linux-kernel - Re: [PATCH 5/6] KVM: Separate rmap_pde from kvm_lpage_info->write

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Message-Id: <20120629104406.5aacc584.yoshikawa.takuya@oss.ntt.co.jp>
Date:	Fri, 29 Jun 2012 10:44:06 +0900
From:	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>
To:	Avi Kivity <avi@...hat.com>
Cc:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	mtosatti@...hat.com, agraf@...e.de, paulus@...ba.org,
	aarcange@...hat.com, kvm@...r.kernel.org, kvm-ppc@...r.kernel.org,
	linux-kernel@...r.kernel.org, takuya.yoshikawa@...il.com
Subject: Re: [PATCH 5/6] KVM: Separate rmap_pde from
 kvm_lpage_info->write_count

On Thu, 28 Jun 2012 20:39:55 +0300
Avi Kivity <avi@...hat.com> wrote:

> > Note: write_count: 4 bytes, rmap_pde: 8 bytes.  So we are wasting
> > extra paddings by packing them into lpage_info.
> 
> The wastage is quite low since it's just 4 bytes per 2MB.

Yes.

> >> Why not just introduce a function to get the next rmap? Something like this:
> > 
> > I want to eliminate this kind of overheads.
> 
> I don't think the overhead is significant.  rmap walk speed is largely a
> function of cache misses IMO, and we may even be adding cache misses by
> splitting lpage_info.

Maybe.  But as far as I can see, write_count does not gain much from
being close to rmap_pde.

> But I still think it's the right thing since it simplifies the code.

After the rmap integration, we can remove
	if (likely(level == PT_PAGE_TABLE_LEVEL))
heuristics from __gfn_to_rmap().

As a bonus, the helper will become enough simple to be always inlined
which reduces some function calls.

> Maybe we should add a prefetch() on write_count do mitigate the
> overhead, if it starts showing up in profiles.

OK, I will post rmap integration work as soon as possible, but it
still needs to be synced with unrelated ppc works at some point in
the future: so please take that separately from this series.

Thanks,
	Takuya
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