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@apollographql, @meteorjs, etc.

Recently, Avi and David tracked down a surprising Clearance Genuine Jeans On Sale Denim Cotton 2017 31 32 34 35 36 Fay Cheap Really RY2VKh
in Meteor’s live HTML template rendering system. The fix will be in the 0.6.5 release (in its final stages of QA right now).

I searched the web for variations on and came up with nothing relevant, so it seemed like this is a relatively little-known issue in the JavaScript context. (Most of what you find for that query talks about the bad garbage collection algorithm in old versions of IE, but this problem affects even my current Chrome install!) I later found a great blog post on this subject by a V8 developer, but it still seems like most users of JavaScript don't know they have to watch out for this.

JavaScript is secretly a functional programming language, and its functions are : function objects get access to variables defined in their enclosing scope, even when that scope is finished. Local variables which are captured by a closure are garbage collected once the function they are defined in has finished and all functions defined inside their scope are themselves GCed.

Now, consider this code:

Every second, we’ll execute the function. It replaces with a new object containing a newly allocated giant string, saving the original value of in the local variable . After it returns, the old value of can be garbage collected, including the long string inside it, since nothing remains that refers to it. So the memory used by this code is roughly constant: it does keep allocating big strings, but each time it can delete the previous big string.

But what if we had a closure that outlasted ?

The body of could theoretically refer to to , so shouldn't be retained for as long as the is alive? This would lead to boundless memory growth, since each version of would essentially hang on to a pointer to the previous version.

Fortunately, modern JavaScript implementations (including, say, the current V8 in Chrome and Node) are smart enough to notice that isn't used in the closure, so it's not put into 's lexical environment, and it's OK to GC the previous when finishes.

(But wait, you ask! What if somebody had earlier run , and so the seemingly innocuous line was actually ing some code that refers to ? Well, the JavaScript standard is one step ahead of you. If you use in a sneaky fashion like this (in any way other than by just calling it ), it's called an "indirect ", and it doesn't actually get to access the lexical environment! If, on the other hand, did contain a direct call to with that name, it could in fact access, and the JavaScript environment is forbidden from keeping out of the lexical environment and this would end up leaking.)

Well, great! JavaScript protects us from memory leaks, right? Well, let’s try one more version, combining the first two examples.

Open up the Timeline tab in your Chrome Developer Tools, switch to the Memory view, and hit record:

Looks like we’re using an extra megabyte every second! And even clicking the garbage can icon to force a manual GC doesn’t help. So it looks like we are leaking .

But isn’t this just the same situation as before? is only referenced in the main body of , and in . itself (which we never even run!) gets cleaned up once ends... the only thing from that escapes is the second closure, . And doesn't refer to at all!

So even though there’s no way for any code to ever refer to again, it never gets garbage collected! Why? Well, the typical way that closures are implemented is that every function object has a link to a dictionary-style object representing its lexical scope. If both functions defined inside actually used , it would be important that they both get the same object, even if gets assigned to over and over, so both functions share the same lexical environment. Now, Chrome's V8 JavaScript engine is apparently smart enough to keep variables out of the lexical environment if they aren't used by closures: that's why the first example doesn't leak.

But as soon as a variable is used by closure, it ends up in the lexical environment shared by closures in that scope. And that can lead to memory leaks.

You could imagine a more clever implementation of lexical environments that avoids this problem. Each closure could have a dictionary containing only the variables which it actually reads and writes; the values in that dictionary would themselves be mutable cells that could be shared among the lexical environments of multiple closures. Based on my casual reading of the ECMAScript 5th Edition standard, this would be legitimate: its description of Lexical Environment describes them as being “purely specification mechanisms [which] need not correspond to any specific artefact of an ECMAScript implementation”. That said, this standard doesn’t actually contain the word “garbage” and only says the word “memory” once. (Here’s a paper about this issue with an optimal implementation , though not in a JavaScript context. Some other language implementations do get this right, such as Go.)

Fixing memory leaks of this form, once you notice them, is straightforward, as demonstrated by the fix to the Meteor bug . Just add to the end of . That way, even though the name is still in the lexical environment of , there won't be a link to the big old value.

So in summary: If you have a large object that is used by some closures, but not by any closures that you need to keep using, just make sure that the local variable no longer points to it once you’re done with it. Unfortunately, these bugs can be pretty subtle; it would be much better if JavaScript engines didn’t require you to have to think about them.

(Originally posted at with interesting discussion on Hacker News .)

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Epithelial cells sequestered virus We speculate that the increasing levels of p24 in the supernatant were the result of virus attachment to epithelial cells during the infection. To determine the amount of virus bound to Ect1 cells, we measured total p24 associated with Ect1 cells, after the cells were thoroughly washed following exposure to virus. To serve as a comparison, similar numbers of H9 cells were treated the same way. To control for possible viral replication in H9 cells, an aldrithiol-2 (AT-2)–inactivated RF virus was included in the analysis. AT-2 is a nucleocapsid protein zinc finger-targeting compound that inactivates the virus without affecting the viral surface proteins either conformationally or functionally [ 18 ]. In general, Ect1 cell lysates contained at least 10-fold more p24 than did H9 cells, on a per cell basis, as shown in figure 4A . We also analyzed the virus released from the Ect1 cells, compared with that released from HeLa CD4 cells, a cervical-carcinoma epithelial cell line that forms monolayers in culture. HeLa cells were known to bind HIV independent of CD4 cells [ 19 ]

Epithelial cells sequestered virus
Figure 4
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Sequestration of virus by Ect1 cell monolayers. Ect1 cell monolayers or H9 cells were exposed to live or aldrithiol-2 (AT-2)–inactivated RF virus for 1 h at 37°C. The cells were washed thoroughly and were treated with trypsin, and the cells were lysed for measurement of p24. Monolayers of Ect1 or HeLa CD4 cells, at similar confluence, were exposed to RF virus. The cells were washed thoroughly, were refed medium or were treated with trypsin, washed, and recultured. Culture supernatants were collected 5 days after exposure to virus, and RNA copy was measured by real-time polymerase chain reaction. Each data point represents mean of replicate wells and the bars represents SEs

Figure 4
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Sequestration of virus by Ect1 cell monolayers. Ect1 cell monolayers or H9 cells were exposed to live or aldrithiol-2 (AT-2)–inactivated RF virus for 1 h at 37°C. The cells were washed thoroughly and were treated with trypsin, and the cells were lysed for measurement of p24. Monolayers of Ect1 or HeLa CD4 cells, at similar confluence, were exposed to RF virus. The cells were washed thoroughly, were refed medium or were treated with trypsin, washed, and recultured. Culture supernatants were collected 5 days after exposure to virus, and RNA copy was measured by real-time polymerase chain reaction. Each data point represents mean of replicate wells and the bars represents SEs

Real-time PCR analysis was performed to quantify the virus released from Ect1 and HeLa CD4 cells that had been exposed to virus. To remove any viral particles nonspecifically associated with the cells, the monolayer was treated with trypsin and replated, and the released virus was measured. Ect1 cells released 23-fold more virus than did HeLa CD4 cells, as shown in figure 4B . Even when Ect1 cells were treated with trypsin and replated, there was almost 3-fold more virus detected in Ect1 culture than in HeLa CD4 cells that were not treated with trypsin. The observation indicated that Ect1 cells sequestered large amount of virus, even after thorough washing and treatment with trypsin

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