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# IRC, freenode, #hurd, 2012-02-14

    <slpz> Open question: what do you think about dropping the memory object
      model and implementing a simple block-level cache?

[[microkernel/mach/memory_object]].

    <kilobug> 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), ...
    <slpz> kilobug: user processes usually make their way to data with POSIX
      operations, so memory objects are only needed for mmap'ed files
    <slpz> kilobug: and swap can be replaced for an in-kernel system or even
      could still use the memory object
    <braunr> slpz: memory objects are used for the page cache
    <kilobug> 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
    <slpz> braunr: and page cache could be moved to a lower level, near to the
      devices
    <braunr> not likely
    <braunr> well, it could, but then you'd still have the file system overhead
    <slpz> 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)
    <braunr> a unified buffer/VM cache as all modern systems have is probably
      the most efficient approach
    <slpz> braunr: I agree. I want to look at *BSD/Linux vfs systems to seem
      how much cache policy depends on the filesystem
    <slpz> braunr: Are you aware of any good papers on this matter?
    <braunr> netbsd UVM, the linux virtual memory system
    <braunr> both a bit old bit still relevant
    <slpz> braunr: Thanks.
    <slpz> the problem in our case is that having FS and cache information at
      different contexts (kernel vs. translator), I find hard to coordinate
      them.
    <slpz> that's why I though about a block-level cache that GNU Mach could
      manage by itself
    <slpz> I wonder how QNX deals with this
    <braunr> the point of having a simple page cache is explicitely about not
      caring if those pages are blocks or files or whatever
    <braunr> the kernel (at least, mach) normally has all the accounting
      information it needs to implement its cache policy
    <braunr> file system translators shouldn't cache much
    <braunr> the pager interface could be refined, but it looks ok to me as it
      is
    <slpz> Mach has the accounting info, but it's not able to purge the cache
      without coordination with translators
    <braunr> which is normal
    <slpz> And this is a big problem when memory pressure increases, as it
      doesn't know for sure when memory is going to be freed
    <braunr> Mach flushes its cache when it decides to, and sends back dirty
      pages if needed by the pager
    <braunr> that's the case with every paging implementation
    <braunr> the main difference is security with untrusted pagers
    <braunr> but that's another issue
    <slpz> 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
    <braunr> that's not true
    <braunr> they're not process, they're threads, but the timing issue is the
      same
    <braunr> see pdflush on linux
    <slpz> no, it isn't.
    <braunr> 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
    <slpz> a kernel thread can access all the information in the kernel, and
      synchronization is pretty easy.
    <braunr> on mach, synchronization is done with messages, that's even easier
      than shared kernel locks
    <slpz> with processes in different spaces, resource coordination becomes
      really difficult
    <braunr> and what kind of info would an external pager need when simply
      asked to take back its dirty pages
    <braunr> what resources ?
    <slpz> just take a look at the thread storm problem when GNU Mach needs to
      clean a bunch of pages
    <braunr> Mach is big enough to correctly account memory
    <braunr> there can be thread storms on monolithic systems
    <braunr> that's a Mach issue, not a microkernel issue
    <braunr> that's why linux limits the number of pdflush thread instances
    <slpz> Mach can account memory, but can't assure when be freed by any
      means, in a lesser degree than a monolithic system
    <braunr> again i disagree
    <braunr> no system can guarantee when memory will be freed with paging
    <slpz> a block level cache can, for most situations
    <braunr> slpz: why ?
    <braunr> slpz: or how i mean ?
    <slpz> 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.
    <slpz> 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.
    <slpz> memory is not really freed, but transformed from page cache to
      anonymous memory pertaining to the corresponding translator
    <slpz> and GNU Mach never knows for sure when this memory is released, if
      it ever is.
    <slpz> not being able to flush dirty pages synchronously is a big problem
      when you need to throttle memory usage
    <slpz> 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
    <braunr> your idea of a block level cache means in kernel block drivers
    <braunr> that's not the direction we're taking
    <braunr> i agree flushing should be a synchronous process, which was one of
      the proposed improvements in the thread migration papers
    <braunr> (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)
    <braunr> but it should be possible to have kernel threads similar to
      pdflush and throttle flush requests too
    <braunr> again, i really think it's a mach bug, and having a buffer cache
      would be stepping backward
    <braunr> the real design issue is allocating memory while trying to free
      it, yes
    <slpz> braunr: thread migration doesn't apply to asynchronous IPC, and the
      entire paging mechanism is implemented this way
    <slpz> in fact, trying to do a synchronous m_o_data_return will trigger a
      deadlock for sure
    <slpz> to achieve synchronous flushing with translators, the entire paging
      model must be redesigned
    <slpz> It's true that I'm not very confident of the viability of user space
      drivers
    <slpz> at least, not for every device
    <slpz> 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.
    <braunr> slpz: thread migration for paging requests implies synchronous
