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    SubjectRe: [bisected] pty performance problem

    (Cc:-ed Alan and Linus - mail repeated below. eot.)

    * Robert Swan <swan.r.l@gmail.com> wrote:

    > 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.
    > !
    > [snip]
    >
    > 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 <alan@linux.intel.com>
    > ! 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 <alan@linux.intel.com>
    > ! Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    >
    > 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,
    >
    > Rob.
    > --
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