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<!DOCTYPE html>
<meta charset="utf-8" />
<title>Compute Pressure Level 1</title>
<script src="https://www.w3.org/Tools/respec/respec-w3c" class="remove" defer></script>
<script class="remove">
// All config options at https://respec.org/docs/
const respecConfig = {
shortName: "compute-pressure",
group: "das",
specStatus: "ED",
xref: "web-platform",
github: "https://github.com/w3c/compute-pressure",
formerEditors: [
{
name: "Olivier Yiptong",
company: "Google Inc.",
companyURL: "https://google.com",
w3cid: 81110,
},
{
name: "Victor Costan",
company: "Google Inc.",
companyURL: "https://google.com",
w3cid: 72312,
}
],
editors: [
{
name: "Kenneth Rohde Christiansen",
company: "Intel Corporation",
companyURL: "https://intel.com",
w3cid: 57705,
},
{
name: "Arnaud Mandy",
company: "Intel Corporation",
companyURL: "https://intel.com",
w3cid: 126342,
}
],
testSuiteURI: "https://github.com/web-platform-tests/wpt/labels/compute-pressure",
xref: {
profile: "web-platform",
specs: [
"permissions-policy", "hr-time", "picture-in-picture", "mediacapture-streams"
]
}
};
</script>
<style>
@counter-style pressure-states-emoji {
system: cyclic;
symbols: "\026AA" "\01F7E2" "\01F7E1" "\01F534";
suffix: " ";
}
.pressure-states {
list-style-type: pressure-states-emoji;
}
@counter-style pressure-source-emoji {
system: cyclic;
symbols: "\1F321" "\1F4A1";
suffix: " ";
}
.pressure-source {
list-style-type: pressure-source-emoji;
}
ul li::marker {
font-family: "Segoe UI Emoji", "Noto Color Emoji";
}
</style>
<section id="abstract">
<p>
The <cite>Compute Pressure API</cite> provides a way for websites to react to changes
in the CPU pressure of the target device, such that websites can trade off
resources for an improved user experience.
</p>
</section>
<section id="sotd"></section>
<section class="informative">
<h2>Introduction</h2>
<p>
Modern applications often need to balance the trade offs and advantages of fully utilizing
the system's computing resources, in order to provide a modern and delightful user experience.
</p>
<p>
As an example, many applications can render video effects with varying degrees of sophistication.
These applications aim to provide the best user experience, while avoiding driving the user's
device into a high pressure regime.
</p>
<p>
Utilization of [=processing units=] close to and often reaching 100% can lead to a bad
user experience, as different tasks are fighting for the processing time.
This can lead to slowless, which is especially noticeable with input delay.
Further, a prolonged utilization close 100% can cause the [=processing units=] to heat up due to prolonged
boosting, which can lead to throttling, resulting in an even worse user experience.
</p>
<p>
As a result of thermal limits, many smartphones, tablets and laptops can become uncomfortably hot
to the touch. The fans in laptops and desktops can become so loud that they disrupt conversations
or the users’ ability to focus.
</p>
<p>
In many cases, a device under high pressure appears to be unresponsive, as the operating
system may fail to schedule the threads advancing the task that the user is waiting for. See also
<a href="https://github.com/wicg/compute-pressure/#goals--motivating-use-cases">Use Cases</a>.
</p>
</section>
<section>
<h2>A Note on Feature Detection</h2>
<p><i>This section is non-normative.</i></p>
<p>
Feature detection is an established web development best practice. Resources on the topic are plentiful on- and
offline and the purpose of this section is not to discuss it further, but rather to put it in the context of
detecting hardware-dependent features.
</p>
<p>
Consider the below feature detection examples:
</p>
<aside class="example" title="Checking existence of PressureObserver interface">
<p>
This simple example illustrates how to check whether the [=User Agent=] exposes the
{{PressureObserver}} interface
</p>
<pre class="js">
if ("PressureObserver" in globalThis) {
// Use PressureObserver interface
}
</pre>
</aside>
<aside class="note">
<p>
Checking against {{globalThis/globalThis}} will work on any [=ECMAScript/agent=] as defined by
[[ECMAScript]], thus also in workers.
</p>
<p>
It does however not tell you whether that API is actually connected to a
real [=platform collector=], whether the [=platform collector=] is collecting real telemetry
readings, or whether the user is going to allow you to access it.
