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Posted Mar 30, 2007 5:40 UTC (Fri) by IkeTo (subscriber, #2122)
In reply to: Really? by man_ls
Parent article: Application-friendly kernel interfaces

> I mean, you design a hard-to-use interface, then write your own code which
> presents a friendly interface to userspace -- and you write it in
> userspace. Well, why not present a friendly interface in the kernel in the
> first place?

Perhaps the whole hugetlb thing tells one of the possible reasons. The original /dev/hshm interface is actually more general than the /dev/hugetlb interface: it allows multiple processes unrelated in ancestry to share the same piece of huge page. It is probably preferable for the kernel API to use only the general interface rather than having to implement both, since every time the interface change it needs to have a "global search" for libraries/applications using the interface, and leave enough time for those libraries/applications to change (if Linus does not say "no" to the change right away). So it might be preferable to implement just the general interface, hoping that it will never change at all; and have another library "cast" it to various different forms that are "more friendly" forms like the hugetlb interface. What unclear to me is actually why one would expect that the new library could be exempted from the global search if it needs to be changed.

I think instead of a general liblinux, we should be contented with the tested solutions of, e.g., pthread (futex) and libfam (dnotify): if the functionality fits well into a general audience, the easier interface is implemented in libc, and if it is not, the easier interface is implemented in a functionality specific library. That way, when the generic interface is changed, the kernel developers have fewer places to search for direct users of them; and the specific interface is usable (and thus relied upon) by a more narrow set of end-user applications.

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