Re: [RFC PATCH 0/6] Introduce Copy-On-Write to Page Table

David Hildenbrand
2022-05-21T20:22:18Z

On 21.05.22 22:12, Matthew Wilcox wrote:
> On Sat, May 21, 2022 at 06:07:27PM +0200, David Hildenbrand wrote:
>> I'm missing the most important point: why do we care and why should we
>> care to make our COW/fork implementation even more complicated?
>>
>> Yes, we might save some page tables and we might reduce the fork() time,
>> however, which specific workload really benefits from this and why do we
>> really care about that workload? Without even hearing about an example
>> user in this cover letter (unless I missed it), I naturally wonder about
>> relevance in practice.
>
> As I get older (and crankier), I get less convinced that fork() is
> really the right solution for implementing system().

Heh, I couldn't agree more. IMHO, fork() is mostly a blast from the
past. There *are* still a lot of user and there are a couple of sane use
cases.

Consequently, I am not convinced that it is something to optimize for,
especially if it adds additional complexity. For the use case of
snapshotting, we have better mechanisms nowadays (uffd-wp) that avoid
messing with copying address spaces.

Calling fork()/system() from a big, performance-sensitive process is
usually a bad idea.

Note: there is an (for me) interesting paper about this topic from 2019
("A fork() in the road"), although it might be a bit biased coming from
Microsoft research :). It comes to a similar conclusion regarding fork
and how it should or shouldn't dictate our OS design.

[1] https://www.microsoft.com/en-us/research/publication/a-fork-in-the-road/

--
Thanks,

David / dhildenb