Memories From A Window High Quality
Real-world memories are formed in a particular context and are often not acquired or recalled in isolation1-5. Time is a key variable in the organization of memories, as events that are experienced close in time are more likely to be meaningfully associated, whereas those that are experienced with a longer interval are not1-4. How the brain segregates events that are temporally distinct is unclear. Here we show that a delayed (12-24 h) increase in the expression of C-C chemokine receptor type 5 (CCR5)-an immune receptor that is well known as a co-receptor for HIV infection6,7-after the formation of a contextual memory determines the duration of the temporal window for associating or linking that memory with subsequent memories. This delayed expression of CCR5 in mouse dorsal CA1 neurons results in a decrease in neuronal excitability, which in turn negatively regulates neuronal memory allocation, thus reducing the overlap between dorsal CA1 memory ensembles. Lowering this overlap affects the ability of one memory to trigger the recall of the other, and therefore closes the temporal window for memory linking. Our findings also show that an age-related increase in the neuronal expression of CCR5 and its ligand CCL5 leads to impairments in memory linking in aged mice, which could be reversed with a Ccr5 knockout and a drug approved by the US Food and Drug Administration (FDA) that inhibits this receptor, a result with clinical implications. Altogether, the findings reported here provide insights into the molecular and cellular mechanisms that shape the temporal window for memory linking.
Memories From a Window
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In mice, two fear-associated memories that are created close in time are represented in the brain's amygdala by the activation of overlapping ensembles of neurons. As a result, eliminating the fear of one memory also extinguishes fear of the other.
Debugger windows like Watch, Autos, Locals, and the QuickWatch dialog show you variables, which are stored at specific locations in memory. The Memory window shows you the overall picture. The memory view is convenient for examining large pieces of data (buffers or large strings, for example) that don't display well in the other windows.
To enable the Memory windows, Enable address-level debugging must be selected in Tools > Options (or Debug > Options) > Debugging > General.
Under Debug > Windows > Memory, select Memory 1, Memory 2, Memory 3, or Memory 4. (Some editions of Visual Studio offer only one Memory window.)
You can instantly go to a specified address in the Memory window by using drag-and-drop, or by entering the address in the Address field. The Address field accepts alphanumeric addresses, and expressions that evaluate to addresses, such as e.User.NonroamableId.
By default, the Memory window treats Address expressions as live expressions, which are re-evaluated as the app runs. Live expressions can be useful, for example, to view the memory that is touched by a pointer variable.
Drag and drop the address or pointer in the Memory window. That address then appears in the Address field, and the Memory window adjusts to display that address at the top.
By default, memory contents appear as 1-byte integers in hexadecimal format, and the window width determines the number of columns shown. You can customize the way the Memory window shows memory contents.
When you use a debug command such as Step, the memory address displayed in the Address field and at the top of the Memory window automatically changes as the pointer changes.