</p>
</aside>
</section>
<section>
<h2>Concepts</h2>
<p>
This specification defines the following concepts:
</p>
<section>
<h3>Processing Units</h3>
<p>
Computing devices consist of a multitude of different <dfn>processing units</dfn> such as the Central
Processing Unit (CPU), the Graphics Processing Unit (GPU) and many specialized
processing units. The latter are becoming popular such as ones designed to accelerate specific
tasks like machine learning or computer vision.
</p>
</section>
<section>
<h3>Pressure sources</h3>
<p>
The specification currently defines the <dfn>valid source types</dfn> as
<em>global system thermals</em> and the <em>central [=processing unit=]</em>, also known as the CPU.
Future levels of this specification MAY introduce additional [=source types=].
</p>
<pre class="idl">
enum PressureSource { "thermals", "cpu" };
</pre>
<p>
The <dfn>PressureSource</dfn> enum represents the [=valid source types=]:
</p>
<p>
<ul class="pressure-source">
<li>
{{PressureSource/"thermals"}} represents the global thermal state of the system.
</li>
<li>
{{PressureSource/"cpu"}} represents the average pressure of the central [=processing unit=]
across all its cores.
</li>
</ul>
</p>
<aside class="note">
<p>
If a user calls {{PressureObserver/observe()}} with a [=source type=] not part of
{{PressureSource}}, at the level of this specification the [=user agent=] supports,
the method will throw a {{TypeError}}.
</p>
<p>
If the [=source type=] is known by the [=user agent=] (part of {{PressureSource}}),
but not supported by it, the host OS or the underlying hardware, the method will instead
throw {{NotSupportedError}}.
</p>
<p>
To list the known [=source types=], a user can call the static attribute
{{PressureObserver/knownSources}}.
</p>
</aside>
</section>
<section>
<h3>Sampling and Reporting Rate</h3>
<p>
The <dfn>requested sampling interval</dfn> represents the desired
interval between samples to be obtained from the hardware,
expressed in milliseconds.
</p>
<p>
Interval and frequency are inverses of each other, so the
[=requested sampling interval=] can also be expressed as a
<dfn>requested sampling rate</dfn> in Hertz (cycles per second) by
dividing 1000 by the [=requested sampling interval=] value.
</p>
<p>
The <dfn>sampling rate</dfn> for a [=platform collector=] is defined as a rate
at which the [=user agent=] obtains telemetry readings from the underlying platform,
and it might differ from the pressure observers' [=requested sampling rates=].
The rate is measured in Hertz (cycles per second).
</p>
<p>
The <dfn>reporting rate</dfn> for a pressure observer is the rate at which it runs
the [=data delivery=] steps, and it will never exceed the [=sampling rate=].
</p>
<p>
The [=sampling rate=] differs from the [=requested sampling rate=] when the
[=requested sampling rate=] exceeds upper or lower sampling rate bounds
supported or accepted by the underlying platform and [=user agent=]<sup>†</sup>.
</p>
<p>
<sup>†</sup>It is recommended that the [=user agent=] limits the [=reporting rate=]
as outlined in [[[#rate-limiting-change-notifications]]].
</p>
<p>
In case the user didn't request a [=sampling rate=], the [=sampling rate=]
is [=implementation-defined=].
</p>
<aside class="note">
In case there are multiple instances of {{PressureObserver}} active with different
[=requested sampling intervals=], it is up to the [=user agent=] to set a
[=platform collector=] level [=sampling rate=] that best fulfills these requests,
while making sure that the [=reporting rate=] of all {{PressureObserver}}s does
not exceed their respective [=requested sampling rates=].
</aside>
</section>
</section>
<section> <h2>Platform primitives</h2>
<p>
The [=platform collector=] refers to a platform interface, with which the [=user agent=] interacts to
obtain the telemetry readings required by this specification.
</p>
<p>
A [=platform collector=] can be defined by the underlying platform (e.g. in a native telemetry
framework) or by the [=user agent=], if it has a direct access to hardware counters.
</p>
<p>
A [=platform collector=] can support telemetry for different <dfn>source types</dfn> of computing
devices defined by {{PressureSource}}, or there can be multiple [=platform collectors=].
</p>
<p>
From the implementation perspective [=platform collector=] can be treated as a software proxy for the
corresponding hardware counters. It is possible to have multiple [=platform collector=] simultaneously
interacting with the same underlying hardware if the underlying platform supports it.
