mm/ hacks -- Generic memory store?

[John Richard Moser]

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I want to do a memory management hack for generic memory stores.
Basically, I want to abstract swap from the kernel; instead, I want a
"store" for excess memory before OOM.  I'm wondering currently the
usefulness and feasibility of this idea.

Consider the kernel's current memory management:

RAM <-> swap

Whereas the new management scheme would be only slightly different:

RAM <-> store

Now, the memory store infrastructure would have its own way of handling
memory.  This would be abstracted from the kernel.

store <-> swap

Thus, the kernel would operate in typical fashion:

RAM <-> store <-> swap

By giving some level of extensibility to the memory store
infrastructure, however, more complex pipelines can be created.

RAM <-> store <-> compress <-> swap

The memory store could be used both by disk cache and by process memory
management.  This would be best facilitated by allowing stored pages to
be marked as persistant or non-persistant.  Disk cache would of course
be non-persistant as long as it could be re-read from disk; other data
would be persistant.

With a persistant/non-persistant mark, the memory store could then free
non-persistant data at its discretion, notifying a "destructor" function
pointer of the change so that associated retrieval data can be freed;
the alternative would be to lazy destruct, which means disk cache would
only find out some cache data was gone when that part of a file is
re-accessed.  Unfortunately, this lazy approach would leave files that
are cached and never revisited with a handle in the disk cache subsystem
forever, unless you want to periodically garbage collect.

The handle in the memory store to the page would probably be handled in
the same way as it is for swap currently.  In any case, the handle has
to uniquely identify the exact page being stored.  This handle could be
used by the data store's swap module when writing the data to disk as well.

Suspend-to-Disk would have to be rewritten to work through the data
store, as swap would now be abstracted from the kernel.  This requires
two things of the data store.  First, it has to be aware of off-RAM
persistant storage options, such as swap.  Second, it has to be able to
take a signal that means "flush to persistant storage."

Depending on implementation, it may be required to have the data store
be told to store certain things certain ways for Suspend-to-Disk to
work.  For example, the swap file would have to be given the attributes
of holding a suspended session for a specific kernel version.  Reading
this data would happen through the data store, possibly via signalling
from an initrd script.

The advantages to this approach are that storage in tight memory
situations can be altered easily.  For example, a plug-in for the data
store would be capable of providing compression or encryption.  These
plug-ins would be given priorities (compression first, encryption
second) and attributed as transient/persistant and terminal/transparent.

A transient data store is one that stores data in memory.  When the
power goes, so does the data.  Persistant data stores, on the other
hand, store data in ways that can handle a power cycle.  Typically a
tranisent store should be transparent, and a persistant store should be
terminal; but this may not always be true.

A terminal store stores data in a final state.  Data in a terminal store
will be pushed no further down the chain; it has to be read back out of
that store before put into a new one.  On the other hand, a transparent
store would host data such that it could be directly processed into
another store.  Transparent stores should be in-memory so that they can
be operated on as memory.

Basically, a transparent store would allocate pages of memory through
the memory store API.  This would simply allocate memory through the
real memory management APIs, marking it as being non-swappable (or
rather non-storable).  The transparent stores would then do their thing
on the pages given them, store them in allocated memory (locking the
pages they're accessing), and be done with it.  The memory store would
operate directly on these pages when passing them to the next level.

The memory store would have to implement its own prioritising; of course
the prioritising is basically FIFO here anyway.  A page enters the store
when it's put there, and leaves it when it's requested; it doesn't get
read at will and still persist.  Thus, the store would assume that the
kernel is doing all the prioritising and telling the memory store in
order what data it thinks has the least value.

When memory gets tight, the store pushes the most aged stored data to
the next level.  So for example the oldest data may be compressed, then
swapped out (or freed, in the case of disk cache).  A more interesting
scheme WRT compression would be to have multiple levels of compression,
where data is stored; compressed (terminal); compressed more intensively
(terminal); compressed even more intensively (transparent); encrypted
(transparent), and then swapped (terminal).  This gives the following
growth model:


RAM <-> store <-> Compress 1 (TERM)
 4k      4k    |   3k
               -> Compress 2 (TERM)
               |   2k
               -> Compress 3 <-
                   1k         |
                              -> Encrypt <-> Swap
                                  1k          0k (swapped out)

  NOTE:  The chunks that are less than 4k are managed with other chunks
         in the stores.  In the Compress 3 and Encrypt transparent data
         stores, these 1k chunks are passed as part of a full 4k page
         that includes other stored pieces of data.

Here we can see that Compress 1 and 2 are terminal, and get
de-compressed and fed into each in turn, finally to Compress 3, then
Encrypt, then Swap.  We can also see that it may be wise to mark some of
these as destructor points for transient data; for example, disk cache
may get freed at Encrypt.

Overall, implementing this would probably be rather tough.  The most
difficult design issue would likely be suspend-to-disk through this
using swap.  The solution to that particular issue should be abstracted
through the data store; this would allow for storage methods like
network storage (swap to an encrypted swap network protocol or NFS),
although psychotic in nature, to be used for such things.

So the questions that arise are:

1.  Is this useful?
    I think it would be useful to be able to add abstraction to kernel
    memory storage at low memory levels, to allow easy implementation of
    compression and encryption pre-swapping.  This would even allow for
    modules that compress disk cache, or anything that gets swapped out
    typically.

2.  Is this worth implementing?
    I'm unsure of the amount of work that would go into implementing
    this.  It'd involve tearing the swap handling stuff out of the
    kernel and rewriting in its place this thing, then placing the swap
    code in as a module.  More importantly, suspending to swap would
    be a large consideration.

3.  What usefulness is of what's given as examples here?
    Encrypt before swap is a nice idea, and typically in small amounts
    it should be practical on most systems.  That isn't to say it would
    be friendly to microbenchmarks -- swapping would become a good deal
    more expensive.

    Compression is of course an all win.  The multi-stage compression
    shown here would use CPU muscle power to avoid the pains of swap;
    but would take a conservative approach and try to avoid using excess
    CPU for heavy compression unless needed.  As all but the last level
    of compression are terminal, it would be possible also to skip to
    Compress 3 in periods of rapidly growing memory usage to avoid
    faulting through Compress 2 and Compress 1 and using 3 times as
    much CPU.  In any case the result is better utilization of RAM;
    even systems with ample RAM would benefit from the increase in
    available disk cache.

4.  What other uses are possible?
    What other things do you want to do to RAM when you're low?  Is
    there any other use?  If not, is this still the cleanest route to
    take for the uses that are there?

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    Creative brains are a valuable, limited resource. They shouldn't be
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