From mboxrd@z Thu Jan  1 00:00:00 1970
From: Avi Kivity <avi@redhat.com>
Subject: Re: [PATCH 03/15] qom: introduce reclaimer to release
	obj
Date: Thu, 09 Aug 2012 11:18:34 +0300
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Cc: kvm@vger.kernel.org, Jan Kiszka <jan.kiszka@siemens.com>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	liu ping fan <qemulist@gmail.com>, qemu-devel@nongnu.org,
	Blue Swirl <blauwirbel@gmail.com>, Anthony Liguori <anthony@codemonkey.ws>,
	Stefan Hajnoczi <stefanha@gmail.com>,
	=?ISO-8859-1?Q?Andreas_F=E4rber?= <afaerber@suse.de>
To: Paolo Bonzini <pbonzini@redhat.com>
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On 08/09/2012 10:49 AM, Paolo Bonzini wrote:
> Il 09/08/2012 09:33, liu ping fan ha scritto:
>> Yes, it is to defer destructors.
>> See 0009-memory-prepare-flatview-and-radix-tree-for-rcu-style.patch
>> When MemoryRegion is _del_subregion from mem in updater, it may be
>> still in use by reader -- radix or flatview, so defer its destructors
>> to the reclaimer --phys_map_release(PhysMap *map)
> 
> How are you sure that the reader is already out of its critical section
> by the time the reclaimer runs?
> 
>> If we have rcu, it could be elegant to do this.
> 
> Yeah, I think inventing primitives is dangerous and difficult to review;
> and it may be difficult to replace it with proper call_rcu.
> 
> You should probably make a proof-of-concept using liburcu.  Then we can
> decide how to implement RCU in a way that is portable enough for QEMU's
> needs.

IMO we should start with a simple mutex (which will cover only the
lookup and map rebuild).  This should reduce the contention to basically
nothing (still leaving a cache line bounce).  If a profile shows the
cache line bounce hurting us, or perhaps contention in ultralarge
guests, then we should switch to rcu.


-- 
error compiling committee.c: too many arguments to function