Re: [RFC PATCH 0/4] KVM: ioctl for populating guest_memfd
From: Nikita Kalyazin
Date: Wed Nov 20 2024 - 12:22:40 EST
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