Paging and the Windows CE Paging Pool

Posted by: Sue Loh

I’d like to explain a little more about memory management in Windows CE. I already explained a bit about paging in Windows CE when I discussed virtual memory. In short, the OS will delay committing memory as long as possible by only allocating pages on first access (known as demand paging). And when memory is running low, the OS will page data out of memory if it corresponds to a file – a DLL or EXE, or a file-backed memory-mapped file – because the OS can always page the data from the file back into memory later. (Win32 allows you to create “memory-mapped files” which do or do not correspond to files on disk – I call these file-backed and RAM-backed memory-mapped files, respectively.) Windows CE does not use a page file, which means that non file-backed data such as heap and thread stacks is never paged out to disk. So for the discussion of paging in this blog post I’m really talking only about memory that is used by executables and by file-backed memory-mapped files.

It’s relatively easy to guess how the OS decides when to page data in to memory – it doesn’t page it in until it absolutely has to, when you actually access it. But how does the OS decide when to remove pageable data from memory? Ahh, that’s the question!

The Paging Pool and How It Works

Back in the old days of CE 3.0 or so (I’m not sure) – Windows CE did not have a paging pool. What that means is that the OS had no limit on the number of pages it could use for holding executables and memory-mapped files. If you ran a lot of programs or accessed large memory-mapped files, you’d see memory usage climb correspondingly. Usage would continue to go up until the system ran out of memory. Other allocations could fail; memory would appear to be nearly gone when really there was actually a lot of potential to free up space by paging data out again. Until finally when the system hit a low memory limit, the kernel would walk through all of the pageable data, paging everything (yes, everything) out again. Then suddenly there would be a lot of free memory, and you’d take page faults to page in any data you’re still actually using.

The algorithm is simple, but it has a few bad effects. First, a bad effect of the simple paging algorithm was, obviously, that the system could encounter preventable RAM shortages. Also, it was really tough for applications or tools to measure free memory – where “free” includes currently-unused pages plus “temporary” pages that could be decommitted when necessary. Conversely, it was difficult for users to determine how much of an application’s memory usage is fixed in RAM vs. “temporary” pageable pages. Even today it is tough to answer the question “how much memory is my process using?” in simple terms without diving into explanations of paging, cross-process shared memory, etc. Another possible problem you can encounter when there’s no paging pool is that the rest of the system can take up all of the free memory, and leave you thrashing over just a few pages.

So we introduced the paging pool. The purpose of the paging pool is to serve as a limit on the amount of memory that could be consumed by pageable data. It also includes the algorithm for choosing the order in which to remove pageable data from memory. Pool behavior is under the OEM’s control – Microsoft sets default parameters for the paging pool, but OEMs have the ability to change those settings. Applications do not have the ability to set the behavior for their own executables or memory-mapped files.

Up to and including CE 5.x, the paging pool behavior was fairly simple.

· The pool only managed read-only pageable data. Executable code is read-only so it used the pool, and so did read-only file-backed memory-mapped files. Read-write memory-mapped files did not use the pool, however. The reason is that paging out read-write data can involve writing back to a file. This is more complicated to implement and requires more care to avoid file system deadlocks and other undesirable situations. So read-write memory-mapped files had no memory usage limitations and could still consume all of the available system RAM.

· The pool had one parameter, the size. OEMs could turn the pool off by setting the size to 0. Turning off the paging pool meant that the OS did not limit pageable data – behavior would follow the pattern described above from before we had a paging pool. Turning on the pool meant that the OS would reserve a fixed amount of RAM for paging. Setting the pool size too low meant that pages could be paged out too early, while they’re still in use. Setting the pool size too high meant that the OS would reserve too much RAM for paging. Pool memory would NOT be available for applications to use if the pool was underutilized. A 4MB pool took 4MB of physical RAM, no matter whether there was only 2MB of pageable data in use or 100MB. Setting the size of the pool was a tricky job, because you had to decide whether to optimize a typical steady-state situation with several applications running (and judge how much pool those applications would need), or optimize “spike” situations such as system boot where many more pages were needed for a short period of time.

· The kernel kept a round-robin FIFO ring of pool pages: the oldest page in memory – the earliest one to be paged in – was the first one paged out when something else needed to be paged in, regardless of whether the oldest page was still in use or not.


So the short roll-up of how the paging pool worked up through CE 5.x is that the paging pool allowed OEMs to set aside a fixed amount of memory to hold read-only pageable data, and it was freed in simple round-robin fashion.

In CE 6.0, the virtual memory architecture changes involved major rewriting of the Windows CE memory system, including the paging pool. The CE 6.0 paging pool behavior is still fairly simplistic, but is a little bit more flexible.