</p>
<p>
In simple cases, a [=platform collector=] represents individual hardware counters, but if the provided
counter readings are a product of data fusion performed in software, the [=platform collector=]
represents the results of the data fusion process. This may happen in user space or in kernel space.
</p>
<p>
As collecting telemetry data often means polling hardware counters, it is not a free operation and thus,
it should not happen if there are no one observing the data. See [[[#life-cycle]]] for more information.
</p>
<p>
A [=platform collector=] samples data at a specific rate. A [=user agent=] may modify this rate
(if possible) for privacy reasons, or ignore and fuse certain readings.
</p>
</section>
<section>
<h3>
User notifications
</h3>
<p>
It is RECOMMENDED that a [=user agent=] show some form of user-visible
notification that informs the user when a pressure observer is active,
as well as provides the user with the means to block the ongoing operation,
or simply dismiss the notification.
</p>
</section>
<section>
<h3>
Policy control
</h3>
<p>
The Compute Pressure API defines a [=policy-controlled feature=]
identified by the token "compute-pressure".
Its [=policy-controlled feature/default allowlist=] is `'self'`.
</p>
<p>
Workers (dedicated and shared) adhere to the permission policy set by their
owning document(s).
</p>
<p>
Shared workers often have multiple owning documents as they can be obtained
by other documents with the [=same origin=].
In this case, all owning documents must be [=allowed to use=] the [=policy-controlled
feature=] defined by this specification.
</p>
<p>
Dedicated workers can be created from other workers,
in which case the permission policy of the first owning document
(or owning documents, in case of a shared worker) up the owner
chain will be used.
</p>
<aside class="note">
There has been discussion on allowing setting permission policy directly
on a worker on creation, in which case that would have to be consulted
as well.
</aside>
<aside class="note">
<p>
The [=policy-controlled feature/default allowlist=] of `'self'` allows usage in
same-origin nested frames but prevents third-party content from using
the feature.
</p>
<p>
Third-party usage can be selectively enabled by adding
`allow="compute-pressure"` attribute to the frame container element:
</p>
<pre class="example html" title=
"Enabling compute pressure on remote content">
<iframe src="https://third-party.com" allow="compute-pressure"/></iframe>
</pre>
<p>
Alternatively, the Compute Pressure API can be disabled completely by
specifying the permissions policy in a HTTP response header:
</p>
<pre class="example http" title="Feature Policy over HTTP">
Permissions-Policy: {"compute-pressure": []}
</pre>
<p>
See [[[PERMISSIONS-POLICY]]] for more details.
</p>
</aside>
</section>
<section>
<h3>
Internal Slot Definitions
</h3>
<p>
Each [=global object=] has:
<ul>
<li>
a <dfn>current pressure state</dfn> (a string), initialized to the empty string.
</li>
<li>
a <dfn>pressure observer task queued</dfn> (a boolean), which is initially false.
</li>
<li>
a <dfn>registered observer list</dfn> per supported [=source type=], which is initially empty.
</li>
<li>
a reference to an underlying <dfn>platform collector</dfn> as detailed in [[[#platform-primitives]]].
</li>
</ul>
A <dfn>registered observer</dfn> consists of an <dfn>observer</dfn> (a {{PressureObserver}} object).
</p>
<p>
The [=user agent=] has:
<ul>
<li>
a <dfn>max queued records</dfn> integer, which is set to an [=implementation-defined=] value, greater than 0.
</li>
<li>
a <dfn>supported source types</dfn> [=list=] of [=implementation-defined=] {{PressureSource}} values.
</li>
</ul>
</p>
<p>
A constructed {{PressureObserver}} object has the following internal slots:
</p>
<ul data-dfn-for="PressureObserver">
<li>
a <dfn>[[\Callback]]</dfn> of type {{PressureUpdateCallback}} set on creation.
</li>
<li>
a <dfn>[[\PendingObservePromises]]</dfn> [=list=] of zero or more source-promise [=tuples=], initially empty,
where source holds a {{PressureSource}} string and promise holds a {{Promise}} object.
</li>
<li>
a <dfn>[[\QueuedRecords]]</dfn> [=queue=] of zero or more {{PressureRecord}}
objects, which is initially empty.
