The [Debugger API][debugger] can help tools observe the debuggee's memory use in various ways:
It can mark each new object with the JavaScript call stack at which it was allocated.
It can log all object allocations, yielding a stream of JavaScript call stacks at which allocations have occurred.
It can compute a census of items belonging to the debuggee, categorizing items in various ways, and yielding item counts.
If dbg is a [Debugger][debugger-object] instance, then the methods and accessor properties of dbg.memory control how dbg observes its debuggees' memory use. The dbg.memory object is an instance of Debugger.Memory; its inherited accesors and methods are described below.
The JavaScript engine marks each new object with the call stack at which it was allocated, if:
the object is allocated in the scope of a global object that is a debuggee of some [Debugger][debugger-object] instance dbg; and
dbg.memory.[trackingAllocationSites][tracking-allocs] is set to true.
A [Bernoulli trial][bernoulli-trial] succeeds, with probability equal to the maximum of [d.memory.allocationSamplingProbability][alloc-sampling-probability] of all Debugger instances d that are observing the global that this object is allocated within the scope of.
Given a [Debugger.Object][object] instance dobj referring to some object, dobj.[allocationSite][allocation-site] returns a [saved call stack][saved-frame] indicating where dobj's referent was allocated.
If dbg is a [Debugger][debugger-object] instance, and dbg.memory.[trackingAllocationSites][tracking-allocs] is set to true, then the JavaScript engine logs each object allocated by dbg‘s debuggee code. You can retrieve the current log by calling dbg.memory.[drainAllocationsLog][drain-alloc-log]. You can control the limit on the log’s size by setting dbg.memory.[maxAllocationsLogLength][max-alloc-log].
A census is a complete traversal of the graph of all reachable memory items belonging to a particular Debugger‘s debuggees. It produces a count of those items, broken down by various criteria. If dbg is a [Debugger][debugger-object] instance, you can call dbg.memory.[takeCensus][take-census] to conduct a census of its debuggees’ possessions.
Debugger.Memory.prototype ObjectIf dbg is a [Debugger][debugger-object] instance, then <i>dbg</i>.memory is a Debugger.Memory instance, which inherits the following accessor properties from its prototype:
trackingAllocationSites : A boolean value indicating whether this Debugger.Memory instance is capturing the JavaScript execution stack when each Object is allocated. This accessor property has both a getter and setter: assigning to it enables or disables the allocation site tracking. Reading the accessor produces true if the Debugger is capturing stacks for Object allocations, and false otherwise. Allocation site tracking is initially disabled in a new Debugger.
Assignment is fallible: if the Debugger cannot track allocation sites, it throws an `Error` instance. You can retrieve the allocation site for a given object with the [`Debugger.Object.prototype.allocationSite`][allocation-site] accessor property.
allocationSamplingProbability : A number between 0 and 1 that indicates the probability with which each new allocation should be entered into the allocations log. 0 is equivalent to “never”, 1 is “always”, and .05 would be “one out of twenty”.
The default is 1, or logging every allocation. Note that in the presence of multiple <code>Debugger</code> instances observing the same allocations within a global's scope, the maximum <code>allocationSamplingProbability</code> of all the <code>Debugger</code>s is used.
maxAllocationsLogLength : The maximum number of allocation sites to accumulate in the allocations log at a time. This accessor can be both fetched and stored to. Its default value is 5000.
allocationsLogOverflowed : Returns true if there have been more than [maxAllocationsLogLength][#max-alloc-log] allocations since the last time [drainAllocationsLog][#drain-alloc-log] was called and some data has been lost. Returns false otherwise.
trackingTenurePromotions : A boolean value indicating whether this Debugger.Memory instance is observing promotions from the nursery to the tenured heap. It is an accessor property that has both a getter and setter: assigning to it enables or disables the tenure promotion tracking. Reading the accessor produces true if the Debugger is logging promotions, and false otherwise. Tenure promotion tracking is initially disabled in a new Debugger.
maxTenurePromotionsLogLength : The maximum number of entries to accumulate in the tenure promotions log at a time. This accessor can be both fetched and stored to. Its default value is 5000.
tenurePromotionsLogOverflowed : Returns true if there have been more than [maxTenurePromotionsLogLength][#max-tenure-log] allocations since the last time [drainTenurePromotionsLog][#drain-tenure-log] was called and some data has been lost. Returns false otherwise.
