Hmmm.... I now realize you are thinking of applying frontswap tobackend.Much easier to simulate an asynchronous API with a synchronous
Swapping is inherently asynchronous, so we'll have to wait for that toIndeed. But an asynchronous API is not appropriate for frontswap
(or cleancache). The reason the hooks are so simple is because they
are assumed to be synchronous so that the page can be immediately
freed/reused.
complete anyway (as frontswap does not guarantee swap-in will succeed).
I don't doubt it makes things simpler, but also less flexible and
useful.
Something else that bothers me is the double swapping. Sure we're
making swapin faster, but we we're still loading the io subsystem with
writes. Much better to make swap-to-ram authoritative (and have the
hypervisor swap it to disk if it needs the memory).
a hosted hypervisor (e.g. KVM). Using frontswap with a bare-metal
hypervisor (e.g. Xen) works fully synchronously, guarantees swap-in
will succeed, never double-swaps, and doesn't load the io subsystem
with writes. This all works very nicely today with a fully
synchronous "backend" (e.g. with tmem in Xen 4.0).
If I understand correctly, SSDs work much more efficiently whenI don't understand. Please clarify.Well, copying memory so you can use a zero-copy dma engine isYes, but for something like an SSD where copying can be used to
counterproductive.
build up a full 64K write, the cost of copying memory may not be
counterproductive.
writing 64KB blocks. So much more efficiently in fact that waiting
to collect 16 4KB pages (by first copying them to fill a 64KB buffer)
will be faster than page-at-a-time DMA'ing them. If so, the
frontswap interface, backed by an asynchronous "buffering layer"
which collects 16 pages before writing to the SSD, may work
very nicely. Again this is still just speculation... I was
only pointing out that zero-copy DMA may not always be the best
solution.