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[[microkernel/mach/memory_object]]. slpz: AFAIK the memory object has more purpose than just cache, it's allow used for passing chunk of data between processes, handling swap (which similar to cache, but still slightly different), ... kilobug: user processes usually make their way to data with POSIX operations, so memory objects are only needed for mmap'ed files kilobug: and swap can be replaced for an in-kernel system or even could still use the memory object slpz: memory objects are used for the page cache slpz: translators (especially diskfs based) make heavy use of memory objects, and if "user processes" use POSIX semantics, Hurd process (translators, pagers, ...) shouldn't be bound to POSIX braunr: and page cache could be moved to a lower level, near to the devices not likely well, it could, but then you'd still have the file system overhead kilobug: but the use of memory objects it's not compulsory, you can easily write a fs translator without implementing memory objects at all (except to mmap) a unified buffer/VM cache as all modern systems have is probably the most efficient approach braunr: I agree. I want to look at *BSD/Linux vfs systems to seem how much cache policy depends on the filesystem braunr: Are you aware of any good papers on this matter? netbsd UVM, the linux virtual memory system both a bit old bit still relevant braunr: Thanks. the problem in our case is that having FS and cache information at different contexts (kernel vs. translator), I find hard to coordinate them. that's why I though about a block-level cache that GNU Mach could manage by itself I wonder how QNX deals with this the point of having a simple page cache is explicitely about not caring if those pages are blocks or files or whatever the kernel (at least, mach) normally has all the accounting information it needs to implement its cache policy file system translators shouldn't cache much the pager interface could be refined, but it looks ok to me as it is Mach has the accounting info, but it's not able to purge the cache without coordination with translators which is normal And this is a big problem when memory pressure increases, as it doesn't know for sure when memory is going to be freed Mach flushes its cache when it decides to, and sends back dirty pages if needed by the pager that's the case with every paging implementation the main difference is security with untrusted pagers but that's another issue but in a monolithic implementation, the kernel is able for force a chunk of cache memory to be freed without hoping for other process to do the job that's not true they're not process, they're threads, but the timing issue is the same see pdflush on linux no, it isn't. when memory is scarce, threads that request memory can either wait or immediately fail, and if they wait, they're usually woken by one of the vm threads once flushing is done a kernel thread can access all the information in the kernel, and synchronization is pretty easy. on mach, synchronization is done with messages, that's even easier than shared kernel locks with processes in different spaces, resource coordination becomes really difficult and what kind of info would an external pager need when simply asked to take back its dirty pages what resources ? just take a look at the thread storm problem when GNU Mach needs to clean a bunch of pages Mach is big enough to correctly account memory there can be thread storms on monolithic systems that's a Mach issue, not a microkernel issue that's why linux limits the number of pdflush thread instances Mach can account memory, but can't assure when be freed by any means, in a lesser degree than a monolithic system again i disagree no system can guarantee when memory will be freed with paging a block level cache can, for most situations slpz: why ? slpz: or how i mean ? braunr: with a block-level page cache, GNU Mach should be able to flush dirty pages directly to the underlaying device without all the complexity and resource cost involved in a m_o_data_return message. It can also throttle the rate at which pages are being cleaned, and do all this while blocking new page allocations to deal with memory exhaustion cases. braunr: in the current state, when cleaning dirty pages, GNU Mach sends a bunch on m_o_data_return to the corresponding pagers, hoping they will do their job as soon and as fast as possible. memory is not really freed, but transformed from page cache to anonymous memory pertaining to the corresponding translator and GNU Mach never knows for sure when this memory is released, if it ever is. not being able to flush dirty pages synchronously is a big problem when you need to throttle memory usage and needing allocating more memory when you're trying to free (which is the case for the m_o_data_return mechanism) makes the problem even worse your idea of a block level cache means in kernel block drivers that's not the direction we're taking i agree flushing should be a synchronous process, which was one of the proposed improvements in the thread migration papers (they didn't achieve it but thought