Re: [GIT PULL] mm: frontswap (for 3.2 window)

[Rik van Riel <riel@redhat.com>]

On 10/31/2011 07:36 PM, Dan Magenheimer wrote:
>> From: Andrea Arcangeli [mailto:aarcange@redhat.com]

>>> real work to do instead and (2) that vmexit/vmenter is horribly
>>
>> Sure the CPU has another 1000 VM to schedule. This is like saying
>> virtio-blk isn't needed on desktop virt becauase the desktop isn't
>> doing much I/O. Absurd argument, there are another 1000 desktops doing
>> I/O at the same time of course.
>
> But this is truly different, I think at least for the most common
> cases, because the guest is essentially out of physical memory if it
> is swapping.  And the vmexit/vmenter (I assume, I don't really
> know KVM) gives the KVM scheduler the opportunity to schedule
> another of those 1000 VMs if it wishes.

I believe the problem Andrea is trying to point out here is
that the proposed API cannot handle a batch of pages to be
pushed into frontswap/cleancache at one time.

Even if the current back-end implementations are synchronous
and can only do one page at a time, I believe it would still
be a good idea to have the API able to handle a vector with
a bunch of pages all at once.

That way we can optimize the back-ends as required, at some
later point in time.

If enough people start using tmem, such bottlenecks will show
up at some point :)