linux-kernel - Re: [PATCH 4/7] KVM: sort memslots and use binary search to search the right slot

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Message-ID: <4D64632E.1040609@cn.fujitsu.com>
Date:	Wed, 23 Feb 2011 09:30:22 +0800
From:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>,
	Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [PATCH 4/7] KVM: sort memslots and use binary search to search
 the right slot

On 02/22/2011 10:25 PM, Avi Kivity wrote:
> On 02/22/2011 10:12 AM, Xiao Guangrong wrote:
>> Sort memslots then search the slot with binary search to speed up the
>> slot searching
>>
> 
> I'm not sure if a binary search is the right algorithm here.  It introduces a lot of branches which may be mispredicted.
> 
> Options we've discussed are:
> 
> - Sort slots by size, use linear search (so the largest slots are found quickly)
> - Weighted balanced tree http://en.wikipedia.org/wiki/Weight-balanced_tree, use weight == slot size
> 

Yeah, there are the better chooses.

> Both options still make the miss case (mmio) slow.  We could cache the result of a miss in an spte by using a reserved bit, and checking the page fault error code (or seeing if we get an ept violation or ept misconfiguration), so if we get repeated mmio on a page, we don't need to search the slot list/tree.
> 

Sounds good! Will do it, thanks!

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