Re: [RFC][PATCH v11 0/2] mm: Support for page hinting

[Alexander Duyck]

On Wed, Jul 10, 2019 at 12:52 PM Nitesh Narayan Lal <nitesh@xxxxxxxxxx> wrote:

The results up here were redundant with what is below so I am just
dropping them. I would suggest only including one set of results in
any future cover page as it is confusing to duplicate it like that.

> This approach tracks all freed pages of the order MAX_ORDER - 2 in bitmaps.
> A new hook after buddy merging is used to set the bits in the bitmap.
> Currently, the bits are only cleared when pages are hinted, not when pages are
> re-allocated.
>
> Bitmaps are stored on a per-zone basis and are protected by the zone lock. A
> workqueue asynchronously processes the bitmaps as soon as a pre-defined memory
> threshold is met, trying to isolate and report pages that are still free.
>
> The isolated pages are reported via virtio-balloon, which is responsible for
> sending batched pages to the host synchronously. Once the hypervisor processed
> the hinting request, the isolated pages are returned back to the buddy.
>
> Changelog in v11:
> * Added logic to take care of multiple NUMA nodes scenarios.
> * Simplified the logic for reporting isolated pages to the host. (Eg. replaced
> dynamically allocated arrays with static ones, introduced wait event instead of
> the loop in order to wait for a response from the host)
> * Added a mutex to prevent race condition when page hinting is enabled by
> multiple drivers.
> * Simplified the logic responsible for decrementing free page counter for each
> zone.
> * Simplified code structuring/naming.
>
> Known work items for the future:
> * Test device assigned guests to ensure that hinting doesn't break it.
> * Follow up on VIRTIO_BALLOON_F_PAGE_POISON's device-side support.
> * Decide between MADV_DONTNEED and MADV_FREE.
> * Look into memory hotplug, more efficient locking, better naming conventions to
> avoid confusion with VIRTIO_BALLOON_F_FREE_PAGE_HINT support.
> * Come up with proper/traceable error-message/logs and look into other code
> simplifications. (If necessary).
>
> Benefit analysis:
> 1. Number of 5GB guests (each touching 4GB memory) that can be launched without
> swap usage on a system with 15GB:
> unmodified kernel - 2, 3rd with 2.5GB
> v11 page hinting - 6, 7th with 26MB
> v1 bubble hinting - 6, 7th with 1.8GB
>
> Conclusion - In this particular testcase on using v11 page hinting and
> v1 bubble-hinting 4 more guests could be launched without swapping compared
> to an unmodified kernel.
> For the 7th guest launch, v11 page hinting is slightly better than v1 Bubble
> hinting as it touches lesser swap space.

I'm confused by the comment. From what I can tell bubble hinting came
up with 1.8GB of memory while page hinting only managed to achieve
.026GB (Using the same units makes it easier to visualize the
difference). Also your test says "can be launched without swap usage",
yet you say the bubble hinting is touching swap which makes not sense
to me.

> Setup & procedure -
> Total NUMA Node Memory ~ 15 GB (All guests are run on a single NUMA node)
> Guest Memory = 5GB
> Number of CPUs in the guest = 1
> Host swap = 4GB
> Workload = test allocation program that allocates 4GB memory, touches it via
> memset and exits.
> 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. It is continued until the swap is not used.
>
> 2. Memhog execution time (For 3 guests each of 6GB on a system with 15GB):
> unmodified kernel - Guest1:21s, Guest2:27s, Guest3:2m37s swap used = 3.7GB
> v11 page hinting - Guest1:23s, Guest2:26s, Guest3:21s swap used = 0
> v1 bubble hinting - Guest1:23, Guest2:11s, Guest3:26s swap used = 0
>
> For this particular test-case in a guest which doesn't require swap access
> "memhog 6G" execution time lies within a range of 15-30s.
> Conclusion -
> In the above test case for an unmodified kernel on executing memhog in the
> third guest execution time rises to above 2minutes due to swap access.
> Using either page-hinting or bubble hinting brings this execution time to a
> a normal range of 15-30s.

So really this test doesn't add much in value. The whole reason why
Guest3 runs so much slower is because it is going to swap. I initially
did this to demonstrate a point, but now running this test doesn't
prove much as it isn't really meant to be a performance test. It is
essentially just a duplicate of the "how many guests can you run" test
that is passing itself off as some sort of performance test.

We could probably just drop this from future version of this as long
as we verify that the memory hinting is freeing most of the memory
back and the guest is reporting a size less than the total guest
memory size.

