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

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Message-ID: <0d12ccec-d05f-80b8-9498-710d521c81d2@intel.com>
Date:   Mon, 11 Feb 2019 09:48:11 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Alexander Duyck <alexander.duyck@...il.com>
Cc:     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 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.

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