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    SubjectRe: [GIT PULL] kdbus for 4.1-rc1
    On Wed, Apr 15, 2015 at 03:28:58PM -0700, Andy Lutomirski wrote:
    > On Wed, Apr 15, 2015 at 3:18 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
    > > On Wed, Apr 15, 2015 at 03:11:17PM -0700, Andy Lutomirski wrote:
    > >
    > >> This is functionally identical to passing AF_UNIX socket fds over
    > >> SCM_RIGHTS, but I want something much lighter weight.
    > >
    > > Most of the weight in SCM_RIGHTS comes from the fact that you can
    > > pass AF_UNIX sockets over it, which requires a garbage collector.
    > > Exclude that and suddenly it becomes very cheap...
    >
    > I should have been more specific. I don't mean the performance of
    > SCM_RIGHTS itself; I mean the memory overhead of keeping tons of fds
    > around, each with their socket data structures and buffers.
    >
    > I think that dbus could be quite efficiently implemented with a
    > userspace daemon that just introduces peers to each other, but the fd
    > explosion could be rather bad for some use cases.
    >
    > I'll be the first to admit that I don't have a clean API in mind.
    > There was a lightweight fd proposal way back when, but it never went
    > anywhere, and it might not be suitable anyway.

    Wait, are you talking about the overhead of descriptors used for capability
    tokens (essentially zero - one system-wide struct file per capability +
    one pointer in descriptor table of anyone who holds it + two bits in
    bitmaps in the sam descriptor tables) or about the overhead of descriptors
    used to send/receive those over? The latter don't have to be sockets
    at all - they could bloody well be files on some ipcfs, or character device,
    or FIFOs, etc.


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