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Subject: Re: [PATCH v11 0/6] mm / virtio: Provide support for unused page reporting

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

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



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