</li>
<li>
a <dfn>[[\LastRecordMap]]</dfn> [=ordered map=] of {{PressureSource}} to
the latest {{PressureRecord}}.
</li>
<li>
a <dfn>[[\SampleIntervalMap]]</dfn> [=ordered map=] of {{PressureSource}} to
positive numbers. It represents the sample interval given source type.
</li>
</ul>
<p>
For the [=rate obfuscation=] mitigation the constructed {{PressureObserver}} object additionally
has the following internal slots:
</p>
<ul data-dfn-for="PressureObserver">
<li>
an <dfn>[[\ObservationWindow]]</dfn> integer set as part of the [=reset observation window=] steps.
</li>
<li>
a <dfn>[[\MaxChangesThreshold]]</dfn> integer set as part of the [=reset observation window=] steps.
</li>
<li>
a <dfn>[[\PenaltyDuration]]</dfn> integer set as part of the [=reset observation window=] steps.
</li>
<li>
a <dfn>[[\ChangesCountMap]]</dfn> [=ordered map=], [=map/keyed=] on a {{PressureSource}},
representing the [=source type=] that triggered transition to the [=current pressure state=].
The [=ordered map=]'s [=map/value=] is an integer representing the number of state changes in the
current observation window timeframe.
</li>
<li>
a <dfn>[[\AfterPenaltyRecordMap]]</dfn> [=ordered map=], [=map/keyed=] on a {{PressureSource}},
representing the [=source type=] of the last {{PressureRecord}}.
The [=ordered map=]'s [=map/value=] is a {{PressureRecord}}.
</li>
</ul>
</section>
<section> <h2>Pressure States</h2>
<p>
<dfn>Pressure states</dfn> represents the minimal set of useful states that allows websites
to react to changes in compute and system pressure with minimal degration in quality or service,
or user experience.
</p>
<pre class="idl">
enum PressureState { "nominal", "fair", "serious", "critical" };
</pre>
<p>
The <dfn>PressureState</dfn> enum represents the [=pressure state=] with the following states:
</p>
<p>
<ul class="pressure-states">
<li>
{{PressureState/"nominal"}}: The conditions of the target device are at an acceptable level with no noticeable
adverse effects on the user.
</li>
<li>
{{PressureState/"fair"}}: Target device pressure, temperature and/or energy usage are slightly elevated, potentially
resulting in reduced battery-life, as well as fans (or systems with fans) becoming active and audible.
Apart from that the target device is running flawlessly and can take on additional work.
</li>
<li>
{{PressureState/"serious"}}: Target device pressure, temperature and/or energy usage is consistently highly elevated.
The system may be throttling as a countermeasure to reduce thermals.
</li>
<li>
{{PressureState/"critical"}}: The temperature of the target device or system is significantly elevated and it requires
cooling down to avoid any potential issues.
</li>
</ul>
</p>
</section>
<section> <h2>Contributing Factors</h2>
<p>
<dfn>Contributing factors</dfn> represent the underlying hardware metrics contributing to the [=current pressure state=]
and can be [=implementation-defined=].
</p>
<p>
The <dfn>adjusted pressure state</dfn> is a [=pressure state=] determined by an [=implementation-defined=]
algorithm that takes as input [=source type=] and any other [=implementation-defined=] data from
[=contributing factors=]. This algorithm MUST not be deterministic to ensure [=break calibration=] mitigation effectiveness.
</p>
<p>
The <dfn>change in contributing factors is substantial</dfn> steps are as follows:
<ol>
<li>
If [=implementation-defined=] low-level hardware metrics that contribute to the
[=current pressure state=] drop below or exceed an, per metric,
[=implementation-defined=] threshold
for the [=current pressure state=], return true.
</li>
<li>
Return false.
</li>
</ol>
<aside class="note">
<p>
The [=change in contributing factors is substantial=] algorithm allows [=user agents=] to avoid flip-
flopping between states in certain circumstances. For example, a state might otherwise change too rapidly
in response to a certain system metric that fluctuates around a boundary condition that triggers a state
change.
</p>
<p>
This specification does not define the precise algorithm
to allow implementations optimize this algorithm to match the underlying hardware platform's behavior.