about it for future works, so that the thread at the origin of the fault would handle it itself) but it should be possible to have kernel threads similar to pdflush and throttle flush requests too again, i really think it's a mach bug, and having a buffer cache would be stepping backward the real design issue is allocating memory while trying to free it, yes braunr: thread migration doesn't apply to asynchronous IPC, and the entire paging mechanism is implemented this way in fact, trying to do a synchronous m_o_data_return will trigger a deadlock for sure to achieve synchronous flushing with translators, the entire paging model must be redesigned It's true that I'm not very confident of the viability of user space drivers at least, not for every device I know this is against the current ideas for most ukernel designs, but if we want to achieve real work functionality, I think some sacrifices must be done. Or at least a reasonable compromise. slpz: thread migration for paging requests implies synchronous RPC, we don't care much about the IPC layer there and it requires large changes of the VM code in addition, yes let's not talk about this, we don't have thread migration anyway :p except the allocation-on-free-path issue, i really don't see how the current pager interface or the page cache creates problems wrt flushing .. monolithic systems also have that problem, with lower impacts though, but still braunr: because as it doesn't know when memory is really freed, 1) it just blindly sends a bunch of m_o_data_return to the pagers, usually overloading them (causing thread storms), and 2) it can't properly throttle new page requests to deal with resource exhaustion it does know when memory is really freed and yes, it blindly sends a bunch of requests, they can and should be trottled but dirty pages freed become indistinguishable from common anonymous chunks released, so it doesn't really know if page flushes are really working or not (i.e. doesn't know how fast a device is processing write requests) memory is freed when the pager deallocates it the speed of the operation is irrelevant no system can rely on disk speed to guarantee correct page flushes disk or anything else requests can't be throttled if Mach doesn't know when they are being processed it can easily know it they are processed as soon as the request is sent from the kernel and processing is done when the pager acknowledges the end of the flush memory backing the flushed pages should be released before acknowleding that to avoid starting new requests too soon AFAIK pagers doesn't acknowledge the end of the flush well that's where the interface should be refined Mach just sends the m_o_data_return and continues on its own that's why flushing should be synrhconous are you sure about that however ? so the entire paging system needs a new design... :) pretty sure not a new design .. there is m_o_supply_completed, i don't see how difficult it would be to add m_o_data_return_completed it's not a small change, but not a difficult one either i'm more worried about the allocation problem the default pager should probably be wired in memory maybe others let's suppose a case in which Mach needs to free memory due to an increase in its pressure. vm_pageout_daemon starts running, clean pages are freed easily, but for each dirty one a m_o_data_return in sent. 1) when should this daemon stop sending m_o_data_return and start waiting for m_o_data_return_completed? 2) what happens if the translator needs to read new blocks to fulfill a write request (pretty common in ext2fs)? it should stop after an arbitrary limit is reached a reasonable one linux limits the number of pdflush threads for that reason as i mentioned (to 8 iirc) the problem of reading blocks while flushing is what i'm worried about too, hence the need to wire that code well, i'm nto sure it's needed again, a reasonable about of free memory should be reserved for that at all times but the work for pdflush seems to be a lot easier, as it only deals directly with block devices (if I understood it correctly, I just started looking at it). i don't know how other systems compute that, but this is how they seem to do as well no, i don't think so well, I'll try to invest a few days understanding how pdflush work, to see if some ideas can be borrowed for Hurd iirc, freebsd has thresholds in percent for each part of its cache (active, inactive, free, dirty) but I still think simple solutions work better, and using the memory object for page cache is tremendously complex. the amount of free cache pages is generally sufficient to guarantee much memory can be released at once if needed, without flushing anything yes but that's the whole point of the Mach VM and its greatest advance .. what, memory objects? yes using physical memory as a cache for anything, not just block buffers memory objects work great as a way to provide a shared image of objects between processes, but as page cache they are an overkill (IMHO). or, at least, in the way we're using them probably http://lwn.net/Articles/326552/ this can help udnerstand the problems we may have without better knowledge