Similar to [Debugger's handler functions][debugger], Debugger.Memory inherits accessor properties that store handler functions for SpiderMonkey to call when given events occur in debuggee code.
Unlike Debugger's hooks, Debugger.Memory‘s handlers’ return values are not significant, and are ignored. The handler functions receive the Debugger.Memory's owning Debugger instance as their this value. The owning Debugger's uncaughtExceptionHandler is still fired for errors thrown in Debugger.Memory hooks.
On a new Debugger.Memory instance, each of these properties is initially undefined. Any value assigned to a debugging handler must be either a function or undefined; otherwise a TypeError is thrown.
Handler functions run in the same thread in which the event occurred. They run in the compartment to which they belong, not in a debuggee compartment.
onGarbageCollection(statistics) : A garbage collection cycle spanning one or more debuggees has just been completed.
The *statistics* parameter is an object containing information about the GC
cycle. It has the following properties:
`collections`
: The `collections` property's value is an array. Because SpiderMonkey's
collector is incremental, a full collection cycle may consist of
multiple discrete collection slices with the JS mutator running
interleaved. For each collection slice that occurred, there is an entry
in the `collections` array with the following form:
<pre class='language-js'><code>
{
"startTimestamp": <i>timestamp</i>,
"endTimestamp": <i>timestamp</i>,
}
</code></pre>
Here the *timestamp* values are [timestamps][] of the GC slice's start
and end events.
`reason`
: A very short string describing the reason why the collection was
triggered. Known values include the following:
* "API"
* "EAGER_ALLOC_TRIGGER"
* "DESTROY_RUNTIME"
* "DESTROY_CONTEXT"
* "LAST_DITCH"
* "TOO_MUCH_MALLOC"
* "ALLOC_TRIGGER"
* "DEBUG_GC"
* "COMPARTMENT_REVIVED"
* "RESET"
* "OUT_OF_NURSERY"
* "EVICT_NURSERY"
* "FULL_STORE_BUFFER"
* "SHARED_MEMORY_LIMIT"
* "PERIODIC_FULL_GC"
* "INCREMENTAL_TOO_SLOW"
* "DOM_WINDOW_UTILS"
* "COMPONENT_UTILS"
* "MEM_PRESSURE"
* "CC_WAITING"
* "CC_FORCED"
* "LOAD_END"
* "PAGE_HIDE"
* "NSJSCONTEXT_DESTROY"
* "SET_NEW_DOCUMENT"
* "SET_DOC_SHELL"
* "DOM_UTILS"
* "DOM_IPC"
* "DOM_WORKER"
* "INTER_SLICE_GC"
* "REFRESH_FRAME"
* "FULL_GC_TIMER"
* "SHUTDOWN_CC"
* "FINISH_LARGE_EVALUATE"
* "USER_INACTIVE"
`nonincrementalReason`
: If SpiderMonkey's collector determined it could not incrementally
collect garbage, and had to do a full GC all at once, this is a short
string describing the reason it determined the full GC was necessary.
Otherwise, `null` is returned. Known values include the following:
* "GC mode"
* "malloc bytes trigger"
* "allocation trigger"
* "requested"
`gcCycleNumber`
: The GC cycle's "number". Does not correspond to the number
of GC cycles that have run, but is guaranteed to be monotonically
increasing.
Debugger.Memory.prototype ObjectdrainAllocationsLog() : When trackingAllocationSites is true, this method returns an array of recent Object allocations within the set of debuggees. Recent is defined as the maxAllocationsLogLength most recent Object allocations since the last call to drainAllocationsLog. Therefore, calling this method effectively clears the log.