One possible implementation of this algorithm is to use a
<a href="https://en.wikipedia.org/wiki/Preisach_model_of_hysteresis#Nonideal_relay">nonideal relay</a>
as a model and identify appropriate lower threshold α and upper threshold β for each
[=pressure state=] taking special characteristics of each contributing factors into consideration.
</p>
</aside>
</p>
</section>
<section> <h2>Pressure Observer</h2>
The Compute Pressure API enables developers to understand the pressure
of system resources such as the CPU.
<section data-dfn-for="PressureObserverCallback">
<h3>The <a>PressureUpdateCallback</a> callback</h3>
<pre class="idl">
callback PressureUpdateCallback = undefined (
sequence<PressureRecord> changes,
PressureObserver observer
);
</pre>
This callback will be invoked when the [=pressure state=] changes.
</section>
<section data-dfn-for="PressureObserver"> <h2>The <a>PressureObserver</a> object</h2>
<p>
The {{PressureObserver}} can be used to observe changes in the [=pressure states=].
</p>
<pre class="idl">
[Exposed=(DedicatedWorker,SharedWorker,Window), SecureContext]
interface PressureObserver {
constructor(PressureUpdateCallback callback);
Promise<undefined> observe(PressureSource source, optional PressureObserverOptions options = {});
undefined unobserve(PressureSource source);
undefined disconnect();
sequence<PressureRecord> takeRecords();
[SameObject] static readonly attribute FrozenArray<PressureSource> knownSources;
};
</pre>
<p>The <dfn>PressureObserver</dfn> interface represents a {{PressureObserver}}.</p>
<section>
<h3>The <dfn>constructor()</dfn> method</h3>
<p>
The `new` {{PressureObserver(callback)}} constructor steps are:
<ol class="algorithm">
<li>
Set |this|.{{PressureObserver/[[Callback]]}} to |callback:PressureUpdateCallback|.
</li>
</ol>
</p>
</section>
<section>
<h3>The <dfn>observe()</dfn> method</h3>
<p>
The {{PressureObserver/observe(source, options)}} method steps are:
<ol class="algorithm">
<li>
Let |relevantGlobal| be [=this=]'s [=relevant global object=].
</li>
<li>
[=list/For each=] |document:Document| in |relevantGlobal|'s [=owning document set=]:
<ol>
<li>
If |document| is not [=allowed to use=] the [=policy-controlled
feature=] token "compute-pressure", return [=a promise rejected with=] {{NotAllowedError}}.
</li>
</ol>
<aside class="issue">
<a href="https://github.com/w3c/compute-pressure/issues/110">
Permission policy does not support workers directly yet #110</a>, so they cannot be set per
individual worker, so for now we consult the policy for all owning documents. For shared
workers this means that all owning documents have to have the policy enabled, which should
be easy to coordinate as shared workers require same origin.
</aside>
</li>
</li>
<li>
Set [=this=].{{PressureObserver/[[SampleIntervalMap]]}}[|source|] to |options:PressureObserverOptions|'s {{PressureObserverOptions/sampleInterval}}.
</li>
<li>
Let |promise:Promise| be [=a new promise=].
</li>
<li>
Let |pendingPromiseTuple| be (|source|, |promise|).
</li>
<li>
[=list/Append=] |pendingPromiseTuple| to [=this=].{{PressureObserver/[[PendingObservePromises]]}}.
</li>
<li>
[=promise/React=] to |promise|:
<ul>
<li>
If |promise| was [=resolved|fulfilled=] or [=rejected=], then:
<ol>
<li>
[=list/Remove=] |tuple| from [=this=].{{PressureObserver/[[PendingObservePromises]]}}.
</li>
</ol>
</li>
</ul>
</li>
<li>
Run the following steps [=in parallel=]:
<ol>
<li>
If |source:PressureSource| is not a [=valid source type=],
[=queue a global task=] on the [=PressureObserver task source=]
given |relevantGlobal|
to reject |promise| {{NotSupportedError}} and abort these steps.
</li>
<li>
Activate [=data delivery=] of |source| data to |relevantGlobal|.
</li>
<li>
[=Queue a global task=] on the [=PressureObserver task source=] given
|relevantGlobal| to run these steps:
<ol>
<li>
If |promise| was rejected, run the following substeps:
<ol>
<li>
If |relevantGlobal|'s [=registered observer list=] for |source| is [=list/empty=],
deactivate [=data delivery=] of |source| data to |relevantGlobal|.
</li>
<li>
Return.
