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    Subject[bisected] pty performance problem
    I posted this to the kernel-newbies list, but have graduated to the
    adults forum:

    ! Two C programs are having a query-response conversation through a
    ! pseudo terminal:
    ! A (client) -- forever { send query; read response }
    ! B (server) -- forever { read query; send response }
    ! Neither has any I/O apart from the pty conversation, so I'd expect to
    ! see CPU usage at 100%. When I ran it, the CPU was pretty well idle.
    ! After a fair bit of fiddling, it turned out that both sides were
    ! taking about 8ms for their read() calls. At that point it seemed
    ! pretty clear that this was a delay in the kernel, not the code.

    2.6.31-rc2-00205-gb4b21ca good
    2.6.31-rc2-00206-gd945cb9 bad

    and still bad with the latest: 2.6.32-rc8-00011-ga8a8a66

    the git log says:
    ! commit d945cb9cce20ac7143c2de8d88b187f62db99bdc
    ! Author: Alan Cox <>
    ! Date: Tue Jul 7 16:39:41 2009 +0100
    ! pty: Rework the pty layer to use the normal buffering logic
    ! This fixes the ppp problems and various other issues with call locking
    ! caused by one side of a pty called in one locking context trying to match
    ! another with differing rules on the other side. We also get a big slack
    ! space to work with that means we can bury the flow control deadlock case
    ! for any conceivable real world situation.
    ! Signed-off-by: Alan Cox <>
    ! Signed-off-by: Linus Torvalds <>

    I can provide reasonably stripped down code which demonstrates the
    problem. It has been reproduced by one other person, though his delay
    was about 2ms.

    Have fun,


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