Messages in this thread | | | Date | Mon, 21 Dec 2009 19:16:41 +0530 | From | Nitin Gupta <> | Subject | Re: Tmem [PATCH 0/5] (Take 3): Transcendent memory |
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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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