</li>
</ol>
</li>
<li>
[=list/Append=] a new [=registered observer=] whose [=observer=] is [=this=]
to |relevantGlobal|'s [=registered observer list=] for |source|.
</li>
<li>
Resolve |promise|.
</li>
</ol>
</li>
</ol>
</li>
<li>
Return |promise|.
</li>
</ol>
</p>
</section>
<section>
<h3>The <dfn>unobserve()</dfn> method</h3>
<p>
The {{PressureObserver/unobserve(source)}} method steps are:
<ol class="algorithm">
<li>
If |source:PressureSource| is not a [=supported source type=], throw {{"NotSupportedError"}}.
</li>
<li>
[=list/Remove=] from |this|.{{PressureObserver/[[QueuedRecords]]}} all
|records| associated with |source|.
</li>
<li>
[=map/Remove=] |this|.{{PressureObserver/[[SampleIntervalMap]]}}[|source|].
</li>
<li>
[=map/Remove=] |this|.{{PressureObserver/[[LastRecordMap]]}}[|source|].
</li>
<li>
[=map/Remove=] |this|.{{PressureObserver/[[AfterPenaltyRecordMap]]}}[|source|].
</li>
<li>
[=list/For each=] (|promiseSource|, |pendingPromise|) of [=this=].{{PressureObserver/[[PendingObservePromises]]}},
if |source| is equal to |promiseSource|, [=reject=] |pendingPromise| with an {{AbortError}}.
</li>
<li>
Let |relevantGlobal| be [=this=]'s [=relevant global object=].
</li>
<li>
Remove any [=registered observer=] from |relevantGlobal|'s [=registered observer list=] for
|source| for which [=this=] is the [=registered observer=].
</li>
<li>
If the above [=registered observer list=] is [=list/empty=],
deactivate [=data delivery=] of |source| data to |relevantGlobal|.
</li>
</ol>
</p>
</section>
<section>
<h3>The <dfn>disconnect()</dfn> method</h3>
<p>
The {{PressureObserver/disconnect()}} method steps are:
<ol class="algorithm">
<li>
[=list/Empty=] |observer|.{{PressureObserver/[[QueuedRecords]]}}.
</li>
<li>
[=map/Clear=] |this|.{{PressureObserver/[[SampleIntervalMap]]}}.
</li>
<li>
[=map/Clear=] |this|.{{PressureObserver/[[LastRecordMap]]}}.
</li>
<li>
[=map/Clear=] |this|.{{PressureObserver/[[AfterPenaltyRecordMap]]}}.
</li>
<li>
[=list/For each=] (|promiseSource|, |pendingPromise|) of [=this=].{{PressureObserver/[[PendingObservePromises]]}},
[=reject=] |pendingPromise| with an {{AbortError}}.
</li>
<li>
Let |relevantGlobal| be [=this=]'s [=relevant global object=].
</li>
<li>
Remove any [=registered observer=] from |relevantGlobal|'s' [=registered observer list=] for
all supported [=source types=] for which [=this=] is the [=observer=].
</li>
<li>
If the above [=registered observer list=] is [=list/empty=],
deactivate [=data delivery=] of |source| data to |relevantGlobal|.
</li>
</ol>
</p>
</section>
<section>
<h3>The <dfn>takeRecords()</dfn> method</h3>
<aside class="note">
<p>
A common use case for this is to immediately fetch all pending state change records
immediately prior to disconnecting the observer, so that any pending state changes
can be processed when shutting down the observer.
</p>
<p>
Another use case is the ability to receive any pending, already-generated states
changes without waiting for the callbacks to be invoked. Callbacks are invoked from
a task queue as part of the event loop cycle, so this allows for conferring the
current state outside of the event loop cycle.
</p>
</aside>
<p>
The {{PressureObserver/takeRecords()}} method steps are:
<ol class="algorithm">
<li>
Let |records| be a [=list/clone=] of |observer|.{{PressureObserver/[[QueuedRecords]]}}.
</li>
<li>
[=list/Empty=] |observer|.{{PressureObserver/[[QueuedRecords]]}}.
</li>
<li>
Return |records|.
</li>
</ol>
</p>
</section>
<section>
<h3>The <dfn>knownSources</dfn> attribute</h3>
<p>
The {{PressureObserver/knownSources}} getter steps are:
<ol class="algorithm">
<li>
Return [=user agent=]'s [=supported source types=] in alphabetical order.
