which are not (yet) associated with any domain

The Squeezed daemon considers memory to be unused (i.e. not allocated for any useful purpose) if it is neither in use by a domain nor reserved.

The main loop

The main loop is triggered by either:

  1. the arrival of an allocation request on the toolstack interface; or

  2. the policy engine – polled every 10s – deciding that a target adjustment is needed.

Each iteration of the main loop generates the following actions:

  1. Domains which were active but have failed to make progress towards their target in 5s are declared inactive. These domains then have: maxmem set to the minimum of target and `totpages.

  2. Domains which were inactive but have started to make progress towards their target are declared active. These domains then have: maxmem set to target.

  3. Domains which are currently active have new targets computed according to the policy (see Section Ballooning policy). Note that inactive domains are ignored and not expected to balloon.

Note that domains remain classified as inactive only during one run of the main loop. Once the loop has terminated all domains are optimistically assumed to be active again. Therefore should a domain be classified as inactive once, it will get many later chances to respond.

The targets are set in two phases. The maxmem is used to prevent domains suddenly allocating more memory than we want them to.

The main loop has a notion of a host free memory “target”, similar to the existing domain memory target. When we are trying to free memory (e.g. for starting a new VM), the host free memory “target” is increased. When we are trying to distribute memory among guests (e.g. after a domain has shutdown and freed lots of memory), the host free memory “target” is low. Note the host free memory “target” is always at least several MiB to ensure that some host free memory with physical address less than 4GiB is free (see Two phase target setting for related information).

The main loop terminates when all active domains have reached their targets (this could be because all domains responded or because they all wedged and became inactive); and the policy function hasn’t suggested any new target changes. There are three possible results:

  1. Success if the host free memory is near enough its “target”;

  2. Failure if the operation is simply impossible within the policy limits (i.e. dynamic_min values are too high;

  3. Failure if the operation failed because one or more domains became inactive and this prevented us from reaching our host free memory “target”.

Note that, since only active domains have their targets set, the system effectively rewards domains which refuse to free memory (inactive) and punishes those which do free memory (active). This effect is countered by signalling to the admin which domains/VMs aren’t responding so they can take corrective action. To achieve this, the daemon monitors the list of inactive domains and if a domain is inactive for more than 20s it writes a flag into xenstore memory/uncooperative. This key can be monitored and used to generate an alert, if desired.

Two phase target setting

The following diagram shows how a system with two domains can evolve if domain memory/target values are increased for some domains and decreased for others, at the same time. Each graph shows two domains (domain 1 and domain 2) and a host. For a domain, the square box shows its adjusted-totpages and the arrow indicates the direction of the memory/target. For the host the square box indicates total free memory. Note the highlighted state where the host’s free memory is temporarily exhausted

Two phase target setting

In the initial state (at the top of the diagram), there are two domains, one which has been requested to use more memory and the other requested to use less memory. In effect the memory is to be transferred from one domain to the other. In the final state (at the bottom of the diagram), both domains have reached their respective targets, the memory has been transferred and the host free memory is at the same value it was initially. However the system will not move atomically from the initial state to the final: there are a number of possible transient in-between states, two of which have been drawn in the middle of the diagram. In the left-most transient state the domain which was asked to free memory has freed all the memory requested: this is reflected in the large amount of host memory free. In the right-most transient state the domain which was asked to allocate memory has allocated all the memory requested: now the host’s free memory has hit zero.

If the host’s free memory hits zero then Xen has been forced to give all memory to guests, including memory less than 4GiB which is critical for allocating certain structures. Even if we ask a domain to free memory via the balloon driver there is no guarantee that it will free the useful memory. This leads to an annoying failure mode where operations such as creating a domain free due to ENOMEM despite the fact that there is apparently lots of memory free.

The solution to this problem is to adopt a two-phase memory/target setting policy. The Squeezed daemon forces domains to free memory first before allowing domains to allocate, in-effect forcing the system to move through the left-most state in the diagram above.

