Each page is either in frontswap OR on the normal swap device,
Perhaps I misunderstood. Isn't frontswap in front of the normal swap
device? So we do have double swapping, first to frontswap (which is in
memory, yes, but still a nonzero cost), then the normal swap device.
The io subsystem is loaded with writes; you only save the reads.
Better to swap to the hypervisor, and make it responsible for
committing
to disk on overcommit or keeping in RAM when memory is available. This
way we avoid the write to disk if memory is in fact available (or at
least defer it until later). This way you avoid both reads and writes
if memory is available.
never both. So, yes, both reads and writes are avoided if memory
is available and there is no write issued to the io subsystem if
memory is available. The is_memory_available decision is determined
by the hypervisor dynamically for each page when the guest attempts
a "frontswap_put". So, yes, you are indeed "swapping to the
hypervisor" but, at least in the case of Xen, the hypervisor
never swaps any memory to disk so there is never double swapping.
In many cases, this is true. For the swap subsystem, it may not alwaysIf I understand correctly, SSDs work much more efficiently whenThe guest can easily (and should) issue 64k dmas using scatter/gather.
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.
No need for copying.
be true, though I see recent signs that it may be headed in that
direction.
In any case, unless you see this SSD discussion as
critical to the proposed acceptance of the frontswap patchset,
let's table it until there's some prototyping done.