linux-kernel - Re: [virtio-dev] Re: [PATCH v11 0/6] mm / virtio: Provide support for unused page reporting

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Date:   Wed, 2 Oct 2019 10:41:13 -0400
From:   Nitesh Narayan Lal <nitesh@...hat.com>
To:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        virtio-dev@...ts.oasis-open.org, kvm@...r.kernel.org,
        mst@...hat.com, dave.hansen@...el.com,
        linux-kernel@...r.kernel.org, willy@...radead.org,
        mhocko@...nel.org, linux-mm@...ck.org, akpm@...ux-foundation.org,
        mgorman@...hsingularity.net, vbabka@...e.cz, osalvador@...e.de
Cc:     yang.zhang.wz@...il.com, pagupta@...hat.com,
        konrad.wilk@...cle.com, riel@...riel.com, lcapitulino@...hat.com,
        wei.w.wang@...el.com, aarcange@...hat.com, pbonzini@...hat.com,
        dan.j.williams@...el.com
Subject: Re: [virtio-dev] Re: [PATCH v11 0/6] mm / virtio: Provide support for
 unused page reporting


On 10/1/19 4:25 PM, Alexander Duyck wrote:
> On Tue, 2019-10-01 at 15:16 -0400, Nitesh Narayan Lal wrote:
>> On 10/1/19 12:21 PM, Alexander Duyck wrote:
>>> On Tue, 2019-10-01 at 17:35 +0200, David Hildenbrand wrote:
>>>> On 01.10.19 17:29, Alexander Duyck wrote:
> <snip>
>
>>>>> As far as possible regressions I have focused on cases where performing
>>>>> the hinting would be non-optimal, such as cases where the code isn't
>>>>> needed as memory is not over-committed, or the functionality is not in
>>>>> use. I have been using the will-it-scale/page_fault1 test running with 16
>>>>> vcpus and have modified it to use Transparent Huge Pages. With this I see
>>>>> almost no difference with the patches applied and the feature disabled.
>>>>> Likewise I see almost no difference with the feature enabled, but the
>>>>> madvise disabled in the hypervisor due to a device being assigned. With
>>>>> the feature fully enabled in both guest and hypervisor I see a regression
>>>>> between -1.86% and -8.84% versus the baseline. I found that most of the
>>>>> overhead was due to the page faulting/zeroing that comes as a result of
>>>>> the pages having been evicted from the guest.
>>>> I think Michal asked for a performance comparison against Nitesh's
>>>> approach, to evaluate if keeping the reported state + tracking inside
>>>> the buddy is really worth it. Do you have any such numbers already? (or
>>>> did my tired eyes miss them in this cover letter? :/)
>>>>
>>> I thought what Michal was asking for was what was the benefit of using the
>>> boundary pointer. I added a bit up above and to the description for patch
>>> 3 as on a 32G VM it adds up to about a 18% difference without factoring in
>>> the page faulting and zeroing logic that occurs when we actually do the
>>> madvise.
>>>
>>> Do we have a working patch set for Nitesh's code? The last time I tried
>>> running his patch set I ran into issues with kernel panics. If we have a
>>> known working/stable patch set I can give it a try.
>> Did you try the v12 patch-set [1]?
>> I remember that you reported the CPU stall issue, which I fixed in the v12.
>>
>> [1] https://lkml.org/lkml/2019/8/12/593
>>
>>> - Alex
>>>
> I haven't tested it. I will pull the patches and give it a try. It works
> with the same QEMU changes that mine does right? If so we should be able
> to get an apples-to-apples comparison.

Yes.

>
> Also, instead of providing lkml.org links to your patches in the future it
> might be better to provide a link to the lore.kernel.org version of the
> thread. So for example the v12 set would be:
> https://lore.kernel.org/lkml/20190812131235.27244-1-nitesh@redhat.com/

I see, I will keep that in mind. Thanks for pointing this out.

>
> The advantage is you can just look up the message ID in your own inbox to
> figure out the link, and it provides raw access to the email if needed.
>
> Thanks.
>
> - Alex
>
>
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-- 
Thanks
Nitesh