· CE 6.0 has two paging pools – the “loader” pool for executable code, and the “file” pool which is used by all file-backed memory-mapped files as well as the new CE 6.0 file cache filter, or “cache manager.” This way, OEMs can put limitations on memory usage for read-write data in addition to read-only data. And they can set separate limitations for memory usage by code vs. data.

· The two pools have several parameters. Primary of these are target and maximum sizes. The idea is that the OS always guarantees the pool will have at least its target amount of memory to use. If memory is available, the kernel allows the pool to consume memory above its target. But when that happens, it also wakes up a low-priority thread which starts paging data out again, back down to slightly below the target. That way, during busy “spikes” of memory usage, such as during system boot, the system can consume more memory for pageable data. But in the steady-state, the system will hover near its target pool memory usage. The maximum size puts a hard limit on the memory consumption – or OEMs could set the maximum to be very large to avoid placing a limit on the pool. OEMs can also get the old pre-CE6 behavior by setting the pool target and maximum to the same size.

· Due to the details of the new CE6 memory implementation, the FIFO ring of pages by age was not possible. The CE6 kernel pages out memory by walking the lists of modules and files, paging out one module/file at a time. This is no better than the FIFO ring, but still leaves us potential for implementing better use-based algorithms in the future.


There are some more details in our documentation under “Paging Pool” and “Paging Pool: Windows CE 5.0 vs. Windows Embedded CE 6.0.”

Overall, enabling the paging pool means that there is always some RAM reserved for code paging and we will be less likely to reach low-memory conditions. In general it's better to turn on the paging pool because it gives you more predictable performance, rather than occasional long delays you’d hit when cleaning up memory when you run out. But it does need to be sized based on the applications in use, which leads to my next point...

Choosing a Pool Size

In Windows CE (embedded) 5.0, the pool is turned off by default. In Windows Mobile, the pool is turned on and set to a default size chosen by Microsoft. I believe it varies between versions, but is somewhere in the neighborhood of 4-6 MB. In CE6, the loader pool has a target size of 3MB and the file pool has a target size of 1MB. Only the OEM of a device can set the pool size; applications cannot change it.

So how do you decide on the right pool size for your platform? I’m afraid it’s still a bit of a black art. :-( There aren’t many tools to help. You can turn on CeLog during boot and see how many page faults it records. You can see the page faults in Remote Kernel Tracker, but in truth that kind of view isn’t much help here. The best tool I know is that readlog.exe will print you a page fault report if you turn on the “verbose” and “summary” options. If you get multiple faults on the same pages, your pool may be too small (you may also be unloading and re-loading the same module, ejecting its pages from memory, so look for module load events in the log too). If you don’t get many repeats, your pool may be bigger than you need. In CE6 you can use IOCTL_KLIB_GET_POOL_STATE to get additional information about how many pages are currently in your pool and how many times the kernel has had to free up pool pages to get down to the target size. There aren’t any tools like “mi” that query the pool state, so you’ll have to call the IOCTL yourself. On debug builds of the OS, there is also a debug zone in the kernel you can turn on to see a lot of detail about paging and when the pool trim thread is running. But CeLog is probably a better choice to collect all of that data.

As I already mentioned, as of CE6 you can set separate “target” and “max” values for the paging pools. I don’t really like the semantics of having a “max” – it isn’t dependent on the other usage or availability in the system. If some application takes most of the available memory in the system, you’d want the pool to let go of more pages. If you have a lot of free memory, and some application is reading a lot of file data, you’d want the pool to grow to use most of the available memory. We supported the “max” as an option to limit the pool size, but I’m starting to think the best idea is to set your max to infinity, to let the pool grow up to the size of available memory. We’ll still page out down to the target in the background. I’d have liked to add more sophisticated settings like “leave at least X amount of free memory” but that’s quite difficult to implement.

You’ll want to examine your pool behavior during important “user scenarios” like boot or running a predefined set of applications. If the user runs a lot of applications at once, or a really big application, or one that reads a lot of file data, they could go through pool pages pretty quickly. There isn’t really a lot you can do about that. We don’t even have a set of recommended scenarios for you to examine. I wish we had more information and more tools for this, but I’ve described about all we have.

The approach I think most OEMs take is that they leave the pool at the default size until they discover a perf problem with too much paging (by profiling or otherwise observing) in a scenario that's important to users. Then they bump it up until the problem goes away. Not very scientific but it works, and it's not like we have any answer that's more scientific anyway.

