Re: [RFC PATCH 0/4] KVM: ioctl for populating guest_memfd

Nikita Kalyazin
2024-11-20T17:21:44Z

On 20/11/2024 16:44, David Hildenbrand wrote:
>> If the problem is the "pagecache" overhead, then yes, it will be a
>> harder nut to crack. But maybe there are some low-hanging fruits to
>> optimize? Finding the main cause for the added overhead would be
>> interesting.

Agreed, knowing the exact root cause would be really nice.

> Can you compare uffdio_copy() when using anonymous memory vs. shmem?
> That's likely the best we could currently achieve with guest_memfd.

Yeah, I was doing that too. It was about ~28% slower in my setup, while
with guest_memfd it was ~34% slower. The variance of the data was quite
high so the difference may well be just noise. In other words, I'd be
much happier if we could bring guest_memfd (or even shmem) performance
closer to the anon/private than if we just equalised guest_memfd with
shmem (which are probably already pretty close).

> There is the tools/testing/selftests/mm/uffd-stress benchmark, not sure
> if that is of any help; it SEGFAULTS for me right now with a (likely)
> division by 0.

Thanks for the pointer, will take a look!

> Cheers,
>
> David / dhildenb
>