Re: [PATCH 2/2 V2] ksm: take dirty bit as reference to avoidvolatile pages scanning

From: Chris Wright
Date: Tue Jun 21 2011 - 20:36:33 EST

* Nai Xia (nai.xia@xxxxxxxxx) wrote:
> (Sorry for repeated mail, I forgot to Cc the list..)
> On Wednesday 22 June 2011 06:38:00 you wrote:
> > * Nai Xia (nai.xia@xxxxxxxxx) wrote:
> > > Introduced ksm_page_changed() to reference the dirty bit of a pte. We clear
> > > the dirty bit for each pte scanned but don't flush the tlb. For a huge page,
> > > if one of the subpage has changed, we try to skip the whole huge page
> > > assuming(this is true by now) that ksmd linearly scans the address space.
> >
> > This doesn't build w/ kvm as a module.
> I think it's because of the name-error of a related kvm patch, which I only sent
> in a same email thread.
> The patch split is not clean...I'll redo it.

It needs an export as it is.
ERROR: "kvm_dirty_update" [arch/x86/kvm/kvm-intel.ko] undefined!

Although perhaps could be done w/out that dirty_update altogether (as I
mentioned in other email)?

> >
> > > A NEW_FLAG is also introduced as a status of rmap_item to make ksmd scan
> > > more aggressively for new VMAs - only skip the pages considered to be volatile
> > > by the dirty bits. This can be enabled/disabled through KSM's sysfs interface.
> >
> > This seems like it should be separated out. And while it might be useful
> > to enable/disable for testing, I don't think it's worth supporting for
> > the long term. Would also be useful to see the value of this flag.
> I think it maybe useful for uses who want to turn on/off this scan policy explicitly
> according to their working sets?

Can you split it out, and show the benefit of it directly? I think it
only benefits:

p = mmap()
memset(p, $value, entire buffer);
very slowly (w.r.t scan times) touch bits of buffer and trigger cow to
break sharing.

Would you agree?

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