of the underlying devices, yes (e.g. not being able to send multiple requests to pagers that don't share the same disk) slpz: actually i'm not sure it's that overkill the linux vm uses struct vm_file to represent memory objects iirc there are many links between that structure and some vfs related subsystems when a system very actively uses the page cache, the kernel has to maintain a lot of objects to accurately describe the cache content you could consider this overkill at first too the mach way of doing it just implies some ipc messages instead of function calls, it's not that overkill for me the main problems are recursion (allocation while freeing, handling page faults in order to handle flushes, that sort of things) struct file and struct address_space actually slpz: see struct address_space, it contains a set of function pointers that can help understanding the linux pager interface they probably sufferred from similar caveats and worked around them, adjusting that interface on the way but their strategy makes them able to treat the relationship between the page cache and the block devices in a really simple way, almost as a traditional storage cache. meanwhile on Mach+pager scenario, the relationship between a block in a file and its underlying storage becomes really blurry this is a huge advantage when flusing out data, specially when resources are scarce I think the idea of using abstract objects for page cache, loses a bit the point that we just want to avoid accessing constantly to a slow device and breaking the tight relationship between the device and its cache, makes things a lot harder this also manifest itself when flushing clean pages, as things like having an static maximum for cached memory objects we shouldn't care about the number of objects, we just need to control the number of pages but as we need the pager to flush pages, we need to keep alive a lot of control ports to them slpz: When mo_data_return is called, once the memory manager no longer needs supplied data, it should be deallocated using vm_deallocate. So this way pagers acknowledges the end of flush. # IRC, freenode, #hurd, 2013-08-26 < Spyro> Ok, so < Spyro> idiot question: in a nutshell, what is a memory object? < Spyro> and how is swapping/paging handled? < braunr> Spyro: a memory object is how the virtual memory system views a file < braunr> so it's a sequence of bytes with a length < braunr> "swapping" is just a special case of paging that applies to anonymous objects < braunr> (which are named so because they're not associated with a file and have no name) < Spyro> Who creates a memory object, and when? < braunr> pagers create memory objects when needed, e.g. when you open a file < Spyro> and this applies both to mmap opens as well as regular I/O opens as in for read() and write()? < braunr> basically, all file systems capable of handling mmap requests and/or caching in physical memory are pagers < braunr> yes < braunr> read/write will go through the page cache when possible < Spyro> and who owns the page cache? < Spyro> also, who decides what pages ot evict to swap/file if physical memory gets tight? < braunr> the kernel < braunr> that's one of the things that make mach a hybrid < Spyro> so the kernel owns the page cage? < Spyro> ...fml < Spyro> cache! < braunr> yes ## IRC, freenode, #hurd, 2013-08-27 < Spyro> so braunr: So, who creates the memory object, and how does it get populated? < Spyro> and how does a process accessing a file get hooked up to the memory object? < braunr> Spyro: i told you, pagers create memory objects < braunr> memory objects are how the VM system views files, so they're populated from the content of files < braunr> either true files or virtual files such as in /proc < braunr> Spyro: processes don't directly access memory objects unless memory mapping them with vm_map() < braunr> pagers (basically = file systems) do ok, so how is a pager/fs involved in handling a fault? ## IRC, freenode, #hurd, 2013-08-28 Spyro: each object is linked to a pager Spyro: when a fault occurs, the kernel looks up the VM map (kernel or a user one), and the address in this map, then the map entry, checks access and lots of other details ok, so it's pager -> object -> vmem ? Spyro: then finds the object mapped at that address (similar to how a file is mapped with mmap) from the object, it finds the pager ok and asks the pager about the data at the appropriate offset so how does a user process do normal file I/O? is faulting just a special case of it? it's completely separate eww normal I/O is done with message passing the hurd io interface ok so who talks to who on a file I/O? a client (e.g. cat) talks to a file system server (e.g. ext2fs) ok so it's client to the pager for regular file I/O? Spyro: i don't understand the question Spyro: it's client to server, the server might not be a pager ok just trying to figure out the difference between paging/faulting and regular I/O regular I/O is just message passing page fault handling is dealt with by pagers and I have a hunch that the fs/pager is involved somehow in both, because the server is the source of the data I'm