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

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Date:	Wed, 25 May 2016 11:57:23 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	"Li, Liang Z" <liang.z.li@...el.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

On Wed, May 25, 2016 at 08:48:17AM +0000, Li, Liang Z wrote:
> > > > 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,

Just to make sure. Do you mean a 32Kbyte bitmap?
Covering 1Gbyte of memory?

> and inflate the balloon to 3GB
> of an idle guest with 4GB RAM.

Should take 3 requests then, right?

> Now: 
> total inflating time: 338ms
> the count of virtio data transmission:  373

Why was this so high? I would expect 3 transmissions.

> 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

Either my math is wrong or there's an implementation bug.

> > >
> > > > 	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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