Re: [RFC][Patch v9 0/6] KVM: Guest Free Page Hinting

From: David Hildenbrand
Date: Tue Mar 19 2019 - 09:33:22 EST

On 18.03.19 16:57, Nitesh Narayan Lal wrote:
> On 3/14/19 12:58 PM, Alexander Duyck wrote:
>> On Thu, Mar 14, 2019 at 9:43 AM Nitesh Narayan Lal <nitesh@xxxxxxxxxx> wrote:
>>> On 3/6/19 1:12 PM, Michael S. Tsirkin wrote:
>>>> On Wed, Mar 06, 2019 at 01:07:50PM -0500, Nitesh Narayan Lal wrote:
>>>>> On 3/6/19 11:09 AM, Michael S. Tsirkin wrote:
>>>>>> On Wed, Mar 06, 2019 at 10:50:42AM -0500, Nitesh Narayan Lal wrote:
>>>>>>> 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 5 guests(each with 5 GB memory) when page hinting was enabled and 3 without it. (Detailed explanation of the test procedure is provided at the bottom under Test - 1).
>>>>>>> Changelog in v9:
>>>>>>> * Guest free page hinting hook is now invoked after a page has been merged in the buddy.
>>>>>>> * Free pages only with order FREE_PAGE_HINTING_MIN_ORDER(currently defined as MAX_ORDER - 1) are captured.
>>>>>>> * Removed kthread which was earlier used to perform the scanning, isolation & reporting of free pages.
>>>>>>> * Pages, captured in the per cpu array are sorted based on the zone numbers. This is to avoid redundancy of acquiring zone locks.
>>>>>>> * Dynamically allocated space is used to hold the isolated guest free pages.
>>>>>>> * All the pages are reported asynchronously to the host via virtio driver.
>>>>>>> * Pages are returned back to the guest buddy free list only when the host response is received.
>>>>>>> Pending items:
>>>>>>> * Make sure that the guest free page hinting's current implementation doesn't break hugepages or device assigned guests.
>>>>>>> * Follow up on VIRTIO_BALLOON_F_PAGE_POISON's device side support. (It is currently missing)
>>>>>>> * Compare reporting free pages via vring with vhost.
>>>>>>> * Decide between MADV_DONTNEED and MADV_FREE.
>>>>>>> * Analyze overall performance impact due to guest free page hinting.
>>>>>>> * Come up with proper/traceable error-message/logs.
>>>>>>> Tests:
>>>>>>> 1. Use-case - Number of guests we can launch
>>>>>>> NUMA Nodes = 1 with 15 GB memory
>>>>>>> Guest Memory = 5 GB
>>>>>>> Number of cores in guest = 1
>>>>>>> Workload = test allocation program allocates 4GB memory, touches it via memset and exits.
>>>>>>> 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.
>>>>>>> Results:
>>>>>>> Without hinting = 3
>>>>>>> With hinting = 5
>>>>>>> 2. Hackbench
>>>>>>> Guest Memory = 5 GB
>>>>>>> Number of cores = 4
>>>>>>> Number of tasks Time with Hinting Time without Hinting
>>>>>>> 4000 19.540 17.818
>>>>>> How about memhog btw?
>>>>>> Alex reported:
>>>>>> My testing up till now has consisted of setting up 4 8GB VMs on a system
>>>>>> with 32GB of memory and 4GB of swap. To stress the memory on the system I
>>>>>> would run "memhog 8G" sequentially on each of the guests and observe how
>>>>>> long it took to complete the run. The observed behavior is that on the
>>>>>> systems with these patches applied in both the guest and on the host I was
>>>>>> able to complete the test with a time of 5 to 7 seconds per guest. On a
>>>>>> system without these patches the time ranged from 7 to 49 seconds per
>>>>>> guest. I am assuming the variability is due to time being spent writing
>>>>>> pages out to disk in order to free up space for the guest.
>>>>> Here are the results:
>>>>> Procedure: 3 Guests of size 5GB is launched on a single NUMA node with
>>>>> total memory of 15GB and no swap. In each of the guest, memhog is run
>>>>> with 5GB. Post-execution of memhog, Host memory usage is monitored by
>>>>> using Free command.
>>>>> Without Hinting:
>>>>> Time of execution Host used memory
>>>>> Guest 1: 45 seconds 5.4 GB
>>>>> Guest 2: 45 seconds 10 GB
>>>>> Guest 3: 1 minute 15 GB
>>>>> With Hinting:
>>>>> Time of execution Host used memory
>>>>> Guest 1: 49 seconds 2.4 GB
>>>>> Guest 2: 40 seconds 4.3 GB
