On 14.02.23 17:58, David Hildenbrand wrote:
Honestly, for improving the fork(), I have an idea to skip the per-page
operation without breaking the logic. However, this will introduce the
complicated mechanism and may has the overhead for other features. It
might not be worth it. It's hard to strike a balance between the
over-complicated mechanism with (probably) better performance and data
consistency with the page status. So, I would focus on the safety and
stable approach at first.
Yes, it is most probably possible, but complexity, robustness and
maintainability have to be considered as well.
Thanks for implementing this approach (only deduplication without other
optimizations) and evaluating it accordingly. It's certainly "cleaner", such
that we only have to mess with unsharing and not with other
accounting/pinning/mapcount thingies. But it also highlights how intrusive
even this basic deduplication approach already is -- and that most benefits
of the original approach requires even more complexity on top.
I am not quite sure if the benefit is worth the price (I am not to decide
and I would like to hear other options).
I'm looking at the discussion of page table sharing in 2002 [1].
It looks like in 2002 ~ 2006, there also have some patches try to
improve fork().
After that, I also saw one thread which is about another shared page
table patch's benchmark. I can't find the original patch though [2].
But, I found the probably same patch in 2005 [3], it also mentioned
the previous benchmark discussion:
"
For those familiar with the shared page table patch I did a couple of years
ago, this patch does not implement copy-on-write page tables for private
mappings. Analysis showed the cost and complexity far outweighed any
potential benefit.
"
Thanks for the pointer, interesting read. And my personal opinion is
that part of that statement still hold true :)
However, it might be different right now. For example, the implemetation
. We have split page table lock now, so we don't have to consider the
page_table_share_lock thing. Also, presently, we have different use
cases (shells [2] v.s. VM cloning and fuzzing) to consider.
Oh, and because I stumbled over it, just as an interesting pointer on
QEMU devel:
"[PATCH 00/10] Retire Fork-Based Fuzzing" [1]
[1] https://lore.kernel.org/all/20230205042951.3570008-1-alxndr@xxxxxx/T/#u
--
Thanks,
David / dhildenb
