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

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Stefan Roesch <shr@xxxxxxxxxxxx> writes:

> David Hildenbrand <david@xxxxxxxxxx> writes:
>
>>>>> Obviously we could spend months analysing which exact allocations are
>>>>> identical, and then more months or years reworking the architecture to
>>>>> deduplicate them by hand and in userspace. But this isn't practical,
>>>>> and KSM is specifically for cases where this isn't practical.
>>>>> Based on your request in the previous thread, we investigated whether
>>>>> the boost was coming from the unintended side effects of KSM splitting
>>>>> THPs. This wasn't the case.
>>>>> If you have other theories on how the results could be bogus, we'd be
>>>>> happy to investigate those as well. But you have to let us know what
>>>>> you're looking for.
>>>>>
>>>>
>>>> Maybe I'm bad at making such requests but
>>>>
>>>> "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?"
>>>>
>>>> "In any case, it would be nice to get a feeling for how much variety in
>>>> these 20% of deduplicated pages are. "
>>>>
>>>> is pretty clear to me. And shouldn't take months.
>>>>
>>
>> Just to clarify: the details I requested are not meant to decide whether to
>> reject the patch set (I understand that it can be beneficial to have); I
>> primarily want to understand if we're really dealing with a workload where KSM
>> is able to deduplicate pages that are non-trivial, to maybe figure out if there
>> are other workloads that could similarly benefit -- or if we could optimize KSM
>> for these specific cases or avoid the memory deduplication altogether.
>>
>> In contrast to e.g.:
>>
>> 1) THP resulted in many zeropages we end up deduplicating again. The THP
>>    placement was unfortunate.
>>
>> 2) Unoptimized memory allocators that leave many identical pages mapped
>>    after freeing up memory (e.g., zeroed pages, pages all filled with
>>    poison values) instead of e.g., using MADV_DONTNEED to free up that
>>    memory.
>>
>>
>
> I repeated an experiment with and without KSM. In terms of THP there is
> no huge difference between the two. On a 64GB main memory machine I see
> between 100 - 400MB in AnonHugePages.
>
>>> /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.
>>
>> Thanks! To which value is /sys/kernel/mm/ksm/max_page_sharing set in that
>> environment?
>>
>
> It's set to the standard value of 256.
>
> In the meantime I have run experiments with different settings for
> pages_to_scan. With the default value of 100, we only get a relatively
> small benefit of KSM. If I increase the value to for instance to 2000 or
> 3000 the savings are substantial. (The workload is memory bound, not
> CPU bound).
>
> Here are some stats for setting pages_to_scan to 3000:
>
> full_scans: 560
> general_profit: 20620539008
> max_page_sharing: 256
> merge_across_nodes: 1
> pages_shared: 125446
> pages_sharing: 5259506
> pages_to_scan: 3000
> pages_unshared: 1897537
> pages_volatile: 12389223
> run: 1
> sleep_millisecs: 20
> stable_node_chains: 176
> stable_node_chains_prune_millisecs: 2000
> stable_node_dups: 2604
> use_zero_pages: 0
> zero_pages_sharing: 0
>
>
>> What would be interesting is pages_shared after max_page_sharing was set to a
>> very high number such that pages_shared does not include duplicates. Then
>> pages_shared actually expresses how many different pages we deduplicate. No need
>> to run without THP in that case.
>>
>
> Thats on my list for the next set of experiments.
>

In the new experiment I increased the max_page_sharing value to 16384.
This reduced the number of stable_node_dups considerably (its around 3%
of the previous value). However pages_sharing is still very high for
this workload.

full_scans: 138
general_profit: 24442268608
max_page_sharing: 16384
merge_across_nodes: 1
pages_shared: 144590
pages_sharing: 6230983
pages_to_scan: 3000
pages_unshared: 2120307
pages_volatile: 14590780
run: 1
sleep_millisecs: 20
stable_node_chains: 23
stable_node_chains_prune_millisecs: 2000
stable_node_dups: 78
use_zero_pages: 0
zero_pages_sharing: 0

>> Similarly, enabling "use_zero_pages" could highlight if your workload ends up
>> deduplciating a lot of zeropages. But maxing out max_page_sharing would be
>> sufficient to understand what's happening.
>>
>>
>
> I already run experiments with use_zero_pages, but they didn't make a
> difference. I'll repeat the experiment with a higher pages_to_scan
> value.
>
>>> Each of these individual processes has around 500MB in KSM pages.
>>>
>>
>> That's really a lot, thanks.
>>
>>> Also to give some idea for individual VMA's
>>> 7ef5d5600000-7ef5e5600000 rw-p 00000000 00:00 0 (Size: 262144 KB, KSM:
>>> 73160 KB)
>>>
>>
>> I'll have a look at the patches today.



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