What goes into the paging pool

This is repeating some of the information above, but in more detail – how do you know exactly what pages will use the pool and what pages won’t? Keep in mind that paging is actually independent of the paging pool. Paging can happen with or without the paging pool. If you turn off the paging pool then you turn off the limit that we set on the amount of RAM that can be taken up for paging. But pages can still be paged. If you turn ON the paging pool then we enforce some limits, that’s all. So this isn’t really a question of what pages can use the pool, it’s a question of what pages are “pageable.”

Executables from the FILES section of ROM will use the paging pool for their code and R/O data. R/W data from executables can’t be paged out, so it will not be part of the pool. Compressed executables from the MODULES section of ROM will use the pool for their code and R/O data. If the image is running from NOR or from RAM, uncompressed executables from MODULES will run directly out of the image without using any pool memory. Executables from MODULES in images on NAND will be paged using the pool. (And by the way, I’m not terribly familiar with how we manage data on NAND/IMGFS so I might be missing some details here.)

Executables that would otherwise page but are marked as “non-pageable” will be paged fully into RAM as soon as they’re loaded, and not paged out again until they’re unloaded. These pages don’t use the pool. You can also create “partially pageable” executables by telling the linker to make individual sections of the executable non-pageable. Generally code and data can’t be pageable if it’s part of an interrupt service routine (ISR) or if it’s called during suspend/resume or other power management, because paging could cause crashes and deadlocks. And code/data shouldn’t be pageable if it’s accessed by an interrupt service thread (IST) because paging would negatively impact real-time performance.

Memory-mapped files which don’t have a file underneath them (a.k.a. RAM-backed mapfiles) will not use the pool. In CE5 and earlier, R/O file-backed mapfiles will use the pool while R/W mapfiles will not. In CE6, all file-backed memory-mapped files use the file pool. And the new file cache filter (cache manager) essentially memory-maps all open files, so the cached file data uses the file pool.

To look at that information from the opposite angle, if you are running all executables directly out of your image – all are uncompressed in the MODULES section of ROM, and the image is executing out of NOR or RAM, then the loader paging pool is probably a waste. You might still want to use the file pool to limit RAM use for file caching and memory-mapped files, but in that case you might want to turn off the loader pool.

Other Paging Pool Details

Someone once asked me whether the pool size affects demand paging. It doesn’t change demand paging behavior or timing. Demand paging is about delaying committing pages as long as possible, and it applies to pages regardless of the paging pool. Pages can be demand paged without being part of the pool; they won’t be paged in until absolutely necessary, and then they’ll stay in RAM without being paged out. Pool pages will be demand paged in, and may eventually be paged out again.

Another question was whether the paging pool uses up virtual address space. Actually, no, it doesn’t. The pool pages that are currently in use are assigned to virtual addresses that are already reserved. For example, when you load a DLL, you reserve virtual address space for the DLL; and when you touch a page in the DLL, a physical page from the pool is assigned to the already-reserved virtual address in your DLL. The pool pages that are NOT in use are not assigned virtual addresses. The kernel tracks them using their physical addresses only. The pool *does* use up physical RAM. In CE5 it uses the whole size of the paging pool; on CE6 it consumes physical memory equal to the “target” size of the pool. This guarantees that you have at least a minimum number of pages to page with, to avoid heavy thrashing over just a few pages when the rest of the memory in the system is taken.

Other Paging-Related Details

A related detail that occasionally confuses people is the “Paging” flag on file system drivers. This flag doesn’t control whether the driver code itself is pageable. Rather, it controls whether the file system allows files to be loaded into memory a page at a time or all at once. On typical file systems like FATFS the “Paging” flag is turned on, allowing executables and memory-mapped files to be accessed a page at a time. On other file system drivers, such as our release directory file system (RELFSD) and our network redirector, it’s turned off by default, causing executables and memory-mapped files to be read into memory all at once. I believe the reasoning is to improve performance and minimize problems when the network connection is lost.

This flag actually derives from the original Windows CE implementation of memory-mapped files. If the file system supported a couple of APIs, ReadFileWithSeek and WriteFileWithSeek, memory-mapped files on that file system would be pageable. If the file system did not support those APIs, the memory-mapped files would be non-pageable, in which case they’d be read entirely into RAM at load time and never paged out until the memory-mapped file is unloaded. The OS required pageability for special memory-mapped files like registry hives and CEDB database volumes, so file systems that did not support the required APIs could not hold these files. (If you ask me, there is no real need to require the seek + read/write to occur in one atomic API call, so the requirement on the “WithSeek” APIs was unnecessary, but perhaps there was a good reason back in the old days.)

As I already mentioned, the new CE6 file cache also uses the paging pool. The file cache is basically just memory-mapping files to hold the file data in RAM for a while. The file cache is enabled by default on top of FATFS volumes.