Use of maxmem

The Xen domain maxmem value is used to limit memory allocations by the domain. The rules are:

  1. if the domain has never been run and is paused then maxmem is set to `reservation (reservations were described in the Toolstack interface section above);
-   these domains are probably still being built and we must let
    them allocate their `startmem`

-   **FIXME**: this "never been run" concept pre-dates the
    `feature-balloon` flag: perhaps we should use the
    `feature-balloon` flag instead.
  1. if the domain is running and the balloon driver is thought to be working then maxmem is set to target; and
-   there may be a delay between lowering a target and the domain
    noticing so we prevent the domain from allocating memory when it
    should in fact be deallocating.
  1. if the domain is running and the balloon driver is thought to be inactive then maxmem is set to the minimum of target and actual.
-   if the domain is using more memory than it should then we allow
    it to make progress down towards its target; however

-   if the domain is using less memory than it should then we must
    prevent it from suddenly waking up and allocating more since we
    have probably just given it to someone else

-   **FIXME**: should we reduce the target to leave the domain in a
    neutral state instead of asking it to allocate and fail forever?

Example operation

The diagram shows an initial system state comprising 3 domains on a single host. The state is not ideal; the domains each have the same policy settings (dynamic-min and dynamic-max) and yet are using differing values of adjusted-totpages. In addition the host has more memory free than desired. The second diagram shows the result of computing ideal target values and the third diagram shows the result after targets have been set and the balloon drivers have responded.


The scenario above includes 3 domains (domain 1, domain 2, domain 3) on a host. Each of the domains has a non-ideal adjusted-totpages value.

Recall we also have the policy constraint that: dynamic-min <= target <= dynamic-max Hypothetically if we reduce target by target-dynamic-min (i.e. by setting target to dynamic-min) then we should reduce totpages by the same amount, freeing this much memory on the host. In the upper-most graph in the diagram above, the total amount of memory which would be freed if we set each of the 3 domain’s target to dynamic-min is: d1 + d2 + d3. In this hypothetical situation we would now have x + s + d1 + d2 + d3 free on the host where s is the host slush fund and x is completely unallocated. Since we always want to keep the host free memory above $s$, we are free to return x + d1 + d2 + d3 to guests. If we use the default built-in proportional policy then, since all domains have the same dynamic-min and dynamic-max, each gets the same fraction of this free memory which we call g: definition of g For each domain, the ideal balloon target is now target = dynamic-min + g. Squeezed does not set all the targets at once: this would allow the allocating domains to race with the deallocating domains, potentially allowing all low memory to be allocated. Therefore Squeezed sets the targets in two phases.

The structure of the daemon

Squeezed is a single-threaded daemon which is started by an init.d script. It sits waiting for incoming requests on its toolstack interface and checks every 10s whether all domain targets are set to the ideal values (recall the Ballooning policy). If an allocation request arrives or if the domain targets require adjusting then it calls into the module squeeze_xen.ml.

The module src/squeeze_xen.ml contains code which inspects the state of the host (through hypercalls and reading xenstore) and creates a set of records describing the current state of the host and all the domains. Note this snapshot of state is not atomic – it is pieced together from multiple hypercalls and xenstore reads – we assume that the errors generated are small and we ignore them. These records are passed into the squeeze.ml module where they are processed and converted into a list of actions i.e. (i) updates to memory/target and; (ii) declarations that particular domains have become inactive or active. The rationale for separating the Xen interface from the main ballooning logic was to make testing easier: the module test/squeeze_test.ml contains a simple simulator which allows various edge-cases to be checked.


  • If a linux domU kernel has the netback, blkback or blktap modules then they away pages via alloc_empty_pages_and_pagevec() during boot. This interacts with the balloon driver to break the assumption that, reducing the target by x from a neutral value should free x amount of memory.

  • Polling the state of the host (particular the xenstore contents) is a bit inefficient. Perhaps we should move the policy values dynamic_min and dynamic_max to a separate place in the xenstore tree and use watches instead.

  • The memory values given to the domain builder are in units of MiB. We may wish to similarly quantise the target value or check that the memory-offset calculation still works.

  • The Xen patch queue reintroduces the lowmem emergency pool. This was an attempt to prevent guests from allocating lowmem before we switched to a two-phase target setting procedure. This patch can probably be removed.

  • It seems unnecessarily evil to modify an inactive domain’s maxmem leaving maxmem less than target}, causing the guest to attempt allocations forwever. It’s probably neater to move thetarget` at the same time.

  • Declaring a domain active just because it makes small amounts of progress shouldn’t be enough. Otherwise a domain could free 1 byte (or maybe 1 page) every 5s.

  • Likewise, declaring a domain “uncooperative” only if it has been inactive for 20s means that a domain could alternate between inactive for 19s and active for 1s and not be declared “uncooperative”.

Document history

Version Date Change
0.2 10th Nov 2014 Update to markdown
0.1 9th Nov 2009 Initial version