Re: Tmem [PATCH 0/5] (Take 3): Transcendent memory

[Nitin Gupta]

Hi Dan,

(I'm not sure if gmane.org interface sends mail to everyone in CC list, so
sending again. Sorry if you are getting duplicate mail).


Dan Magenheimer <dan.magenheimer <at> oracle.com> writes:

> 
> Tmem [PATCH 0/5] (Take 3): Transcendent memory
> Transcendent memory
<snip>
> 
> Normal memory is directly addressable by the kernel, of a known
> normally-fixed size, synchronously accessible, and persistent (though
> not across a reboot).
> 
> What if there was a class of memory that is of unknown and dynamically
> variable size, is addressable only indirectly by the kernel, can be
> configured either as persistent or as "ephemeral" (meaning it will be
> around for awhile, but might disappear without warning), and is still
> fast enough to be synchronously accessible?
> 

I really like the idea of allocating cache memory from hypervisor directly. This
is much more flexible than assigning fixed size memory to guests.

> 
> "Frontswap" is so named because it can be thought of as the opposite of
> a "backing store". Frontswap IS persistent, but for various reasons may not
> always be available for use, again due to factors that may not be visible to
> the kernel. (But, briefly, if the kernel is being "good" and has shared its
> resources nicely, then it will be able to use frontswap, else it will not.)
> Once a page is put, a get on the page will always succeed.  So when the
> kernel finds itself in a situation where it needs to swap out a page, it
> first attempts to use frontswap.  If the put works, a disk write and
> (usually) a disk read are avoided.  If it doesn't, the page is written
> to swap as usual.  Unlike cleancache, whether a page is stored in frontswap
> vs swap is recorded in kernel data structures, so when a page needs to
> be fetched, the kernel does a get if it is in frontswap and reads from
> swap if it is not in frontswap.
> 

I think 'frontswap' part seriously overlaps the functionality provided by
'ramzswap' which is a virtual block device driver recently added to
drivers/staging/ramzswap/. This device acts as a swap disk which compresses and
stores pages in memory itself.

To provide frontswap functionality, ramzswap needs few changes only:
instead of:
  compress --> alloc and store within guest.
do:
  compress --> send out to hypervisor (tmem_put_page).

Also, ramzswap driver supports multiple /dev/ramzswap{0,1,2...} devices. Each of
these devices can have separate backing partition/file which is used to flush
out incompressible pages or when (per-device) memory limit is exceeded.
When used on native systems, it uses custom xvmalloc allocator which is
specially designed to handle these compressed pages.

We can use all this by just a minor change in ramzswap as mentioned above.

> "Cleancache" can be thought of as a page-granularity victim cache for clean
> pages that the kernel's pageframe replacement algorithm (PFRA) would like
> to keep around, but can't since there isn't enough memory.   So when the
> PFRA "evicts" a page, it first puts it into the cleancache via a call to
> tmem.  And any time a filesystem reads a page from disk, it first attempts
> to get the page from cleancache.  If it's there, a disk access is eliminated.
> If not, the filesystem just goes to the disk like normal.  Cleancache is
> "ephemeral" so whether a page is kept in cleancache (between the "put" and
> the "get") is dependent on a number of factors that are invisible to
> the kernel.

Just an idea: as an alternate approach, we can create an 'in-memory compressed
storage' backend for FS-Cache. This way, all filesystems modified to use
fs-cache can benefit from this backend. To make it virtualization friendly like
tmem, we can again provide (per-cache?) option to allocate from hypervisor  i.e.
tmem_{put,get}_page() or use [compress]+alloc natively.

For guest<-->hypervisor interface, maybe we can use virtio so that all
hypervisors can benefit? Not quite sure about this one.

Thanks,
Nitin
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