linux-kernel - Re: [PATCH 2/2 V2] ksm: take dirty bit as reference to avoid volatile pages scanning

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Message-Id: <201106221855.43667.nai.xia@gmail.com>
Date:	Wed, 22 Jun 2011 18:55:43 +0800
From:	Nai Xia <nai.xia@...il.com>
To:	Chris Wright <chrisw@...s-sol.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Izik Eidus <izik.eidus@...ellosystems.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	"linux-mm" <linux-mm@...ck.org>,
	Johannes Weiner <hannes@...xchg.org>,
	"linux-kernel" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2 V2] ksm: take dirty bit as reference to avoid volatile pages scanning

On Wednesday 22 June 2011 08:35:36 Chris Wright wrote:
> * Nai Xia (nai.xia@...il.com) 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@...il.com) 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. http://marc.info/?l=linux-mm&m=130866318804277&w=2
> > 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?

The direct benefit of it is that when merging a very big area, the system
does not be caught in a non-trivial period people see the free memory is 
actually dropping by creating only rmap_items, despite he is 100% sure that
his workset is very duplicated. I think it's puzzling to users and also 
risky of OOM.

Thanks,
Nai

> 
> thanks,
> -chris
> 
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