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

On 10/7/19 12:27 PM, Alexander Duyck wrote:
> On Mon, 2019-10-07 at 12:19 -0400, Nitesh Narayan Lal wrote:
>> On 10/7/19 11:33 AM, Alexander Duyck wrote:
>>> On Mon, 2019-10-07 at 08:29 -0400, Nitesh Narayan Lal wrote:
>>>> On 10/2/19 10:25 AM, Alexander Duyck wrote:
>> [...]
>>>> You  don't have to, I can fix the issues in my patch-set. :)
>>>>> Sounds good. Hopefully the stuff I pointed out above helps you to get
>>>>> a reproduction and resolve the issues.
>>>> So I did observe a significant drop in running my v12 path-set [1] with the
>>>> suggested test setup. However, on making certain changes the performance
>>>> improved significantly.
>>>> I used my v12 patch-set which I have posted earlier and made the following
>>>> changes:
>>>> 1. Started reporting only (MAX_ORDER - 1) pages and increased the number of
>>>>     pages that can be reported at a time to 32 from 16. The intent of making
>>>>     these changes was to bring my configuration closer to what Alexander is
>>>>     using.
>>> The increase from 16 to 32 is valid. No point in working in too small of
>>> batches. However tightening the order to only test for MAX_ORDER - 1 seems
>>> like a step in the wrong direction. The bitmap approach doesn't have much
>>> value if it can only work with the highest order page. I realize it is
>>> probably necessary in order to make the trick for checking on page_buddy
>>> work, but it seems very limiting.
>> If using (pageblock_order - 1) is a better way to do this, then I can probably
>> switch to that.
>> I will agree with the fact that we have to make the reporting order
>> configurable, atleast to an extent.
> I think you mean pageblock_order, not pageblock_order - 1. The problem
> with pageblock_order - 1 is that it will have a negative impact on
> performance as it would disable THP.

Ah, I see. Yes my bad.

>>>> 2. I made an additional change in my bitmap scanning logic to prevent acquiring
>>>>     spinlock if the page is already allocated.
>>> Again, not a fan. It basically means you can only work with MAX_ORDER - 1
>>> and there will be no ability to work with anything smaller.
>>>> Setup:
>>>> On a 16 vCPU 30 GB single NUMA guest affined to a single host NUMA, I ran the
>>>> modified will-it-scale/page_fault number of times and calculated the average
>>>> of the number of process and threads launched on the 16th core to compare the
>>>> impact of my patch-set against an unmodified kernel.
>>>> Conclusion:
>>>> %Drop in number of processes launched on 16th vCPU =     1-2%
>>>> %Drop in number of threads launched on 16th vCPU     =     5-6%
>>> These numbers don't make that much sense to me. Are you talking about a
>>> fully functioning setup that is madvsing away the memory in the
>>> hypervisor?
>> Without making this change I was observing a significant amount of drop
>> in the number of processes and specifically in the number of threads.
>> I did a double-check of the configuration which I have shared.
>> I was also observing the "AnonHugePages" via meminfo to check the THP usage.
>> Any more suggestions about what else I can do to verify?
>> I will be more than happy to try them out.
> 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.

>>>> Other observations:
>>>> - I also tried running Alexander's latest v11 page-reporting patch set and
>>>>   observe a similar amount of average degradation in the number of processes
>>>>   and threads.
>>>> - I didn't include the linear component recorded by will-it-scale because for
>>>>   some reason it was fluctuating too much even when I was using an unmodified
>>>>   kernel. If required I can investigate this further.
>>>> Note: If there is a better way to analyze the will-it-scale/page_fault results
>>>> then please do let me know.
>>> Honestly I have mostly just focused on the processes performance.
>> In my observation processes seems to be most consistent in general.
> Agreed.
>>>  There is
>>> usually a fair bit of variability but a pattern forms after a few runs so
>>> you can generally tell if a configuration is an improvement or not.
>> Yeah, that's why I thought of taking the average of 5-6 runs.
> Same here. I am usually running about 5 iterations.
>>>> Other setup details:
>>>> Following are the configurations which I enabled to run my tests:
>>>> - Set host THP to always
>>>> - Set guest THP to madvise
>>>> - Added the suggested madvise call in page_fault source code.
>>>> @Alexander please let me know if I missed something.
>>> This seems about right.
>>>> The current state of my v13:
>>>> I still have to look into Michal's suggestion of using page-isolation API's
>>>> instead of isolating the page. However, I believe at this moment our objective
>>>> is to decide with which approach we can proceed and that's why I decided to
>>>> post the numbers by making small required changes in v12 instead of posting a
>>>> new series.
>>>> Following are the changes which I have made on top of my v12:
>>>> page_reporting.h change:
>>>> -#define PAGE_REPORTING_MIN_ORDER               (MAX_ORDER - 2)
>>>> -#define PAGE_REPORTING_MAX_PAGES               16
>>>> +#define PAGE_REPORTING_MIN_ORDER              (MAX_ORDER - 1)
>>>> +#define PAGE_REPORTING_MAX_PAGES              32
>>>> page_reporting.c change:
>>>> @@ -101,8 +101,12 @@ static void scan_zone_bitmap(struct page_reporting_config
>>>> *phconf,
>>>>                 /* Process only if the page is still online */
>>>>                 page = pfn_to_online_page((setbit << PAGE_REPORTING_MIN_ORDER) +
>>>>                                           zone->base_pfn);
>>>> -               if (!page)
>>>> +               if (!page || !PageBuddy(page)) {
>>>> +                       clear_bit(setbit, zone->bitmap);
>>>> +                       atomic_dec(&zone->free_pages);
>>>>                         continue;
>>>> +               }
>>> I suspect the zone->free_pages is going to be expensive for you to deal
>>> with. It is a global atomic value and is going to have the cacheline
>>> bouncing that it is contained in. As a result thinks like setting the
>>> bitmap with be more expensive as every tome a CPU increments free_pages it
>>> will likely have to take the cache line containing the bitmap pointer as
>>> well.
>> I see I will have to explore this more. I am wondering if there is a way to
>> measure this If its effect is not visible in will-it-scale/page_fault1. If
>> there is a noticeable amount of degradation, I will have to address this.
> If nothing else you might look at seeing if you can split up the
> structures so that the bitmap and nr_bits is in a different region
> somewhere since those are read-mostly values.

ok, I will try to understand the issue and your suggestion.
Thank you for bringing this up.

> Also you are now updating the bitmap and free_pages both inside and
> outside of the zone lock so that will likely have some impact.

So as per your previous suggestion, I have made the bitmap structure
object as a rcu protected pointer. So we are safe from that side.
The other downside which I can think of is a race where one page
trying to increment free_pages and other trying to decrements it.
However, being an atomic variable that should not be a problem.
Did I miss anything? ÂÂÂ

>>>> @Alexander in case you decide to give it a try and find different results,
>>>> please do let me know.
>>>> [1] https://lore.kernel.org/lkml/20190812131235.27244-1-nitesh@redhat.com/
>>> If I have some free time I will take a look.
>> That would be great, thanks.
>>>  However one thing that
>>> concerns me about this change is that it will limit things much further in
>>> terms of how much memory can ultimately be freed since you are now only
>>> working with the highest order page and that becomes a hard requirement
>>> for your design.
>> I would assume that should be resolved with (pageblock_order - 1).
> There is no need for the - 1. The pageblock_order value is the lowest you
> can go before you start causing THP to be disabled. If you cross that
> threshold the performance will drop significantly. 

Makes sense.


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