</li>
</ol>
</p>
<aside class="note">
<p>
The attribute property is a merely hint about the [=source types=] the [=user agent=] supports.
It is not guaranteed pressure observation will work on the underlying operating system or hardware.
Call {{PressureObserver/observe()}} and check for {{NotSupportedError}} if pressure observation is possible.
</p>
</aside>
</section>
</section>
<section data-dfn-for="PressureRecord">
<h3>The <dfn>PressureRecord</dfn> interface</h3>
<pre class="idl">
[Exposed=(DedicatedWorker,SharedWorker,Window), SecureContext]
interface PressureRecord {
readonly attribute PressureSource source;
readonly attribute PressureState state;
readonly attribute DOMHighResTimeStamp time;
[Default] object toJSON();
};
</pre>
<p>
A constructed {{PressureRecord}} object has the following internal slots:
</p>
<ul data-dfn-for="PressureRecord">
<li>
a <dfn>[[\Source]]</dfn> value of type {{PressureSource}}, which represents the current [=source type=].
</li>
<li>
a <dfn>[[\State]]</dfn> value of type {{PressureState}}, which represents the [=current pressure state=].
</li>
<li>
a <dfn>[[\Time]]</dfn> value of type {{DOMHighResTimeStamp}},
which corresponds to the
time the data was obtained from the system, relative to the [=environment settings object/time origin=] of the global object associated with
the {{PressureObserver}} instance that generated the notification.
</li>
</ul>
<section>
<h3>The <dfn>source</dfn> attribute</h3>
<p>
The {{PressureRecord/source}} [=getter steps=] are to return its {{PressureRecord/[[Source]]}} internal slot.
</p>
</section>
<section>
<h3>The <dfn>state</dfn> attribute</h3>
<p>
The {{PressureRecord/state}} [=getter steps=] are to return its {{PressureRecord/[[State]]}} internal slot.
</p>
</section>
<section>
<h3>The <dfn>time</dfn> attribute</h3>
<p>
The {{PressureRecord/time}} [=getter steps=] are to return its {{PressureRecord/[[Time]]}} internal slot.
</p>
</section>
<section>
<h3>The <dfn>toJSON</dfn> member</h3>
<p>
When {{PressureRecord.toJSON}} is called, run [[[WebIDL]]]'s [=default toJSON steps=].
</p>
</section>
</section>
<section data-dfn-for="PressureObserverOptions">
<h3>The <dfn>PressureObserverOptions</dfn> dictionary</h3>
<pre class="idl">
dictionary PressureObserverOptions {
[EnforceRange] unsigned long sampleInterval = 0;
};
</pre>
<section>
<h3>The <dfn>sampleInterval</dfn> member</h3>
<p>
The {{PressureObserverOptions/sampleInterval}} member represents the [=requested sampling
interval=] expressed in milliseconds.
</p>
<aside class="note">
<p>
A [=user agent=] might not be able to respect the requested sampling interval. For more information
consult [[[#sampling-and-reporting-rate]]].
</p>
</aside>
</section>
</section>
<section id="life-cycle">
<h3>Life-cycle and garbage collection</h3>
<p>
Each [=global object=] has a
strong reference to [=registered observers=] in their [=registered observer list=]
(one per source).
</p>
<aside class="note">
<p>
A {{PressureObserver}} is observing a |source:PressureSource| if it
exists in the [=registered observer list=], modified by invocations of
{{PressureObserver/observe()}},
{{PressureObserver/unobserve()}}
and {{PressureObserver/disconnect()}}.
</p>
<p>
This means that a {{PressureObserver}} object |observer:PressureObserver| will
remain alive (thus not be garbage collection) until both of these conditions hold:
<ul>
<li>
There are no scripting references to the |observer|.
</li>
<li>
The |observer| is not observing any |source|.
</li>
</ul>
</p>
<p>
[[[#cb-observer-example]]] contains an example of how to use the
{{PressureObserver/disconnect()}} (or any other {{PressureObserver}}
method) from the observer callback.
</p>
</aside>
</section>
<section id="processing-model">
<h3>Processing Model</h3>
<p>
This section outlines the steps the user agent must take when implementing the specification.
