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

From: David Hildenbrand <david@redhat.com>
To: Stefan Roesch <shr@devkernel.io>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	kernel-team@fb.com, linux-mm@kvack.org, riel@surriel.com,
	mhocko@suse.com, linux-kselftest@vger.kernel.org,
	linux-doc@vger.kernel.org, Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH v4 0/3] mm: process/cgroup ksm support
Date: Mon, 3 Apr 2023 11:48:59 +0200	[thread overview]
Message-ID: <f969cb1f-651f-592f-7540-89f73e175c7d@redhat.com> (raw)
In-Reply-To: <qvqwv8ii89x6.fsf@dev0134.prn3.facebook.com>

>>> 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.

> 
> /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?

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.

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.

> 
> 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.

-- 
Thanks,

David / dhildenb