linux-kernel - Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting

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Date:   Mon, 18 Feb 2019 21:45:57 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.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: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting

On Mon, Feb 18, 2019 at 03:47:22PM -0800, Alexander Duyck wrote:
> > > So far with my patch set that hints at the PMD level w/ THP enabled I
> > > am not really seeing that much overhead for the hypercalls. The bigger
> > > piece that is eating up CPU time is all the page faults and page
> > > zeroing that is going on as we are cycling the memory in and out of
> > > the guest. Some of that could probably be resolved by using MADV_FREE,
> > > but if we are under actual memory pressure I suspect it would behave
> > > similar to MADV_DONTNEED.
> > >
> >
> > MADV_FREE is certainly the better thing to do for hinting in my opinion.
> > It should result in even less overhead. Thanks for the comment about the
> > hypercall overhead.
> 
> Yeah, no problem. The only thing I don't like about MADV_FREE is that
> you have to have memory pressure before the pages really start getting
> scrubbed with is both a benefit and a drawback. Basically it defers
> the freeing until you are under actual memory pressure so when you hit
> that case things start feeling much slower, that and it limits your
> allocations since the kernel doesn't recognize the pages as free until
> it would have to start trying to push memory to swap.

For sure if someone *wants* to spend cycles freeing memory,
we could add a system call that does exactly that. There's
no reason to force that on the same CPU while VCPU
is stopped though.

-- 
MST