getting a headache nalaginrut: a server like ext2fs is both a file server and a pager oh! oh btw, does a file server make use of memory objects for caching? Spyro: yes or rather, can it? does it have to? memory objects are for caching, and thus for page faults Spyro: for caching, it's a requirement for I/O, it's not you could have I/O without memory objects ok so how does the pager/fileserver use memory objects for caching? does it just map and write to them? basically yes but there is a complete protocol with the kernel for that http://www.gnu.org/software/hurd/gnumach-doc/External-Memory-Management.html#External-Memory-Management heh, lucky guess ty I am in way over my head here btw zero experience with micro kernels in practice it's not trivial that's not a microkernel thing at all that's how it works in monolithic kernels too i recommend netbsd uvm thesis there are nice pictures describing the vm system derrr...preacious? wow just ignore the anonymous memory handling part which is specific to uvm @_@ the rest is common to practically all VM systems out there I know about the linux page cache well it's almost the same with memory objects being the same thing as files in a page cache? memory objects are linux "address spaces" and address spaces are how the linux mm views a file, yes derp ... um... struvt vm_page == struct page * Spyro first must learn what an address_space is struct vm_map == struct mm_struct struct vm_map_entry == struct vm_area_struct * Spyro isn't a linux kernel vm expert either struct vm_object == struct address_space roughly details vary a lot and what's an address_space ? 11:41 < braunr> and address spaces are how the linux mm views a file, yes ok see include/linux/fs.h struct address_space_operations is the pager interface * Spyro should look at the linux kernel sources perhaps, unless you have an easier reference embarrassingly, RVR hired me as an editor for the linux-mm wiki I should know this stuff see http://darnassus.sceen.net/~rbraun/design_and_implementation_of_the_uvm_virtual_memory_system.pdf page 42 page 66 for another nice view i wouldn't recommend using linux source as refernece it's very complicated, filled with a lot of code dealing with details lmao and linux guys have a habit of choosing crappy names I was only going to stoppit except for "linux" and "git" ...make me laugh any more and I'll need rib surgery laugh ? complicated and crappy seriously, "address space" for a file is very very confusing oh I agree with that yes, names are crappy and the code is very complicated it took me half an hour to find where readahead is done once and i'm still not sure it was the right code so in linkern, there is an address_space for each cached file? takes me 30 seconds in netbsd .. yes eww yeah, BAD name but thanks for the explanation now I finally know what an address space is many linux core developers admit they don't care much about names so, in hurd, a memory object is to hurd, what an address_space is to linux? yes notto hurd ok to mach you know what I mean :P easier than for linux I can tell you that much and the bsd vm system is a stripped version of the mach vm ok that's why i think it's important to note it good, I learned something abou tthe linux vm...from the mach guys this is funny linux did too there is a paper about linux page eviction that directly borrows the mach algorithm and improves it mach is the historic motivation behind mmap on posix oh nice! but yes, linux picked a shitty name is all that clearer to you ? I think that address_space connection was a magic bolt of understanding and do you see how I/O and paging are mostly unrelated ? almost but how does a file I/O take advantage of caching by a memory object? does the file server just nudge the core for a hint? the file system copies from the memory object * Spyro noddles I think I understand a bit better now it's message passing but I havfe too much to digest already memory copying if the memory is already there, good, if not, the kernel will ask the file system to bring the data if message passing uses zero copy, data retrieval can be deferred until the client actually accesses it which is a fancy way of saying demand paging? :P it's always demand paging what i mean is that the file system won't fetch data as soon as it copies memory but when this data is actually needed by the client uh... whta's a precious page? let me check quickly If precious is FALSE, the kernel treats the data as a temporary and may throw it away if it hasn't been changed. If the precious value is TRUE, the kernel treats its copy as a data repository and promises to return it to the manager basically, it's used when you want the kernel to keep cached data in memory the cache becomes a lossless container for such pages the kernel may flush them, but not evict them what's the difference? imagine a ramfs point made ok would be pretty hard to flush something that doesn't have a backing store that was quick :) well the normal backing store for anonymous memory is the default pager aka swap eww but if you want your data *either* in swap or in memory and never in both it may be useful