How does the iOS platform handle memory-mapped files during low-memory scenarios? By low-memory scenarios, I mean when the OS sends the UIApplicationDidReceiveMemoryWarningNotification notification to all observers in the application.
Our files are mapped into memory using +[NSData dataWithContentsOfMappedFile:], the documentation for which states:
A mapped file uses virtual memory techniques to avoid copying pages of the file into memory until they are actually needed.
Does this mean that the OS will also unmap the pages when they're no longer in use? Is it possible to mark pages as being no longer in use? This data is read-only, if that changes the scenario. How about if we were to use mmap() directly? Would this be preferable?
Memory-mapped files copy data from disk into memory a page at a time. Unused pages are free to be swapped out, the same as any other virtual memory, unless they have been wired into physical memory using mlock(2). Memory mapping leaves the determination of what to copy from disk to memory and when to the OS.
Dropping from the Foundation level to the BSD level to use mmap is unlikely to make much difference, beyond making code that has to interface with other Foundation code somewhat more awkward.
(This is not an answer, but it would be useful information.)
From #ID_AA_Carmack tweet,
#ID_AA_Carmack are iOS memory mapped files automatically unmapped in low memory conditions? (using +[NSData dataWithContentsOfMappedFile]?)
ID_AA_Carmack replied for this,
#KhrobEdmonds yes, that is one of the great benefits of using mapped files on iOS. I use mmap(), though.
I'm not sure that is true or not...
From my experiments NSData does not respond to memory warnings. I tested by creating a memory mapped NSData and accessing parts of the file so that it would be loaded into memory and finally sending memory warnings. There was no decrease in memory usage after the memory warning. Nothing in the documentation says that a memory will cause NSData to reduce real memory usage in low memory situations so it leads me to believe that it does not respond to memory warnings. For example NSCache documentation says that it will try and play nice with respect to memory usage plus I have been told it responds to the low memory warnings the system raises.
Also in my simple tests on an iPod Touch (4th gen) I was able to map about 600 megs of file data into virtual memory use +[NSData dataWithContentsOfMappedFile:]. Next I started to access pages via the bytes property on the NSData instance. As I did this real memory started to grow however it stopped growing at around 30 megs of real memory usage. So the way it is implemented it seems to cap how much real memory will be used.
In short if you want to reduce memory usage of NSData objects the best bet is to actually make sure they are completely released and not relying on anything the system automagically does on your behalf.
If iOS is like any other Unix -- and I would bet money it is in this regard -- pages in an mmap() region are not "swapped out"; they are simply dropped (if they are clean) or are written to the underlying file and then dropped (if they are dirty). This process is called "evicting" the page.
Since your memory map is read-only, the pages will always be clean.
The kernel will decide which pages to evict when physical memory gets tight.
You can give the kernel hints about which pages you would prefer it keep/evict using posix_madvise(). In particular, POSIX_MADV_DONTNEED tells the kernel to feel free to evict the pages; or as you say, "mark pages as being no longer in use".
It should be pretty simple to write some test programs to see whether iOS honors the "don't need" hint. Since it is derived from BSD, I bet it will.
Standard virtual memory techniques for file-backed memory says that the OS is free to throw away pages whenever it wants because it can always get them again later. I have not used iOS, but this has been the behavior of virtual memory on many other operating systems for a long time.
The simplest way to test it is to map several large files into memory, read through them to guarantee that it pages them into memory, and see if you can force a low memory situation. If you can't, then the OS must have unmapped the pages once it decided that they were no longer in use.
The dataWithContentsOfMappedFile: method is now deprecated from iOS5.
Use mmap, as you will avoid these situations.
Related
I get frequent memory warnings in my application but I don't know why.
Here is the snapshot of allocation instruments.
I know that we don't have any control over virtual memory assigned to us but I am trying to understand what information does that number 26.50 MB means for a developer.
1. What does a high VM means ? Does it lead to a jetsam ? Is that cause of any other concern ?
2. Is this value dependent on device ?
3. Does a low vm means that your app is memory efficient
4. Does a high VM leads to memory warnings in your app ?
5. What cause this value to change ?
6. What steps should a developer take when they see a high vm for their app (like 300 MB) ?
7. Is VM tracker instrument related to this value ?
Anonymous VM covers a lot of things, some of which are things you want to minimize and some that are generally less important. The short version of "anonymous VM" is that it's addresses you have mapped but not named. Heap allocations get "named" which lets you track them as objects. But there are lots (and lots) of non-objecty things that fall into the "anonymous VM" bucket.
Things allocated with malloc can wind up in this region. But also memory mapped files. Your executable is a memory mapped file, but since it's never dirty, parts of it can be swapped out. So "it's complicated." But in big, vague terms, yes, you do care about this section, but you may not care about all of it very much. Heap allocations tends to track your ObjC stuff. Anonymous VM often tracks things that you don't have a lot of direct control over (like CALayer backing storage).
All that said, the Instruments output you provide doesn't look like any major problem. I suspect it's not indicative of a time you're pressuring memory. You'll need to get yourself into a memory warning situation and see what's going on then, and dig into the specifics of what is using memory.
