linux-kernel - Re: Thoughts on simple scanner approach for free page hinting

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Message-ID: <20190409093642-mutt-send-email-mst@kernel.org>
Date:   Tue, 9 Apr 2019 09:37:54 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Paolo Bonzini <pbonzini@...hat.com>, lcapitulino@...hat.com,
        pagupta@...hat.com, wei.w.wang@...el.com,
        Yang Zhang <yang.zhang.wz@...il.com>,
        Rik van Riel <riel@...riel.com>, dodgen@...gle.com,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        dhildenb@...hat.com, Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: Thoughts on simple scanner approach for free page hinting

On Tue, Apr 09, 2019 at 03:36:08PM +0200, David Hildenbrand wrote:
> On 09.04.19 15:31, Michael S. Tsirkin wrote:
> > On Tue, Apr 09, 2019 at 11:20:36AM +0200, David Hildenbrand wrote:
> >> BTW I like the idea of allocating pages that have already been hinted as
> >> last "choice", allocating pages that have not been hinted yet first.
> > 
> > OK I guess but note this is just a small window during which
> > not all pages have been hinted.
> 
> Yes, good point. It might sound desirable but might be completely
> irrelevant in practice.
> 
> > 
> > So if we actually think this has value then we need
> > to design something that will desist and not drop pages
> > in steady state too.
> 
> By dropping, you mean dropping hints of e.g. MAX_ORDER - 1 or e.g. not
> reporting MAX_ORDER - 3?

I mean the issue is host unmaps the pages from guest right?  That is
what makes hinted pages slower than non-hinted ones.  If we do not want
that to happen for some pages, then either host can defer acting on the
hint, or we can defer hinting.

> -- 
> 
> Thanks,
> 
> David / dhildenb