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

[Michael S. Tsirkin]

On Wed, May 25, 2016 at 09:28:58AM +0000, Li, Liang Z wrote:
> > 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

Interesting. How about instead of tell host, we do multiple scans, each
time ignoring pages out of range?

        for (pfn = min pfn; pfn < max pfn; pfn += 1G) {
                foreach page
                        if page pfn < pfn || page pfn >= pfn + 1G
                                continue
                        set bit
                tell host
        }

> 
> > 
> > > 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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