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

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Message-ID: <20190211145531-mutt-send-email-mst@kernel.org>
Date:   Mon, 11 Feb 2019 14:56:39 -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 10:19:17AM -0800, Dave Hansen wrote:
> On 2/11/19 9:58 AM, Michael S. Tsirkin wrote:
> >>> 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.
> 
> In a systems where we have some asynchrounous zeroing of memory where
> freed, non-zeroed memory is sequestered out of the allocator, yeah, that
> could make sense.
> 
> But, that's not what we have today.

Right. I wonder whether it's smart to build this assumption
into a host/guest interface though.

> >> 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.
> 
> Yeah, agreed.  Sounds like we need some more testing to see if these
> approaches hit bottlenecks anywhere.