Messages in this thread | | | Date | Sun, 22 Nov 2009 09:23:19 +1100 | From | Robert Swan <> | Subject | [bisected] pty performance problem |
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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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