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

[Li, Liang Z]

> 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?
Yes.

> 
> > and inflate the balloon to 3GB
> > of an idle guest with 4GB RAM.
> 
> Should take 3 requests then, right?
> 

No,  we can't assign the PFN when allocating page in balloon driver,
So the PFNs of pages allocated may be across a large range,  we will
tell the host once the pfn_max -pfn_min >= 0x40000(1GB range),
so the requests count is most likely to be more than 3. 


> > Now:
> > total inflating time: 338ms
> > the count of virtio data transmission:  373
> 
> Why was this so high? I would expect 3 transmissions.

I follow your suggestion:
------------------------------------------------------------------------------------
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.
        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.
-------------------------------------------------------------------------------------
Because the PFNs of the allocated pages are not linear increased, so 3 
transmissions
are  impossible.


Liang

> 
> > 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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