linux-kernel - Re: [RFC PATCH 3/4] kvm: Add guest side support for free memory hints

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Date:   Mon, 11 Feb 2019 12:58:14 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        rkrcmar@...hat.com, alexander.h.duyck@...ux.intel.com,
        x86@...nel.org, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        pbonzini@...hat.com, tglx@...utronix.de, akpm@...ux-foundation.org
Subject: Re: [RFC PATCH 3/4] kvm: Add guest side support for free memory hints

On Mon, Feb 11, 2019 at 09:48:11AM -0800, Dave Hansen wrote:
> On 2/9/19 4:49 PM, Michael S. Tsirkin wrote:
> > On Mon, Feb 04, 2019 at 10:15:52AM -0800, Alexander Duyck wrote:
> >> From: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
> >>
> >> Add guest support for providing free memory hints to the KVM hypervisor for
> >> freed pages huge TLB size or larger. I am restricting the size to
> >> huge TLB order and larger because the hypercalls are too expensive to be
> >> performing one per 4K page.
> > Even 2M pages start to get expensive with a TB guest.
> 
> Yeah, but we don't allocate and free TB's of memory at a high frequency.
> 
> > Really it seems we want a virtio ring so we can pass a batch of these.
> > E.g. 256 entries, 2M each - that's more like it.
> 
> That only makes sense for a system that's doing high-frequency,
> discontiguous frees of 2M pages.  Right now, a 2M free/realloc cycle
> (THP or hugetlb) is *not* super-high frequency just because of the
> latency for zeroing the page.

Heh but with a ton of free memory, and a thread zeroing some of
it out in the background, will this still be the case?
It could be that we'll be able to find clean pages
at all times.


> A virtio ring seems like an overblown solution to a non-existent problem.

It would be nice to see some traces to help us decide one way or the other.

-- 
MST