Objects in the array are of the form:
<pre class='language-js'><code>
{
"timestamp": <i>timestamp</i>,
"frame": <i>allocationSite</i>,
"class": <i>className</i>,
"constructor": <i>constructorName</i>,
"size": <i>byteSize</i>,
"inNursery": <i>inNursery</i>,
}
</code></pre>
Where
* *timestamp* is the [timestamp][timestamps] of the allocation event.
* *allocationSite* is an allocation site (as a
[captured stack][saved-frame]). Note that this property can be null if the
object was allocated with no JavaScript frames on the stack.
* *className* is the string name of the allocated object's internal
`[[Class]]` property, for example "Array", "Date", "RegExp", or (most
commonly) "Object".
* *constructorName* is the constructor function's display name for objects
created by `new Ctor`. If that data is not available, or the object was
not created with a `new` expression, this property is `null`.
* *byteSize* is the size of the object in bytes.
* *inNursery* is true if the allocation happened inside the nursery. False
if the allocation skipped the nursery and started in the tenured heap.
When `trackingAllocationSites` is `false`, `drainAllocationsLog()` throws an
`Error`.
drainTenurePromotionsLog : When trackingTenurePromotions is true, this method returns an array of recent promotions from the nursery to the tenured heap within this Debugger's set of debuggees. Recent is defined as the maxTenurePromotionsLogLength most recent promotions since the last call to drainTenurePromotionsLog. Therefore, calling this method effectively clears the log.
Objects in the array are of the form:
<pre class='language-js'><code>
{
"timestamp": <i>timestamp</i>,
"frame": <i>allocationSite</i>,
"class": <i>className</i>,
"size": <i>byteSize</i>,
}
</pre>
Where
* *timestamp* is the [timestamp][timestamps] of the allocation event.
* *allocationSite* is an allocation site (as a
[captured stack][saved-frame]) if the promoted object's allocation site
was captured. Note that this property can be `null` if the object was
allocated with no JavaScript frames on the stack, the object's allocation
site was not [sampled](#alloc-sampling-probability), or if allocation
sites are not being [tracked](#trackingallocationsites').
* *className* is the string name of the allocated object's internal
`[[Class]]` property, for example "Array", "Date", "RegExp", or (most
commonly) "Object".
* *byteSize* is the size of the newly tenured object (within the tenured
heap, not the nursery) in bytes.
When `trackingTenurePromotions` is `false`, `drainTenurePromotionsLog()`
throws an `Error`.
takeCensus(options) : Carry out a census of the debuggee compartments' contents. A census is a complete traversal of the graph of all reachable memory items belonging to a particular Debugger's debuggees. The census produces a count of those items, broken down by various criteria.
The <i>options</i> argument is an object whose properties specify how the
census should be carried out.
If <i>options</i> has a `breakdown` property, that determines how the census
categorizes the items it finds, and what data it collects about them. For
example, if `dbg` is a `Debugger` instance, the following performs a simple
count of debuggee items:
dbg.memory.takeCensus({ breakdown: { by: 'count' } })
That might produce a result like:
{ "count": 1616, "bytes": 93240 }
Here is a breakdown that groups JavaScript objects by their class name, and
non-string, non-script items by their C++ type name:
{
by: "coarseType",
objects: { by: "objectClass" },
other: { by: "internalType" }
}
which produces a result like this:
{
"objects": {
"Function": { "count": 404, "bytes": 37328 },
"Object": { "count": 11, "bytes": 1264 },
"Debugger": { "count": 1, "bytes": 416 },
"ScriptSource": { "count": 1, "bytes": 64 },
// ... omitted for brevity...