For much more detail on this, you should watch WWDC 2013 session 704 "Building Efficient OS X Apps" which goes into depth on much of this. While iOS has a somewhat different memory system, and some OS X tools aren't available on iOS, many of the concepts still apply.
I am working on a Browser application in which I use a UIWebView for opening web pages. I run the Instruments tool with Memory Monitor. I am totally confused by the terms which are used in Instruments and why they're important. Please explain some of my questions with proper reasons:
Live Bytes is important for checking memory optimization or memory consumption? Why ?
Why would I care about the Overall Bytes/ Real Memory, if it contains also released objects?
When and why are these terms used (Live Bytes/ Overall Bytes/Real Memory)?
Thanks
"Live Bytes" means "memory which has been allocated, but not yet deallocated." It's important because it's the most easily graspable measure of "how much memory your app is using."
"Overall Bytes" means "all memory which has ever been allocated including memory that has been deallocated." This is less useful, but gives you some idea of "heap churn." Churn leads to fragmentation, and heap fragmentation can be a problem (albeit a pretty obscure one these days.)
"Real Memory" is an attempt to distinguish how much physical RAM is in use (as opposed to how many bytes of address space are valid). This is different from "Live Bytes" because "Live Bytes" could include ranges of memory that correspond to memory-mapped files (or shared memory, or window backing stores, or whatever) that are not currently paged into physical RAM. Even if you don't use memory-mapped files or other exotic VM allocation methods, the system frameworks do, and you use them, so this distinction will always have some importance to every process.
EDIT: Since you're clearly concerned about memory use incurred by using UIWebView, let me see if I can shed some light on that:
There is a certain memory "price" to using UIWebView at all (i.e. global caches and the like). These include various global font caches, JavaScript JIT caches, and stuff like that. Most of these are going to behave like singletons: allocated the first time you use them (indirectly by using UIWebView) and never deallocated until the process ends. There are also some variable size global caches (like those that cache web responses; CFURL typically manages these) but those are expected to be managed by the system. The collective "weight" of these things with respect to UIWebView is, as you've seen, non-trivial.
I don't have any knowledge of UIKit or WebKit internals, but I would expect that if you had a discussion with someone who did, their response to the question of "Why is my use of UIWebView causing so much memory use?" would be two pronged: The first prong would be "this is the price of admission for using UIWebView -- it's basically like running a whole web browser in your process." The second prong would be "system framework caches are automatically managed by the system" by which they would mean that, for instance, the CFURL caches (which is one of the things that using UIWebView causes to be created) are managed by the system, so if a memory warning came in, the system frameworks would be responsible for evicting things from those caches to reduce the memory consumed by them; you have no control over those, and you just have to trust that the system frameworks will do what needs to be done. (That doesn't help you in the case where whatever the system cache managers do isn't aggressive enough for you, but you're not going to get any more control over them, so you need to attack the issue from another angle, either way.) If you're wondering why the memory use doesn't go down once you deallocate your UIWebView, this is your answer. There's a bunch of stuff it's doing behind the scenes, that you can't control.
The expectation that allocating, using, and then deallocating a UIWebView is a net-zero operation ignores some non-trivial, inherent and unavoidable side-effects. The existence of such side-effects is not (in and of itself) indicative of a bug in UIWebView. There are side effects like this all over the place. If you were to create a trivial application that did nothing but launch and then terminate after one spin of the run loop, and you set a breakpoint on exit(), and looked at the memory that had been allocated and never freed, there would be thousands of allocations. This is a very common pattern used throughout the system frameworks and in virtually every application.
What does this mean for you? It means that you effectively have two choices: Use UIWebView and pay the "price of admission" in memory consumption, or don't use UIWebView.
I'm planning an application which will involve loading many pictures at one time and thus requires a large chunk of memory. For example, I might have 50 image objects created at once, taking a total of 1GB of RAM. But when the user goes to load 20 more pictures, I'd like to make sure that amount of memory is already reserved and ready.
Now this part might seem a little backwards from normal. Rather than specifying how much memory my application shall reserve, instead I need to specify how much memory to leave free for other applications, and adjust my application's memory periodically according to this specification. I must say I've never worked with reserving memory at all, and especially won't know how to leave this remaining available memory.
So for example, if the computer has 2048 MB of RAM, and the option is set to leave 50 MB free for other applications, and there is already 10MB of RAM being used by other apps, then it should reserve 2048-50-10 = 1988 MB for my app.
The trouble I foresee is suppose the user opens another application which requires 1GB. My app has to catch this and shrink its self.
Does this even sound like a feasible approach? Basically, I need to make sure there is as much memory reserved as possible at any given time, while leaving a decent amount available for other apps. Would it make a significant impact on performance if I do this, or not much at all? I might be loading and unloading images at rapid paces, and I don't want it to reserve/free this memory on demand, I want it to stay reserved.
+1 for Sertac's mentioning of how SQL Server rides the line of allocating memory it needs, but releasing memory when Windows complains.
Applications can receive Window's complaints by using the CreateMemoryResourceNotification:
hLowMemory := CreateMemoryResourceNotification(LowMemoryResourceNotification);
Applications can use memory resource notification events to scale the
memory usage as appropriate. If available memory is low, the
application can reduce its working set. If available memory is high,
the application can allocate more memory.
