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

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Message-ID: <44740a29-bb14-e6e6-2992-98d0ae58e994@redhat.com>
Date:   Mon, 18 Feb 2019 17:59:06 +0100
From:   David Hildenbrand <david@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Nitesh Narayan Lal <nitesh@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, pbonzini@...hat.com,
        lcapitulino@...hat.com, pagupta@...hat.com, wei.w.wang@...el.com,
        yang.zhang.wz@...il.com, riel@...riel.com, dodgen@...gle.com,
        konrad.wilk@...cle.com, dhildenb@...hat.com, aarcange@...hat.com,
        Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting

On 18.02.19 17:49, Michael S. Tsirkin wrote:
> On Sat, Feb 16, 2019 at 10:40:15AM +0100, David Hildenbrand wrote:
>> It would be worth a try. My feeling is that a synchronous report after
>> e.g. 512 frees should be acceptable, as it seems to be acceptable on
>> s390x. (basically always enabled, nobody complains).
> 
> What slips under the radar on an arch like s390 might
> raise issues for a popular arch like x86. My fear would be
> if it's only a problem e.g. for realtime. Then you get
> a condition that's very hard to trigger and affects
> worst case latencies.

Realtime should never use free page hinting. Just like it should never
use ballooning. Just like it should pin all pages in the hypervisor.

> 
> But really what business has something that is supposedly
> an optimization blocking a VCPU? We are just freeing up
> lots of memory why is it a good idea to slow that
> process down?

I first want to know that it is a problem before we declare it a
problem. I provided an example (s390x) where it does not seem to be a
problem. One hypercall ~every 512 frees. As simple as it can get.

No trying to deny that it could be a problem on x86, but then I assume
it is only a problem in specific setups.

I would much rather prefer a simple solution that can eventually be
disabled in selected setup than a complicated solution that tries to fit
all possible setups. Realtime is one of the examples where such stuff is
to be disabled either way.

Optimization of space comes with a price (here: execution time).

-- 

Thanks,

David / dhildenb