Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting

[Nitesh Narayan Lal]

On 2/16/19 4:40 AM, David Hildenbrand wrote:
> On 04.02.19 21:18, Nitesh Narayan Lal wrote:
>
> Hi Nitesh,
>
> I thought again about how s390x handles free page hinting. As that seems
> to work just fine, I guess sticking to a similar model makes sense.
>
>
> I already explained in this thread how it works on s390x, a short summary:
>
> 1. Each VCPU has a buffer of pfns to be reported to the hypervisor. If I
> am not wrong, it contains 512 entries, so is exactly 1 page big. This
> buffer is stored in the hypervisor and is on page granularity.
>
> 2. This page buffer is managed via the ESSA instruction. In addition, to
> synchronize with the guest ("page reused when freeing in the
> hypervisor"), special bits in the host->guest page table can be
> set/locked via the ESSA instruction by the guest and similarly accessed
> by the hypervisor.
>
> 3. Once the buffer is full, the guest does a synchronous hypercall,
> going over all 512 entries and zapping them (== similar to MADV_DONTNEED)
>
>
> To mimic that, we
>
> 1. Have a static buffer per VCPU in the guest with 512 entries. You
> basically have that already.
>
> 2. On every free, add the page _or_ the page after merging by the buddy
> (e.g. MAX_ORDER - 1) to the buffer (this is where we could be better
> than s390x). You basically have that already.
>
> 3. If the buffer is full, try to isolate all pages and do a synchronous
> report to the hypervisor. You have the first part already. The second
> part would require a change (don't use a separate/global thread to do
> the hinting, just do it synchronously).
>
> 4. One hinting is done, putback all isolated pages to the budy. You
> basically have that already.
>
>
> For 3. we can try what you have right now, using virtio. If we detect
> that's a problem, we can do it similar to what Alexander proposes and
> just do a bare hypercall. It's just a different way of carrying out the
> same task.
>
>
> This approach
> 1. Mimics what s390x does, besides supporting different granularities.
> To synchronize guest->host we simply take the pages off the buddy.
>
> 2. Is basically what Alexander does, however his design limitation is
> that doing any hinting on smaller granularities will not work because
> there will be too many synchronous hints. Bad on fragmented guests.
>
> 3. Does not require any dynamic data structures in the guest.
>
> 4. Does not block allocation paths.
>
> 5. Blocks on e.g. every 512'ed free. It seems to work on s390x, why
> shouldn't it for us. We have to measure.
>
> 6. We are free to decide which granularity we report.
>
> 7. Potentially works even if the guest memory is fragmented (little
> MAX_ORDER - 1) pages.
>
> It would be worth a try. My feeling is that a synchronous report after
> e.g. 512 frees should be acceptable, as it seems to be acceptable on
> s390x. (basically always enabled, nobody complains).

The reason I like the current approach of reporting via separate kernel
thread is that it doesn't block any regular allocation/freeing code path
in anyways.
>
> We would have to play with how to enable/disable reporting and when to
> not report because it's not worth it in the guest (e.g. low on memory).
>
>
> Do you think something like this would be easy to change/implement and
> measure?

I can do that as I figure out a real world guest workload using which
the two approaches can be compared.

> Thanks!
>
>> The following patch-set proposes an efficient mechanism for handing freed memory between the guest and the host. It enables the guests with no page cache to rapidly free and reclaims memory to and from the host respectively.
>>
>> Benefit:
>> With this patch-series, in our test-case, executed on a single system and single NUMA node with 15GB memory, we were able to successfully launch atleast 5 guests 
>> when page hinting was enabled and 3 without it. (Detailed explanation of the test procedure is provided at the bottom).
>>
>> Changelog in V8:
>> In this patch-series, the earlier approach [1] which was used to capture and scan the pages freed by the guest has been changed. The new approach is briefly described below:
>>
>> The patch-set still leverages the existing arch_free_page() to add this functionality. It maintains a per CPU array which is used to store the pages freed by the guest. The maximum number of entries which it can hold is defined by MAX_FGPT_ENTRIES(1000). When the array is completely filled, it is scanned and only the pages which are available in the buddy are stored. This process continues until the array is filled with pages which are part of the buddy free list. After which it wakes up a kernel per-cpu-thread.
>> This kernel per-cpu-thread rescans the per-cpu-array for any re-allocation and if the page is not reallocated and present in the buddy, the kernel thread attempts to isolate it from the buddy. If it is successfully isolated, the page is added to another per-cpu array. Once the entire scanning process is complete, all the isolated pages are reported to the host through an existing virtio-balloon driver.
>>
>> Known Issues:
>> 	* Fixed array size: The problem with having a fixed/hardcoded array size arises when the size of the guest varies. For example when the guest size increases and it starts making large allocations fixed size limits this solution's ability to capture all the freed pages. This will result in less guest free memory getting reported to the host.
>>
>> Known code re-work:
>> 	* Plan to re-use Wei's work, which communicates the poison value to the host.
>> 	* The nomenclatures used in virtio-balloon needs to be changed so that the code can easily be distinguished from Wei's Free Page Hint code.
>> 	* Sorting based on zonenum, to avoid repetitive zone locks for the same zone.
>>
>> Other required work:
>> 	* Run other benchmarks to evaluate the performance/impact of this approach.
>>
>> Test case:
>> Setup:
>> Memory-15837 MB
>> Guest Memory Size-5 GB
>> Swap-Disabled
>> Test Program-Simple program which allocates 4GB memory via malloc, touches it via memset and exits.
>> Use case-Number of guests that can be launched completely including the successful execution of the test program.
>> Procedure: 
>> The first guest is launched and once its console is up, the test allocation program is executed with 4 GB memory request (Due to this the guest occupies almost 4-5 GB of memory in the host in a system without page hinting). Once this program exits at that time another guest is launched in the host and the same process is followed. We continue launching the guests until a guest gets killed due to low memory condition in the host.
>>
>> Result:
>> Without Hinting-3 Guests
>> With Hinting-5 to 7 Guests(Based on the amount of memory freed/captured).
>>
>> [1] https://www.spinics.net/lists/kvm/msg170113.html 
>>
>>
>
-- 
Regards
Nitesh

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