linux-kernel - RE: [PATCH RFC kernel] balloon: speed up inflating/deflating process

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Date:	Wed, 25 May 2016 08:48:17 +0000
From:	"Li, Liang Z" <liang.z.li@...el.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"dgilbert@...hat.com" <dgilbert@...hat.com>,
	"amit.shah@...hat.com" <amit.shah@...hat.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: RE: [PATCH RFC kernel] balloon: speed up inflating/deflating process

> > > Suggestion to address all above comments:
> > > 	1. allocate a bunch of pages and link them up,
> > > 	   calculating the min and the max pfn.
> > > 	   if max-min exceeds the allocated bitmap size,
> > > 	   tell host.
> >
> > I am not sure if it works well in some cases, e.g. The allocated pages
> > are across a wide range and the max-min > limit is very frequently to be
> true.
> > Then, there will be many times of virtio transmission and it's bad for
> > performance improvement. Right?
> 
> It's a tradeoff for sure. Measure it, see what the overhead is.
> 

Hi MST,

I have measured the performance when using a 32K page bitmap, and inflate the balloon to 3GB
of an idle guest with 4GB RAM.

Now: 
total inflating time: 338ms
the count of virtio data transmission:  373
the call count of madvise: 865

before:
total inflating time: 175ms
the count of virtio data transmission: 1
the call count of madvise: 42

Maybe the result will be worse if the guest is not idle, or the guest has more RAM.
Do you want more data?

Is it worth to do that?

Liang

> >
> > > 	2. limit allocated bitmap size to something reasonable.
> > > 	   How about 32Kbytes? This is 256kilo bit in the map, which comes
> > > 	   out to 1Giga bytes of memory in the balloon.
> >
> > So, even the VM has 1TB of RAM, the page bitmap will take 32MB of
> memory.
> > Maybe it's better to use a big page bitmap the save the pages
> > allocated by balloon, and split the big page bitmap to 32K bytes unit, then
> transfer one unit at a time.
> 
> How is this different from what I said?
> 
> >
> > Should we use a page bitmap to replace 'vb->pages' ?
> >
> > How about rolling back to use PFNs if the count of requested pages is a
> small number?
> >
> > Liang
> 
> That's why we have start pfn. you can use that to pass even a single page
> without a lot of overhead.
> 
> > > > --
> > > > 1.9.1
> > > --
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