> Setup & procedure -
> Total NUMA Node Memory ~ 15 GB (All guests are run on a single NUMA node)
> Guest Memory = 6GB
> Number of CPUs in the guest = 4
> Process = 3 Guests are launched and the âmemhog 6Gâ execution time is monitored
> one after the other in each of them.
> Host swap = 4GB
>
> Performance Analysis:
> 1. will-it-scale's page_faul1
> Setup -
> Guest Memory = 6GB
> Number of cores = 24
>
> Unmodified kernel -
> 0,0,100,0,100,0
> 1,514453,95.84,519502,95.83,519502
> 2,991485,91.67,932268,91.68,1039004
> 3,1381237,87.36,1264214,87.64,1558506
> 4,1789116,83.36,1597767,83.88,2078008
> 5,2181552,79.20,1889489,80.08,2597510
> 6,2452416,75.05,2001879,77.10,3117012
> 7,2671047,70.90,2263866,73.22,3636514
> 8,2930081,66.75,2333813,70.60,4156016
> 9,3126431,62.60,2370108,68.28,4675518
> 10,3211937,58.44,2454093,65.74,5195020
> 11,3162172,54.32,2450822,63.21,5714522
> 12,3154261,50.14,2272290,58.98,6234024
> 13,3115174,46.02,2369679,57.74,6753526
> 14,3150511,41.86,2470837,54.02,7273028
> 15,3134158,37.71,2428129,51.98,7792530
> 16,3143067,33.57,2340469,49.54,8312032
> 17,3112457,29.43,2263627,44.81,8831534
> 18,3089724,25.29,2181879,38.69,9351036
> 19,3076878,21.15,2236505,40.01,9870538
> 20,3091978,16.95,2266327,35.00,10390040
> 21,3082927,12.84,2172578,28.12,10909542
> 22,3055282,8.73,2176269,29.14,11429044
> 23,3081144,4.56,2138442,24.87,11948546
> 24,3075509,0.45,2173753,21.62,12468048
>
> page hinting -
> 0,0,100,0,100,0
> 1,491683,95.83,494366,95.82,494366
> 2,988415,91.67,919660,91.68,988732
> 3,1344829,87.52,1244608,87.69,1483098
> 4,1797933,83.37,1625797,83.70,1977464
> 5,2179009,79.21,1881534,80.13,2471830
> 6,2449858,75.07,2078137,76.82,2966196
> 7,2732122,70.90,2178105,73.75,3460562
> 8,2910965,66.75,2340901,70.28,3954928
> 9,3006665,62.61,2353748,67.91,4449294
> 10,3164752,58.46,2377936,65.08,4943660
> 11,3234846,54.32,2510149,63.14,5438026
> 12,3165477,50.17,2412007,59.91,5932392
> 13,3141457,46.05,2421548,57.85,6426758
> 14,3135839,41.90,2378021,53.81,6921124
> 15,3109113,37.75,2269290,51.76,7415490
> 16,3093613,33.62,2346185,48.73,7909856
> 17,3086542,29.49,2352140,46.19,8404222
> 18,3048991,25.36,2217144,41.52,8898588
> 19,2965500,21.18,2313614,38.18,9392954
> 20,2928977,17.05,2175316,35.67,9887320
> 21,2896667,12.91,2141311,28.90,10381686
> 22,3047782,8.76,2177664,28.24,10876052
> 23,2994503,4.58,2160976,22.97,11370418
> 24,3038762,0.47,2053533,22.39,11864784
>
> bubble-hinting v1 -
> 0,0,100,0,100,0
> 1,515272,95.83,492355,95.81,515272
> 2,985903,91.66,919653,91.68,1030544
> 3,1475300,87.51,1353723,87.65,1545816
> 4,1783938,83.36,1586307,83.78,2061088
> 5,2093307,79.20,1867395,79.95,2576360
> 6,2441370,75.05,2055421,76.65,3091632
> 7,2650471,70.89,2246014,72.93,3606904
> 8,2926782,66.75,2333601,70.41,4122176
> 9,3107617,62.60,2383112,68.46,4637448
> 10,3192332,58.44,2441626,65.84,5152720
> 11,3268043,54.32,2235964,62.92,5667992
> 12,3191105,50.18,2449045,60.49,6183264
> 13,3145317,46.05,2377317,57.80,6698536
> 14,3161552,41.91,2395814,53.26,7213808
> 15,3140443,37.77,2333200,51.42,7729080
> 16,3130866,33.65,2150967,46.11,8244352
> 17,3112894,29.52,2372068,45.93,8759624
> 18,3078424,25.39,2336211,39.85,9274896
> 19,3036457,21.27,2224821,35.25,9790168
> 20,3046330,17.13,2199755,37.43,10305440
> 21,2981130,12.98,2214862,28.67,10820712
> 22,3017481,8.84,2195996,29.69,11335984
> 23,2979906,4.68,2173395,25.90,11851256
> 24,2971170,0.52,2134311,21.89,12366528

Okay, so this doesn't match up with the results you gave me last time
(https://lore.kernel.org/lkml/afac6f92-74f5-4580-0303-12b7374e5011@xxxxxxxxxx/),
and actually more closely matches what I was expecting to see. The
bubble-hinting patches are performing within a few percent of what the
baseline kernel was doing. I am assuming the results from before had
some additional debugging enabled for the bubble-hinting test that
wasn't enabled for the other ones.

> Conclusion -
> For an unmodified kernel, with every fresh boot, there is 3-4% delta observed
> in the results wrt the numbers mentioned above. For both bubble-hinting and
> page-hinting, there was no noticeable degradation observed other than the
> expected variability mentioned earlier.
>
> Page hinting vs bubble hinting:
> From the benefits and performance perspective, both solutions look quite similar
> so far. However, unlike bubble-hinting which is more invasive, the overall core
> mm changes required for page hinting are minimal.
>
> [1] https://lkml.org/lkml/2019/6/19/926

So I think I called it out in the review of the patch but I think we
may want to see what happens if we increase the size of the memory in
the guest to something more like 64G or larger. My main concern is
that as we increase the size of memory the walk through the bitmap is
going to become more and more expensive and I am worried that at some
point it will start impacting the results.