On 10.01.25 16:46, Nikita Kalyazin wrote:
Based on David's suggestion for speeding up guest_memfd memory
population [1] made at the guest_memfd upstream call on 5 Dec 2024 [2],
this adds `filemap_grab_folios` that grabs multiple folios at a time.
Hi,
Motivation
When profiling guest_memfd population and comparing the results with
population of anonymous memory via UFFDIO_COPY, I observed that the
former was up to 20% slower, mainly due to adding newly allocated pages
to the pagecache. As far as I can see, the two main contributors to it
are pagecache locking and tree traversals needed for every folio. The
RFC attempts to partially mitigate those by adding multiple folios at a
time to the pagecache.
Testing
With the change applied, I was able to observe a 10.3% (708 to 635 ms)
speedup in a selftest that populated 3GiB guest_memfd and a 9.5% (990 to
904 ms) speedup when restoring a 3GiB guest_memfd VM snapshot using a
custom Firecracker version, both on Intel Ice Lake.
Does that mean that it's still 10% slower (based on the 20% above), or
were the 20% from a different micro-benchmark?
Limitations
While `filemap_grab_folios` handles THP/large folios internally and
deals with reclaim artifacts in the pagecache (shadows), for simplicity
reasons, the RFC does not support those as it demonstrates the
optimisation applied to guest_memfd, which only uses small folios and
does not support reclaim at the moment.
It might be worth pointing out that, while support for larger folios is
in the works, there will be scenarios where small folios are unavoidable
in the future (mixture of shared and private memory).
How hard would it be to just naturally support large folios as well?
We do have memfd_pin_folios() that can deal with that and provides a
slightly similar interface (struct folio **folios).
For reference, the interface is:
long memfd_pin_folios(struct file *memfd, loff_t start, loff_t end,
struct folio **folios, unsigned int max_folios,
pgoff_t *offset)
Maybe what you propose could even be used to further improve
memfd_pin_folios() internally? However, it must do this FOLL_PIN thingy,
so it must process each and every folio it processed.
--
Cheers,
David / dhildenb
