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

[Li, Liang Z]

> Subject: Re: [PATCH v2 repost 4/7] virtio-balloon: speed up inflate/deflate
> process
> 
> On 07/26/2016 06:23 PM, Liang Li wrote:
> > +   vb->pfn_limit = VIRTIO_BALLOON_PFNS_LIMIT;
> > +   vb->pfn_limit = min(vb->pfn_limit, get_max_pfn());
> > +   vb->bmap_len = ALIGN(vb->pfn_limit, BITS_PER_LONG) /
> > +            BITS_PER_BYTE + 2 * sizeof(unsigned long);
> > +   hdr_len = sizeof(struct balloon_bmap_hdr);
> > +   vb->bmap_hdr = kzalloc(hdr_len + vb->bmap_len, GFP_KERNEL);
> 
> 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