Re: [PATCH v4 0/3] mm: process/cgroup ksm support

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David Hildenbrand <david@xxxxxxxxxx> writes:

> On 15.03.23 22:19, Johannes Weiner wrote:
>> On Wed, Mar 15, 2023 at 05:05:47PM -0400, Johannes Weiner wrote:
>>> On Wed, Mar 15, 2023 at 09:03:57PM +0100, David Hildenbrand wrote:
>>>> On 10.03.23 19:28, Stefan Roesch wrote:
>>>>> So far KSM can only be enabled by calling madvise for memory regions. To
>>>>> be able to use KSM for more workloads, KSM needs to have the ability to be
>>>>> enabled / disabled at the process / cgroup level.
>>>>>
>>>>> Use case 1:
>>>>> The madvise call is not available in the programming language. An example for
>>>>> this are programs with forked workloads using a garbage collected language without
>>>>> pointers. In such a language madvise cannot be made available.
>>>>>
>>>>> In addition the addresses of objects get moved around as they are garbage
>>>>> collected. KSM sharing needs to be enabled "from the outside" for these type of
>>>>> workloads.
>>>>>
>>>>> Use case 2:
>>>>> The same interpreter can also be used for workloads where KSM brings no
>>>>> benefit or even has overhead. We'd like to be able to enable KSM on a workload
>>>>> by workload basis.
>>>>>
>>>>> Use case 3:
>>>>> With the madvise call sharing opportunities are only enabled for the current
>>>>> process: it is a workload-local decision. A considerable number of sharing
>>>>> opportuniites may exist across multiple workloads or jobs. Only a higler level
>>>>> entity like a job scheduler or container can know for certain if its running
>>>>> one or more instances of a job. That job scheduler however doesn't have
>>>>> the necessary internal worklaod knowledge to make targeted madvise calls.
>>>>>
>>>>> Security concerns:
>>>>> In previous discussions security concerns have been brought up. The problem is
>>>>> that an individual workload does not have the knowledge about what else is
>>>>> running on a machine. Therefore it has to be very conservative in what memory
>>>>> areas can be shared or not. However, if the system is dedicated to running
>>>>> multiple jobs within the same security domain, its the job scheduler that has
>>>>> the knowledge that sharing can be safely enabled and is even desirable.
>>>>>
>>>>> Performance:
>>>>> Experiments with using UKSM have shown a capacity increase of around 20%.
>>>>
>>>> Stefan, can you do me a favor and investigate which pages we end up
>>>> deduplicating -- especially if it's mostly only the zeropage and if it's
>>>> still that significant when disabling THP?
>>>>
>>>>
>>>> I'm currently investigating with some engineers on playing with enabling KSM
>>>> on some selected processes (enabling it blindly on all VMAs of that process
>>>> via madvise() ).
>>>>
>>>> One thing we noticed is that such (~50 times) 20MiB processes end up saving
>>>> ~2MiB of memory per process. That made me suspicious, because it's the THP
>>>> size.
>>>>
>>>> What I think happens is that we have a 2 MiB area (stack?) and only touch a
>>>> single page. We get a whole 2 MiB THP populated. Most of that THP is zeroes.
>>>>
>>>> KSM somehow ends up splitting that THP and deduplicates all resulting
>>>> zeropages. Thus, we "save" 2 MiB. Actually, it's more like we no longer
>>>> "waste" 2 MiB. I think the processes with KSM have less (none) THP than the
>>>> processes with THP enabled, but I only took a look at a sample of the
>>>> process' smaps so far.
>>>
>>> THP and KSM is indeed an interesting problem. Better TLB hits with
>>> THPs, but reduced chance of deduplicating memory - which may or may
>>> not result in more IO that outweighs any THP benefits.
>>>
>>> That said, the service in the experiment referenced above has swap
>>> turned on and is under significant memory pressure. Unused splitpages
>>> would get swapped out. The difference from KSM was from deduplicating
>>> pages that were in active use, not internal THP fragmentation.
>> Brainfart, my apologies. It could have been the ksm-induced splits
>> themselves that allowed the unused subpages to get swapped out in the
>> first place.
>
> Yes, it's not easy to spot that this is implemented. I just wrote a simple
> reproducer to confirm: modifying a single subpage in a bunch of THP ranges will
> populate a THP whereby most of the THP is zeroes.
>
> As long as you keep accessing the single subpage via the PMD I assume chances of
> getting it swapped out are lower, because the folio will be references/dirty.
>
> KSM will come around and split the THP filled mostly with zeroes and deduplciate
> the resulting zero pages.
>
> [that's where a zeropage-only KSM could be very valuable eventually I think]
>

We can certainly run an experiment where THP is turned off to verify if we
observe similar savings,

>> But no, I double checked that workload just now. On a weekly average,
>> it has about 50 anon THPs and 12 million regular anon. THP is not a
>> factor in the reduction results.
>
> You mean with KSM enabled or with KSM disabled for the process? Not sure if your
> observation reliably implies that the scenario described couldn't have happened,
> but it's late in Germany already :)
>
> In any case, it would be nice to get a feeling for how much variety in these 20%
> of deduplicated pages are. For example, if it's 99% the same page or just a wild
> collection.
>
> Maybe "cat /sys/kernel/mm/ksm/pages_shared" would be expressive already. But I
> seem to be getting "126" in my simple example where only zeropages should get
> deduplicated, so I have to take another look at the stats tomorrow ...

/sys/kernel/mm/ksm/pages_shared is over 10000 when we run this on an
Instagram workload. The workload consists of 36 processes plus a few
sidecar processes.

Also to give some idea for individual VMA's

7ef5d5600000-7ef5e5600000 rw-p 00000000 00:00 0 (Size: 262144 KB, KSM: 73160 KB)



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