This is an example of a detached window. The popup window was closed, but our code has a reference to it that prevents the browser from being able to destroy it and reclaim that memory.When a page calls window.open() to create a new browser window or tab, a Window object is returned that represents the window or tab. Even after such a window has been closed or the user has navigated it away, the Window object returned from window.open() can still be used to access information about it. This is one type of detached window: because JavaScript code can still potentially access properties on the closed Window object, it must be kept in memory. If the window included a lot of JavaScript objects or iframes, that memory can't be reclaimed until there are no remaining JavaScript references to the window's properties. Using Chrome DevTools to demonstrate how it's possible to retain a document after a window has been closed.The same issue can also occur when using <iframe> elements. Iframes behave like nested windows that contain documents, and their contentWindow property provides access to the contained Window object, much like the value returned by window.open(). JavaScript code can keep a reference to an iframe's contentWindow or contentDocument even if the iframe is removed from the DOM or its URL changes, which prevents the document from being garbage collected since its properties can still be accessed. Demonstration of how an event handler can retain an iframe's document, even after navigating the iframe to a different URL.In cases where a reference to the document within a window or iframe is retained from JavaScript, that document will be kept in-memory even if the containing window or iframe navigates to a new URL. This can be particularly troublesome when the JavaScript holding that reference doesn't detect that the window/frame has navigated to a new URL, since it doesn't know when it becomes the last reference keeping a document in memory.How detached windows cause memory leaks #When working with windows and iframes on the same domain as the primary page, it's common to listen for events or access properties across document boundaries. For example, let's revisit a variation on the presentation viewer example from the beginning of this guide. The viewer opens a second window for displaying speaker notes. The speaker notes window listens forclick events as its cue to move to the next slide. If the user closes this notes window, the JavaScript running in the original parent window still has full access to the speaker notes document:
Imagine we close the browser window created by showNotes() above. There's no event handler listening to detect that the window has been closed, so nothing is informing our code that it should clean up any references to the document. The nextSlide() function is still "live" because it is bound as a click handler in our main page, and the fact that nextSlide contains a reference to notesWindow means the window is still referenced and can't be garbage collected. Illustration of how references to a window prevent it from being garbage collected once closed.See Solution: communicate over postMessage to learn how to fix this particular memory leak.There are a number of other scenarios where references are accidentally retained that prevent detached windows from being eligible for garbage collection:Event handlers can be registered on an iframe's initial document prior to the frame navigating to its intended URL, resulting in accidental references to the document and the iframe persisting after other references have been cleaned up.
A memory-heavy document loaded in a window or iframe can be accidentally kept in-memory long after navigating to a new URL. This is often caused by the parent page retaining references to the document in order to allow for listener removal.
When passing a JavaScript object to another window or iframe, the Object's prototype chain includes references to the environment it was created in, including the window that created it. This means it's just as important to avoid holding references to objects from other windows as it is to avoid holding references to the windows themselves.
Tracking down memory leaks can be tricky. It is often difficult to construct isolated reproductions of these issues, particularly when multiple documents or windows are involved. To make things more complicated, inspecting potential leaked references can end up creating additional references that prevent the inspected objects from being garbage collected. To that end, it's useful to start with tools that specifically avoid introducing this possibility.A great place to start debugging memory problems is to take a heap snapshot. This provides a point-in-time view into the memory currently used by an application - all the objects that have been created but not yet garbage-collected. Heap snapshots contain useful information about objects, including their size and a list of the variables and closures that reference them.A heap snapshot showing the references that retain a large object.To record a heap snapshot, head over to the Memory tab in Chrome DevTools and select Heap Snapshot in the list of available profiling types. Once the recording has finished, the Summary view shows current objects in-memory, grouped by constructor. Demonstration of taking a heap snapshot in Chrome DevTools.Try itOpen this step-by-step walk through in a new window.Analyzing heap dumps can be a daunting task, and it can be quite difficult to find the right information as part of debugging. To help with this, Chromium engineers yossik@ and peledni@ developed a standalone Heap Cleaner tool that can help highlight a specific node like a detached window. Running Heap Cleaner on a trace removes other unnecessary information from the retention graph, which makes the trace cleaner and much easier to read.Measure memory programmatically #Heap snapshots provide a high level of detail and are excellent for figuring out where leaks occur, but taking a heap snapshot is a manual process. Another way to check for memory leaks is to obtain the currently used JavaScript heap size from the performance.memory API:Checking the used JS heap size in DevTools as a popup is created, closed and unreferenced.The performance.memory API only provides information about the JavaScript heap size, which means it doesn't include memory used by the popup's document and resources. To get the full picture, we'd need to use the new performance.measureUserAgentSpecificMemory() API currently being trialled in Chrome.Solutions for avoiding detached window leaks #The two most common cases where detached windows cause memory leaks are when the parent document retains references to a closed popup or removed iframe, and when unexpected navigation of a window or iframe results in event handlers never being unregistered.Example: Closing a popup #The unset references, monitor and dispose, and WeakRef solutions are all based off of this example.In the following example, two buttons are used to open and close a popup window. In order for the Close Popup button to work, a reference to the opened popup window is stored in a variable: 041b061a72