Any thread of the calling
process can specify the memory resource notification handle in a call
to the QueryMemoryResourceNotification function or one of the wait functions.
The state of the object is signaled when the specified
memory condition exists. This is a system-wide event, so all
applications receive notification when the object is signaled. Note
that there is a range of memory availability where neither the
LowMemoryResourceNotification or HighMemoryResourceNotification object
is signaled. In this case, applications should attempt to keep the
memory use constant.
But it's also worth mentioning that you might as well allocate memory that you need. Your operating system has a very sophisiticated set of algorithms to swap out the least used memory when memory pressure is high. You can take advantage of this by simply allocating all the memory that you need. When Windows starts to run low, it will find those pages of memory that you are using the least and swap them out to disk. (This is how a well-known reverse proxy works).
The only thing left is to decide if you want to free some images when Windows says it's running low on RAM. But if you're not using the memory, it is going to be swapped out to disk for you.
It's not realistic to account for other apps. Just ignore them. The system will page things in and out as needed. If you really wanted to do this you'd have to dynamically adapt to other processes as they start and finish. That's really not realistic. What's more it's not practical to inquire of other processes how much memory they need. Leave it all to the system.
Set a budget for your app and make sure you don't exceed it. Keep the most recently used images in memory and when you approach your memory budget throw away the least recently used images to make space.
If you are stressing the available resources then make sure you use FastMM and enable LARGE_ADDRESS_AWARE for your app so that you get 4GB address space when running on a 64 bit OS.
I was reading an article on memory fragmentation when I recalled that there are several examples of software that claim to defragment memory. I got curious, how does it work? Does it work at all?
EDIT:
xappymah gave a good argument against memory defragmentation in that a process might be very surprised to learn that its memory layout suddenly changed. But as I see it there's still the possibility of the OS providing some sort of API for global memory control. It does seem a bit unlikely however since it would give rise to the possibility of using it in malicious intent, if badly designed. Does anyone know if there is an OS out there that supports something of the sort?
The real memory defragmentation on a process level is possible only in managed environments such as, for example, Java VMs when you have some kind of an access to objects allocated in memory and can manage them.
But if we are talking about the unmanaged applications then there is no possibility to control their memory with third-party tools because every process (both the tool and the application) runs in its own address space and doesn't have access to another's one, at least without help from OS.
However even if you get access to another process's memory (by hacking your OS or else) and start modifying it I think the target application would be very "surprised".
Just imagine, you allocated a chunk of memory, got it's starting address and on the next second this chunk of memory is moved somewhere else because of "VeryCoolMemoryDefragmenter" :)
In my opinion memory it's a kind of Flash Drive, and this chip don't get fragmented because there aren't turning disks pins recording and playing information, in a random way, like a lie detector. This is the way that Hard Disk Fragmentation it's done. That's why SSD drives are so fast, effective, reliable and maintenance free. SSD it's a BIG piece of memory and it kind of look alike.
I heard many libraries such as JXTA and PjSIP have smaller footprints. Is this pointing to small resource consumption or something else?
Footprint designates the size occupied by your application in computer RAM memory.
Footprint can have different meaning when speaking about memory consumption.
In my experience, memory footprint often doesn't include memory allocated on the heap (dynamic memory), or resource loaded from disc etc. This is because dynamic allocations are non constant and may vary depending on how the application or module is used. When reporting "low footprint" or "high footprint", a constant or top measure of the required space is usually wanted.
If for example including dynamic memory in the footprint report of an image editor, the footprint would entirely depend on the size of the image loaded into the application by the user.
In the context of a third party library, the library author can optimize the static memory footprint of the library by assuring that you never link more code into your application binary than absolutely needed. A common method used for doing this in for instance C, is to distribute library functions to separate c-files. This is because most C linkers will link all code from a c-file into your application, not just the function you call. So if you put a single function in the c-file, that's all the linker will incoporate into your application when calling it. If you put five functions in the c-file the linker will probably link all of them into your app even if you only use one of them.
All this being said, the general (academic) definition of footprint includes all kinds of memory/storage aspects.
From Wikipedia Memory footprint article:
Memory footprint refers to the amount of main memory that a program uses or references while running.
This includes all sorts of active memory regions like code segment containing (mostly) program instructions (and occasionally constants), data segment (both initialized and uninitialized), heap memory, call stack, plus memory required to hold any additional data structures, such as symbol tables, debugging data structures, open files, shared libraries mapped to the current process, etc., that the program ever needs while executing and will be loaded at least once during the entire run.
Generally it's the amount of memory it takes up - the 'footprint' it leaves in memory when running. However it can also refer to how much space it takes up on your harddrive - although these days that's less of an issue.
If you're writing an app and have memory limitations, consider running a profiler to keep track of how much your program is using.
It does refer to resources. Particularly memory. It requires a smaller amount of memory when running.
yes, resources such as memory or disk
Footprint in Computing i-e for computer programs or Computer machines is referred as the occupied device memory , for a program , process , code ,etc