      RPC, we don't care much about the IPC layer there
    <braunr> and it requires large changes of the VM code in addition, yes
    <braunr> let's not talk about this, we don't have thread migration anyway
      :p
    <braunr> 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 ..
    <braunr> monolithic systems also have that problem, with lower impacts
      though, but still
    <slpz> 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
    <braunr> it does know when memory is really freed
    <braunr> and yes, it blindly sends a bunch of requests, they can and should
      be trottled
    <slpz> 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)
    <braunr> memory is freed when the pager deallocates it
    <braunr> the speed of the operation is irrelevant
    <braunr> no system can rely on disk speed to guarantee correct page flushes
    <braunr> disk or anything else
    <slpz> requests can't be throttled if Mach doesn't know when they are being
      processed
    <braunr> it can easily know it
    <braunr> they are processed as soon as the request is sent from the kernel
    <braunr> and processing is done when the pager acknowledges the end of the
      flush
    <braunr> memory backing the flushed pages should be released before
      acknowleding that to avoid starting new requests too soon
    <slpz> AFAIK pagers doesn't acknowledge the end of the flush
    <braunr> well that's where the interface should be refined
    <slpz> Mach just sends the m_o_data_return and continues on its own
    <braunr> that's why flushing should be synrhconous
    <braunr> are you sure about that however ?
    <slpz> so the entire paging system needs a new design... :)
    <slpz> pretty sure
    <braunr> not a new design ..
    <braunr> there is m_o_supply_completed, i don't see how difficult it would
      be to add m_o_data_return_completed
    <braunr> it's not a small change, but not a difficult one either
    <braunr> i'm more worried about the allocation problem
    <braunr> the default pager should probably be wired in memory
    <braunr> maybe others
    <slpz> 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)?
    <braunr> it should stop after an arbitrary limit is reached
    <braunr> a reasonable one
    <braunr> linux limits the number of pdflush threads for that reason as i
      mentioned (to 8 iirc)
    <braunr> the problem of reading blocks while flushing is what i'm worried
      about too, hence the need to wire that code
    <braunr> well, i'm nto sure it's needed
    <braunr> again, a reasonable about of free memory should be reserved for
      that at all times
    <slpz> 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).
    <braunr> i don't know how other systems compute that, but this is how they
      seem to do as well
    <braunr> no, i don't think so
    <slpz> well, I'll try to invest a few days understanding how pdflush work,
      to see if some ideas can be borrowed for Hurd
    <braunr> iirc, freebsd has thresholds in percent for each part of its cache
      (active, inactive, free, dirty)
    <slpz> but I still think simple solutions work better, and using the memory
      object for page cache is tremendously complex.
    <braunr> the amount of free cache pages is generally sufficient to
      guarantee much memory can be released at once if needed, without flushing
      anything
    <braunr> yes but that's the whole point of the Mach VM
    <braunr> and its greatest advance ..
    <slpz> what, memory objects?
    <braunr> yes
    <braunr> using physical memory as a cache for anything, not just block
      buffers
    <slpz> 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).
    <slpz> or, at least, in the way we're using them
    <braunr> probably
    <braunr> http://lwn.net/Articles/326552/
    <braunr> this can help udnerstand the problems we may have without better
      knowledge of the underlying devices, yes
    <braunr> (e.g. not being able to send multiple requests to pagers that
      don't share the same disk)
    <braunr> slpz: actually i'm not sure it's that overkill
    <braunr> the linux vm uses struct vm_file to represent memory objects iirc
    <braunr> there are many links between that structure and some vfs related
      subsystems
    <braunr> when a system very actively uses the page cache, the kernel has to
      maintain a lot of objects to accurately describe the cache content
    <braunr> you could consider this overkill at first too
    <braunr> the mach way of doing it just implies some ipc messages instead of
      function calls, it's not that overkill for me
    <braunr> the main problems are recursion (allocation while freeing,
      handling page faults in order to handle flushes, that sort of things)
    <braunr> struct file and struct address_space actually
    <braunr> slpz: see struct address_space, it contains a set of function
      pointers that can help understanding the linux pager interface
    <braunr> they probably sufferred from similar caveats and worked around
      them, adjusting that interface on the way
    <slpz> 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.
    <slpz> meanwhile on Mach+pager scenario, the relationship between a block
      in a file and its underlying storage becomes really blurry
    <slpz> this is a huge advantage when flusing out data, specially when
      resources are scarce
    <slpz> 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
    <slpz> and breaking the tight relationship between the device and its
      cache, makes things a lot harder
    <slpz> this also manifest itself when flushing clean pages, as things like
      having an static maximum for cached memory objects
    <slpz> we shouldn't care about the number of objects, we just need to
      control the number of pages
    <slpz> but as we need the pager to flush pages, we need to keep alive a lot
      of control ports to them
    <mcsim> 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.