},
"scripts": { "count": 1, "bytes": 0 },
"strings": { "count": 701, "bytes": 49080 },
"other": {
"js::Shape": { "count": 450, "bytes": 0 },
"js::BaseShape": { "count": 21, "bytes": 0 },
"js::ObjectGroup": { "count": 17, "bytes": 0 }
}
}
In general, a `breakdown` value has one of the following forms:
<code>{ by: "count", count:<i>count<i>, bytes:<i>bytes</i> }</code>
: The trivial categorization: none whatsoever. Simply tally up the items
visited. If <i>count</i> is true, count the number of items visited; if
<i>bytes</i> is true, total the number of bytes the items use directly.
Both <i>count</i> and <i>bytes</i> are optional; if omitted, they
default to `true`. In the result of the census, this breakdown produces
a value of the form:
{ "count":<i>n</b>, "bytes":<i>b</i> }
where the `count` and `bytes` properties are present as directed by the
<i>count</i> and <i>bytes</i> properties on the breakdown.
Note that the census can produce byte sizes only for the most common
types. When the census cannot find the byte size for a given type, it
returns zero.
<code>{ by: "allocationStack", then:<i>breakdown</i>, noStack:<i>noStackBreakdown</i> }</code>
: Group items by the full JavaScript stack trace at which they were
allocated.
Further categorize all the items allocated at each distinct stack using
<i>breakdown</i>.
In the result of the census, this breakdown produces a JavaScript `Map`
value whose keys are `SavedFrame` values, and whose values are whatever
sort of result <i>breakdown</i> produces. Objects allocated on an empty
JavaScript stack appear under the key `null`.
SpiderMonkey only tracks allocation sites for items if requested via the
[`trackingAllocationSites`][tracking-allocs] flag; even then, it does
not record allocation sites for every kind of item that appears in the
heap. Items that lack allocation site information are counted using
<i>noStackBreakdown</i>. These appear in the result `Map` under the key
string `"noStack"`.
<code>{ by: "objectClass", then:<i>breakdown</i>, other:<i>otherBreakdown</i> }</code>
: Group JavaScript objects by their ECMAScript `[[Class]]` internal property values.
Further categorize JavaScript objects in each class using
<i>breakdown</i>. Further categorize items that are not JavaScript
objects using <i>otherBreakdown</i>.
In the result of the census, this breakdown produces a JavaScript object
with no prototype whose own property names are strings naming classes,
and whose values are whatever sort of result <i>breakdown</i> produces.
The results for non-object items appear as the value of the property
named `"other"`.
<code>{ by: "coarseType", objects:<i>objects</i>, scripts:<i>scripts</i>, strings:<i>strings</i>, other:<i>other</i> }</code>
: Group items by their coarse type.
Use the breakdown value <i>objects</i> for items that are JavaScript
objects.
Use the breakdown value <i>scripts</i> for items that are
representations of JavaScript code. This includes bytecode, compiled
machine code, and saved source code.
Use the breakdown value <i>strings</i> for JavaScript strings.
Use the breakdown value <i>other</i> for items that don't fit into any of
the above categories.
In the result of the census, this breakdown produces a JavaScript object
of the form:
<pre class='language-js'><code>
{
"objects": <i>result</i>,
"scripts": <i>result</i>,
"strings": <i>result</i>,
"other": <i>result</i>
}
</code></pre>
where each <i>result</i> is a value of whatever sort the corresponding
breakdown value produces. All breakdown values are optional, and default
to `{ type: "count" }`.
<code>{ by: "internalType", then:<i>breakdown</i> }</code>
: Group items by the names given their types internally by SpiderMonkey.
These names are not meaningful to web developers, but this type of
breakdown does serve as a catch-all that can be useful to Firefox tool
developers.
For example, a census of a pristine debuggee global broken down by
internal type name typically looks like this:
{
"JSString": { "count": 701, "bytes": 49080 },
"js::Shape": { "count": 450, "bytes": 0 },
"JSObject": { "count": 426, "bytes": 44160 },
"js::BaseShape": { "count": 21, "bytes": 0 },
"js::ObjectGroup": { "count": 17, "bytes": 0 },
"JSScript": { "count": 1, "bytes": 0 }
}
In the result of the census, this breakdown produces a JavaScript object
with no prototype whose own property names are strings naming types,
and whose values are whatever sort of result <i>breakdown</i> produces.
