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

Li, Liang Z
2016-07-28T01:13:35Z

> 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