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xen-devel
RE: [Xen-devel] [PATCH] turn off writable page tables
> > I am having a hard time finding any "enterprise" workloads which
have
> > a lot of PTEs/page right before fork. If anyone can point me to
some,
> > that would be great.
> >
> > I will look into batching next, but I am curious if simply using a
> > hypercall in stead of write fault + emulate will make any difference
> > at all. I'll try that first, then implement the batched update.
> > Eventually a hypercall which does more would be nice, but I guess
> > we'll have to convince the Linux maintainers it's a good idea.
>
> The obvious thing to do is emulate the first 4 updates to a particular
> page, and only then switch to batched mode. Slows down the batched
path
> a bit, but stops it firing in many cases where it is no help.
Why? There should be no overhead to just building batches on the stack
(or a per vcpu area) and flushing at the end of the page. Certainly if
we were to keep wrpt it would make sense to take a few emulations faults
first on a page before engaging wrpt, but for explicit batches we don't
need any smarts.
[Although the batching strategy would (currently) work for Linux, we do
have to bare in mind that some OSes (possibly NetBSD) won't rely on a
lock to protect updates to pagetables and will use individual atomic
ops.]
Ian
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