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

[Stefan Roesch <shr@devkernel.io>]

David Hildenbrand <david@redhat.com> 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.
> 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.