</p>
<section>
<h3>Supporting algorithms</h3>
<p>
The <dfn>reset observation window</dfn> steps given the argument |observer:PressureObserver|, are as follows:
<ul>
<li>
set |observer|.{{PressureObserver/[[ObservationWindow]]}} to an [=implementation-defined=] randomized integer value in
milliseconds within an [=implementation-defined=] range.
</li>
<li>
set |observer|.{{PressureObserver/[[MaxChangesThreshold]]}} to an [=implementation-defined=] randomized integer
value of maximum allowed changes within the |observationWindow| within an [=implementation-defined=] range.
</li>
<li>
set |observer|.{{PressureObserver/[[PenaltyDuration]]}} to an [=implementation-defined=] randomized integer value
in milliseconds, within an [=implementation-defined=] range.
</li>
<li>
[=list/Empty=] the observer.{{PressureObserver/[[ChangesCountMap]]}} map.
</li>
</ul>
Run the [=reset observation window=] steps and start a timer to re-run the steps when the observer.{{PressureObserver/[[ObservationWindow]]}}
time has passed, using different randomized values.
</p>
<p>
<aside class="note">
Readings are available for [=documents=] (incl. [=iframes=] and popup windows) and workers,
matching the following criteria:
<ul>
<li>
They are [=Document/fully active=] [=documents=], or
they are <a href="https://html.spec.whatwg.org/multipage/workers.html#active-needed-worker">active needed workers</a>.
</li>
<li>
Their [=origin=] is [=same origin=] with the [=Node/node document|document=] containing the
<a href="https://html.spec.whatwg.org/multipage/interaction.html#focused">focused</a> [=node=], or an
<a href="https://w3c.github.io/picture-in-picture/#initiators-of-active-picture-in-picture-sessions">
initiator of an active Picture-in-Picture session</a>, or the browsing [=context is capturing=],
including microphone, camera and display.
</li>
</ul>
</aside>
<p>
To determine the <dfn>owning document set</dfn> for a [=relevant global object=] |relevantGlobal|:
<ol>
<li>
Let |owningDocumentSet| be an empty [=set=].
</li>
<li>
If |relevantGlobal| is {{Window}}, then [=set/append=] |relevantGlobal|'s [=associated document=] to |owningDocumentSet|.
</li>
<li>
Otherwise, [=list/for each=] |owner| in {{WorkerGlobalScope}} |relevantGlobal|'s [=WorkerGlobalScope/owner set=]:
<ol>
<li>
If |owner| is a {{Document}}, then [=set/append=] |owner| to |owningDocumentSet|.
</li>
<li>
If |owner| is a {{WorkerGlobalScope}}, set |owningDocumentSet| to the [=set/union=] of
|owningDocumentSet| and |owner|'s [=owning document set=].
</li>
</ol>
<li>
Return |owningDocumentSet|.
</li>
</li>
</ol>
</p>
<p>
The <dfn>document has implicit focus</dfn> steps given the argument |document:Document|, are as follows:
<ol>
<li>
If |document| is not [=Document/fully active=], return false.
</li>
<li>
Let |relevantGlobal| be |document|'s [=relevant global object=].
</li>
<li>
[=list/For each=] |origin| in
<a href="https://w3c.github.io/picture-in-picture/#initiators-of-active-picture-in-picture-sessions">
initiators of active Picture-in-Picture sessions</a>:
<ol>
<li>
If |relevantGlobal|'s [=relevant settings object=]'s [=origin=] is [=same origin=] with |origin|, return true.
</li>
</ol>
</li>
<li>
If |relevantGlobal|'s [=browsing context=] is [=context is capturing|capturing=], return true.
</li>
<li>
Let |topLevelBC| be |relevantGlobal|'s [=browsing context=]'s [=top-level browsing context=].
</li>
<li>
If |topLevelBC| does not have [=top-level traversable/system focus=], return false.
</li>
<li>
Let |focusedDocument| be the |topLevelBC|'s
<a href="https://html.spec.whatwg.org/multipage/interaction.html#currently-focused-area-of-a-top-level-browsing-context">
currently focused area</a>'s [=Node/node document=].
</li>
<li>
If |relevantGlobal|'s [=relevant settings object=]'s [=origin=] is [=same origin=] with
|focusedDocument|'s [=origin=], return true.
</li>
<li>
Otherwise, return false.
</li>
</ol>
</p>
<p>