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

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Date:	Thu, 28 Jul 2016 01:13:35 +0000
From:	"Li, Liang Z" <liang.z.li@...el.com>
To:	"Hansen, Dave" <dave.hansen@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"virtio-dev@...ts.oasis-open.org" <virtio-dev@...ts.oasis-open.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
	"dgilbert@...hat.com" <dgilbert@...hat.com>,
	"quintela@...hat.com" <quintela@...hat.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Paolo Bonzini <pbonzini@...hat.com>,
	"Cornelia Huck" <cornelia.huck@...ibm.com>,
	Amit Shah <amit.shah@...hat.com>
Subject: RE: [PATCH v2 repost 4/7] virtio-balloon: speed up inflate/deflate
 process

> 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