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Subject: Re: [PATCH v11 0/6] mm / virtio: Provide support for unused page
 reporting
From:   Nitesh Narayan Lal <nitesh@redhat.com>
To:     Alexander Duyck <alexander.h.duyck@linux.intel.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>,
        Alexander Duyck <alexander.duyck@gmail.com>,
        David Hildenbrand <david@redhat.com>,
        virtio-dev@lists.oasis-open.org, kvm list <kvm@vger.kernel.org>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Michal Hocko <mhocko@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        Vlastimil Babka <vbabka@suse.cz>,
        Oscar Salvador <osalvador@suse.de>,
        Yang Zhang <yang.zhang.wz@gmail.com>,
        Pankaj Gupta <pagupta@redhat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Rik van Riel <riel@surriel.com>, lcapitulino@redhat.com,
        "Wang, Wei W" <wei.w.wang@intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Dan Williams <dan.j.williams@intel.com>
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Message-ID: <5c640ecb-cfef-2fa6-57aa-1352f1036f4e@redhat.com>
Date:   Wed, 9 Oct 2019 11:21:34 -0400
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On 10/7/19 1:06 PM, Nitesh Narayan Lal wrote:
[...]
>> So what was the size of your guest? One thing that just occurred to me is
>> that you might be running a much smaller guest than I was.
> I am running a 30 GB guest.
>
>>>>  If so I would have expected a much higher difference versus
>>>> baseline as zeroing/faulting the pages in the host gets expensive fairly
>>>> quick. What is the host kernel you are running your test on? I'm just
>>>> wondering if there is some additional overhead currently limiting your
>>>> setup. My host kernel was just the same kernel I was running in the guest,
>>>> just built without the patches applied.
>>> Right now I have a different host-kernel. I can install the same kernel to the
>>> host as well and see if that changes anything.
>> The host kernel will have a fairly significant impact as I recall. For
>> example running a stock CentOS kernel lowered the performance compared to
>> running a linux-next kernel. As a result the numbers looked better since
>> the overall baseline was lower to begin with as the host OS was
>> introducing additional overhead.
> I see in that case I will try by installing the same guest kernel
> to the host as well.

As per your suggestion, I tried replacing the host kernel with an
upstream kernel without my patches i.e., my host has a kernel built on top
of the upstream kernel's master branch which has Sept 23rd commit and the guest
has the same kernel for the no-hinting case and same kernel + my patches
for the page reporting case.

With the changes reported earlier on top of v12, I am not seeing any further
degradation (other than what I have previously reported).

To be sure that THP is actively used, I did an experiment where I changed the
MEMSIZE in the page_fault. On doing so THP usage checked via /proc/meminfo also
increased as I expected.

In any case, if you find something else please let me know and I will look into it
again.


I am still looking into your suggestion about cache line bouncing and will reply
to it, if I have more questions.


[...]


-- 
Thanks
Nitesh