>>>>> Guest 3: 50 seconds 6.3 GB
>>>> OK so no improvement. OTOH Alex's patches cut time down to 5-7 seconds
>>>> which seems better. Want to try testing Alex's patches for comparison?
>>> I realized that the last time I reported the memhog numbers, I didn't
>>> enable the swap due to which the actual benefits of the series were not
>>> shown.
>>> I have re-run the test by including some of the changes suggested by
>>> Alexander and David:
>>> * Reduced the size of the per-cpu array to 32 and minimum hinting
>>> threshold to 16.
>>> * Reported length of isolated pages along with start pfn, instead of
>>> the order from the guest.
>>> * Used the reported length to madvise the entire length of address
>>> instead of a single 4K page.
>>> * Replaced MADV_DONTNEED with MADV_FREE.
>>> Setup for the test:
>>> NUMA node:1
>>> Memory: 15GB
>>> Swap: 4GB
>>> Guest memory: 6GB
>>> Number of core: 1
>>> Process: A guest is launched and memhog is run with 6GB. As its
>>> execution is over next guest is launched. Everytime memhog execution
>>> time is monitored.
>>> Results:
>>> Without Hinting:
>>> Time of execution
>>> Guest1: 22s
>>> Guest2: 24s
>>> Guest3: 1m29s
>>> With Hinting:
>>> Time of execution
>>> Guest1: 24s
>>> Guest2: 25s
>>> Guest3: 28s
>>> When hinting is enabled swap space is not used until memhog with 6GB is
>>> ran in 6th guest.
>> So one change you may want to make to your test setup would be to
>> launch the tests sequentially after all the guests all up, instead of
>> combining the test and guest bring-up. In addition you could run
>> through the guests more than once to determine a more-or-less steady
>> state in terms of the performance as you move between the guests after
>> they have hit the point of having to either swap or pull MADV_FREE
>> pages.
> I tried running memhog as you suggested, here are the results:
> Setup for the test:
> NUMA node:1
> Memory: 15GB
> Swap: 4GB
> Guest memory: 6GB
> Number of core: 1
> Process: 3 guests are launched and memhog is run with 6GB. Results are
> monitored after 1st-time execution of memhog. Memhog is launched
> sequentially in each of the guests and time is observed after the
> execution of all 3 memhog is over.
> Results:
> Without Hinting
> ÂÂÂ Time of ExecutionÂÂÂ
> With Hinting
> Array size:16 Minimum Threshold:8
> 1.ÂÂÂ 2m57sÂÂÂ ÂÂÂ ÂÂÂ
> 2.ÂÂÂ 2m20sÂÂÂ ÂÂÂ ÂÂÂ
> The memhog execution time in the case of hinting is still not that low
> as we would have expected. This is due to the usage of swap space.
> Although wrt to non-hinting when swap used space is around 3.5G, with
> hinting it remains to around 1.1-1.5G.
> I did try using a zone free page barrier which prevented hinting when
> free pages of order HINTING_ORDER goes below 256. This further brings
> down the swap usage to 100-150 MB. The tricky part of this approach is
> to configure this barrier condition for different guests.
> Array size:16 Minimum Threshold:8
> 1.ÂÂÂ 1m16sÂÂÂ ÂÂÂ
> 2.ÂÂÂ 1m41s
> Note: Memhog time does seem to vary a little bit on every boot with or
> without hinting.

I don't quite understand yet why "hinting more pages" (no free page
barrier) should result in a higher swap usage in the hypervisor
(1.1-1.5GB vs. 100-150 MB). If we are "hinting more pages" I would have
guessed that runtime could get slower, but not that we need more swap.

One theory:

If you hint all MAX_ORDER - 1 pages, at one point it could be that all
"remaining" free pages are currently isolated to be hinted. As MM needs
more pages for a process, it will fallback to using "MAX_ORDER - 2"
pages and so on. These pages, when they are freed, you won't hint
anymore unless they get merged. But after all they won't get merged
because they can't be merged (otherwise they wouldn't be "MAX_ORDER - 2"
after all right from the beginning).

Try hinting a smaller granularity to see if this could actually be the case.



David / dhildenb