Messages in this thread | | | From | Linus Torvalds <> | Date | Tue, 22 Jan 2019 10:34:06 +1300 | Subject | Re: [RFC PATCH v2 02/13] epoll: introduce user structures for polling from userspace |
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So I'm not entirely convinced, but I guess actual numbers and users might convince me otherwise.
However, a quick comment:
On Tue, Jan 22, 2019 at 9:15 AM Roman Penyaev <rpenyaev@suse.de> wrote: > > +struct epoll_uitem { > + __poll_t ready_events; > + struct epoll_event event; > +};
This really ends up being a horrible data structure.
struct epoll_event is declared as
struct epoll_event { __poll_t events; __u64 data; } EPOLL_PACKED;
and __poll_t is "unsigned". So on pretty much all 64-bit architectures except for x86-64 (which sets that packed attribute), you have a packing hole there in between the events and the data, and "struct epoll_event" has 8-byte alignment.
Now, in "struct epoll_uitem", you end up having *another* packing hold in between "ready_events" and "struct epoll_event".
So this data structure that has 16 bytes of actual data, ends up being 24 bytes in size.
Again, x86-64 happens to be the exception to this, but that's a random small implementation detail, not a design thing.
I think "struct epoll_event" was badly designed to begin with to have this issue, but it shouldn't then be an excuse to make things even worse with this array of "struct epoll_uitem" things.
Hmm?
Linus
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