<code>[ <i>breakdown</i>, ... ]</code>
: Group each item using all the given breakdown values. In the result of
the census, this breakdown produces an array of values of the sort
produced by each listed breakdown.
To simplify breakdown values, all `then` and `other` properties are optional.
If omitted, they are treated as if they were `{ type: "count" }`.
If the `options` argument has no `breakdown` property, `takeCensus` defaults
to the following:
<pre class='language-js'><code>
{
by: "coarseType",
objects: { by: "objectClass" },
other: { by: "internalType" }
}
</code></pre>
which produces results of the form:
<pre class='language-js'><code>
{
objects: { <i>class</i>: <i>count</i>, ... },
scripts: <i>count</i>,
strings: <i>count</i>,
other: { <i>type name</i>: <i>count</i>, ... }
}
</code></pre>
where each <i>count</i> has the form:
<pre class='language-js'><code>
{ "count": <i>count</i>, bytes:<i>bytes</i> }
</code></pre>
Because performing a census requires traversing the entire graph of objects
in debuggee compartments, it is an expensive operation. On developer
hardware in 2014, traversing a memory graph containing roughly 130,000 nodes
and 410,000 edges took roughly 100ms. The traversal itself temporarily
allocates one hash table entry per node (roughly two address-sized words) in
addition to the per-category counts, whose size depends on the number of
categories.
Memory analysis may yield surprising results, because browser implementation details that are transparent to content JavaScript often have visible effects on memory consumption. Web developers need to know their pages' actual memory consumption on real browsers, so it is correct for the tool to expose these behaviors, as long as it is done in a way that helps developers make decisions about their own code.
This section covers some areas where Firefox's actual behavior deviates from what one might expect from the specified behavior of the web platform.
SpiderMonkey objects usually use less memory than the naïve “table of properties with attributes” model would suggest. For example, it is typical for many objects to have identical sets of properties, with only the properties' values varying from one object to the next. To take advantage of this regularity, SpiderMonkey objects with identical sets of properties may share their property metadata; only property values are stored directly in the object.
Array objects may also be optimized, if the set of live indices is dense.
SpiderMonkey has three representations of strings:
Normal: the string's text is counted in its size.
Substring: the string is a substring of some other string, and points to that string for its storage. This representation may result in a small string retaining a very large string. However, the memory consumed by the string itself is a small constant independent of its size, since it is simply a reference to the larger string, a start position, and a length.
Concatenations: When asked to concatenate two strings, SpiderMonkey may elect to delay copying the strings' data, and represent the result simply as a pointer to the two original strings. Again, such a string retains other strings, but the memory consumed by the string itself is a small constant independent of its size, since it is simply a pair of pointers.
SpiderMonkey converts strings from the more complex representations to the simpler ones when it pleases. Such conversions usually increase memory consumption.
SpiderMonkey shares some strings amongst all web pages and browser JS. These shared strings, called atoms, are not included in censuses' string counts.
SpiderMonkey has a complex, hybrid representation of JavaScript code. There are four representations kept in memory:
Source code. SpiderMonkey retains a copy of most JavaScript source code.
Compressed source code. SpiderMonkey compresses JavaScript source code, and de-compresses it on demand. Heuristics determine how long to retain the uncompressed code.
Bytecode. This is SpiderMonkey's parsed representation of JavaScript. Bytecode can be interpreted directly, or used as input to a just-in-time compiler. Source is parsed into bytecode on demand; functions that are never called are never parsed.
Machine code. SpiderMonkey includes several just-in-time compilers, each of which translates JavaScript source or bytecode to machine code. Heuristics determine which code to compile, and which compiler to use. Machine code may be dropped in response to memory pressure, and regenerated as needed.
Furthermore, SpiderMonkey tracks which types of values have appeared in variables and object properties. This type information can be large.
In a census, all the various forms of JavaScript code are placed in the "script" category. Type information is accounted to the "types" category.