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

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.

>> 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.

>  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.

>> 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.

>  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.

>> 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.

>> @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).


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