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

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Date:	Fri, 29 Jul 2016 00:38:18 +0000
From:	"Li, Liang Z" <liang.z.li@...el.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"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>,
	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

> On Thu, Jul 28, 2016 at 03:06:37AM +0000, Li, Liang Z wrote:
> > > > + * VIRTIO_BALLOON_PFNS_LIMIT is used to limit the size of page
> > > > +bitmap
> > > > + * to prevent a very large page bitmap, there are two reasons for this:
> > > > + * 1) to save memory.
> > > > + * 2) allocate a large bitmap may fail.
> > > > + *
> > > > + * The actual limit of pfn is determined by:
> > > > + * pfn_limit = min(max_pfn, VIRTIO_BALLOON_PFNS_LIMIT);
> > > > + *
> > > > + * If system has more pages than VIRTIO_BALLOON_PFNS_LIMIT, we
> > > > +will scan
> > > > + * the page list and send the PFNs with several times. To reduce
> > > > +the
> > > > + * overhead of scanning the page list. VIRTIO_BALLOON_PFNS_LIMIT
> > > > +should
> > > > + * be set with a value which can cover most cases.
> > >
> > > So what if it covers 1/32 of the memory? We'll do 32 exits and not
> > > 1, still not a big deal for a big guest.
> > >
> >
> > The issue here is the overhead is too high for scanning the page list for 32
> times.
> > Limit the page bitmap size to a fixed value is better for a big guest?
> >
> 
> I'd say avoid scanning free lists completely. Scan pages themselves and check
> the refcount to see whether they are free.
> This way each page needs to be tested once.
> 
> And skip the whole optimization if less than e.g. 10% is free.

That's better than rescanning the free list. Will change in next version.

Thanks!

Liang