Re: [Qemu-devel] [virtio-dev] Re: [PATCH v2 repost 4/7] virtio-balloon: speed up inflate/deflate process

[Michael S. Tsirkin]

On Thu, Jul 28, 2016 at 06:36:18AM +0000, Li, Liang Z wrote:
> > > > This ends up doing a 1MB kmalloc() right?  That seems a _bit_ big.
> > > > How big was the pfn buffer before?
> > >
> > > Yes, it is if the max pfn is more than 32GB.
> > > The size of the pfn buffer use before is 256*4 = 1024 Bytes, it's too
> > > small, and it's the main reason for bad performance.
> > > Use the max 1MB kmalloc is a balance between performance and
> > > flexibility, a large page bitmap covers the range of all the memory is
> > > no good for a system with huge amount of memory. If the bitmap is too
> > > small, it means we have to traverse a long list for many times, and it's 
> > > bad
> > for performance.
> > >
> > > Thanks!
> > > Liang
> > 
> > There are all your implementation decisions though.
> > 
> > If guest memory is so fragmented that you only have order 0 4k pages, then
> > allocating a huge 1M contigious chunk is very problematic in and of itself.
> > 
> 
> The memory is allocated in the probe stage. This will not happen if the 
> driver is
>  loaded when booting the guest.
> 
> > Most people rarely migrate and do not care how fast that happens.
> > Wasting a large chunk of memory (and it's zeroed for no good reason, so you
> > actually request host memory for it) for everyone to speed it up when it
> > does happen is not really an option.
> > 
> If people don't plan to do inflating/deflating, they should not enable the 
> virtio-balloon
> at the beginning, once they decide to use it, the driver should provide 
> better performance
> as much as possible.

The reason people inflate/deflate is so they can overcommit memory.
Do they need to overcommit very quickly? I don't see why.
So let's get what we can for free but I don't really believe
people would want to pay for it.

> 1MB is a very small portion for a VM with more than 32GB memory and it's the 
> *worst case*, 
> for VM with less than 32GB memory, the amount of RAM depends on VM's memory 
> size
> and will be less than 1MB.

It's guest memmory so might all be in swap and never touched,
your memset at probe time will fault it in and make hypervisor
actually pay for it.

> If 1MB is too big, how about 512K, or 256K?  32K seems too small.
> 
> Liang

It's only small because it makes you rescan the free list.
So maybe you should do something else.
I looked at it a bit. Instead of scanning the free list, how about
scanning actual page structures? If page is unused, pass it to host.
Solves the problem of rescanning multiple times, does it not?


Another idea: allocate a small bitmap at probe time (e.g. for deflate),
allocate a bunch more on each request. Use something like
GFP_ATOMIC and a scatter/gather, if that fails use the smaller bitmap.



> > --
> > MST
> > 
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