Re: [RFC PATCH 0/2] SKSM: Synchronous Kernel Samepage Merging

[David Hildenbrand <david@redhat.com>]

On 28.02.25 16:01, Mathieu Desnoyers wrote:
> On 2025-02-28 08:59, David Hildenbrand wrote:
>> On 28.02.25 06:17, Linus Torvalds wrote:
>>> On Thu, 27 Feb 2025 at 19:03, Mathieu Desnoyers
>>> <mathieu.desnoyers@efficios.com> wrote:
>>>>
>>>> I'd be fine with SKSM replacing KSM entirely. However, I don't
>>>> think we should try to re-implement the existing KSM userspace ABIs
>>>> over SKSM.
>>>
>>> No, absolutely. The only point (for me) for your new synchronous one
>>> would be if it replaced the kernel thread async scanning, which would
>>> make the old user space interface basically pointless.
>>>
>>> But I don't actually know who uses KSM right now. My reaction really
>>> comes from a "it's not nice code in the kernel", not from any actual
>>> knowledge of the users.
>>>
>>> Maybe it works really well in some cloud VM environment, and we're
>>> stuck with it forever.
>>
>> Exactly that; and besides the VM use-case, lately people stated using it
>> in the context of interpreters (IIRC inside Meta) quite successfully as
>> well.
>>
> 
> I suspect that SKSM is a better fit for JIT and code patching than KSM,
> because user-space knows better when a set of pages is going to become
> invariant for a long time and thus benefit from merging. This removes
> the background scanning from the picture.
 > > Does the interpreter use-case require background scanning, or does
> it know when a set of pages are meant to become invariant for a long
> time ?

To make the JIT/interpreter use case happy, people wanted ways to 
*force* KSM on for *the whole process*, not just individual VMAs like 
the traditional VM use case would have done.

I recall one of the reasons being that you don't really want to modify 
your JIT/interpreter to just make KSM work.

See [1] "KSM at Meta" for some details, and in general, optimization 
work to adapt KSM to new use cases.

Regarding some concerns you raised, Stefan did a lot of optimization 
work like "smart scanning" (slide "Optimization - Smart Scan (6.7)") to 
reduce the scanning overhead and make it much more efficient.

So people started optimizing for that already and got pretty good results.

[1] 
https://lpc.events/event/17/contributions/1625/attachments/1320/2649/KSM.pdf

-- 
Cheers,

David / dhildenb