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    SubjectRe: [patch 1/1] uml: fixes performance regression in activate_mm and thus exec()
    Date
    On Friday 12 August 2005 22:11, Benjamin LaHaise wrote:
    > Is it possible to get an optimization for this case where uml can execute
    > the kernel thread in the same process as it normally executes kernel mode
    > for the given mm? AIO performance on uml is pretty bad when it has to
    > access userspace.
    I thought to another solution to that, i.e. use get_user_pages() and kmap()
    the resulting pages, like other kernel threads do when accessing userspace
    context.

    However, by looking at the code (starting from aio_pread), maybe AIO reuses
    too much normal I/O code to do this.

    Actually, the changes wouldn't maybe be so intrusive, they're limited to
    __generic_file_aio_read (conditionalize access_ok, and change the setup of
    read_descriptor_t) and file_read_actor, plus the write side, plus anything
    not using pagecache - generic_file_aio_read.

    Having a different actor for AIO would work. The real problem is adding a
    field to iovec to specify whether we're passing in a page array (given by
    get_user_pages) rather than a void __user *, or passing an iovec "wrapper"
    structure, which can carry either a normal iovec or the new kind of
    descriptor.

    However, this does not sound like too much added complexity. Instead, allowing
    to move a kernel thread elsewhere on the host is a lot more difficult.
    --
    Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
    Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
    http://www.user-mode-linux.org/~blaisorblade






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